Metabolism of Fatty Acids and Proteins

Fatty acids and amino acids are the MCAT's "backup fuel" molecules — the substrates your body turns to when carbohydrates run low, when you're fasting, or when you're running a marathon past the 90-minute mark. Understanding how these molecules are broken down, built up, and interconverted will not only earn you points on biochemistry passages, but will give you the mechanistic insight to handle physiology passages on diabetes, starvation, obesity, and inborn errors of metabolism. Let's build that understanding from the ground up.

How to read the priority labels: Must know = know it cold. Know the logic = understand the mechanism/why, don't memorize every name. Passage-level = recognize it if a passage gives it; don't memorize. Optional = deeper detail, safe to skip on a first pass.

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Fatty Acid Structure and Classification

Must know

Before you can understand how fatty acids are metabolized, you need to know what they look like and the vocabulary the MCAT uses to describe them.

A fatty acid is a long-chain carboxylic acid with the general formula $\ce{CH3(CH2)_nCOOH}$. The carboxyl carbon is C-1; the carbon adjacent to it is the α carbon, the next is the β carbon, and the terminal methyl carbon is the ω (omega) carbon. This directionality matters when you get to β-oxidation (oxidation occurs at the β carbon).

Saturated vs. Unsaturated Fatty Acids

Must know

Saturated fatty acids have no $\ce{C=C}$ double bonds; unsaturated fatty acids have one (monounsaturated) or more (polyunsaturated). High-yield examples: palmitic acid 16:0, stearic acid 18:0, oleic acid 18:1, linoleic acid 18:2, α-linolenic acid 18:3.

A critical structural point: naturally occurring unsaturated fatty acids have cis double bonds. The cis kink prevents tight packing, lowers the melting point, and increases membrane fluidity. Trans fats are unsaturated fats with trans double bonds; they pack more like saturated fats and decrease membrane fluidity relative to cis unsaturated fats.

Know the logic

Shorthand notation: carbon count : number of double bonds, with the first double bond located by Δ (counting from the carboxyl end) or ω/n (from the methyl end). You don't need to memorize exact Δ positions — recognize that ω-3 vs. ω-6 just states where the double bond falls relative to the methyl tail.

Essential fatty acids — linoleic acid (18:2, ω-6) and α-linolenic acid (18:3, ω-3) — cannot be synthesized by humans and must come from the diet. Arachidonic acid (ω-6), the precursor to prostaglandins and leukotrienes, can be made from linoleic acid.

Fatty Acid Ionization and the Carboxylate Group

Must know

The carboxyl group has $\mathrm{p}K_a \approx 4{-}5$, so at physiological pH (7.4) it is mostly deprotonated — free fatty acids exist mainly as carboxylates ("free fatty acids," FFA). In plasma, these nonesterified fatty acids are transported bound to albumin because their hydrocarbon chains are hydrophobic.

Quick check: Palmitic acid is described as 16:0. What does this tell you?

Answer: It has 16 carbons and zero double bonds — it is a saturated fatty acid. No kinks, high melting point, solid at room temperature (think butter).

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Digestion, Mobilization, and Transport of Fats

Digestion

Must know

Dietary fat is overwhelmingly in the form of triglycerides (triacylglycerols, TAG) — a glycerol backbone esterified to three fatty acids. The digestion of fat begins in the mouth (lingual lipase) and stomach (gastric lipase), but the major enzymatic work happens in the small intestine.

Because fats are hydrophobic, they must be emulsified before enzymes can act on them. Bile salts (synthesized in the liver from cholesterol, stored in the gallbladder, released into the duodenum) act as biological detergents — their amphipathic structure allows them to surround fat droplets and break them into smaller micelles, dramatically increasing the surface area available to lipases.

Pancreatic lipase (with colipase as a cofactor) then cleaves fatty acids from the sn-1 and sn-3 positions of the triglyceride, producing two free fatty acids and one 2-monoacylglycerol per TAG.

These products are packaged into micelles and absorbed across the brush border of enterocytes. Inside the enterocyte, triglycerides are reassembled (re-esterification), packaged with cholesterol, cholesterol esters, phospholipids, and apolipoproteins (especially ApoB-48) to form chylomicrons, which are secreted into the lymph and then the bloodstream via the thoracic duct.

Lipoprotein Transport

Must know

The MCAT tests lipoproteins heavily. Density increases as protein content rises and lipid content falls — chylomicrons are least dense, HDL is most dense.

Lipoprotein	Main Role	Key Apo	MCAT Association
Chylomicron	Carries dietary TAG from intestine	ApoB-48	Lowest density, highest TAG
VLDL	Carries endogenous (liver) TAG	ApoB-100	Precursor to IDL/LDL
IDL	VLDL remnant	ApoB-100	Intermediate particle
LDL	Delivers cholesterol to tissues	ApoB-100	"Bad cholesterol"
HDL	Reverse cholesterol transport	ApoA-I	"Good cholesterol"

The four apolipoproteins worth knowing cold: ApoB-48 (chylomicrons), ApoB-100 (VLDL/IDL/LDL), ApoC-II (activates LPL), ApoA-I (HDL). Passage-level ApoE mediates remnant uptake.

Lipoprotein lipase (LPL), found on the luminal surface of capillary endothelium in muscle and adipose, is the enzyme that hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids for uptake into tissues. ApoC-II activates LPL.

Mobilization of Stored Fat

Must know

In adipocytes, triglycerides are stored in lipid droplets. When energy is needed (fasting, exercise, stress), hormone-sensitive lipase (HSL) is activated by glucagon/epinephrine and inhibited by insulin; it hydrolyzes stored TAG → fatty acids + glycerol. (The whole-body hormonal control of the fed/fasted switch is covered in the Metabolism (whole-body integration) guide; here the point is that HSL is the enzyme that releases the fatty-acid substrate for β-oxidation.)

Free fatty acids are insoluble in plasma and are transported bound to albumin to target tissues (muscle, liver, kidney) for oxidation.

Glycerol, the other product, travels to the liver where it can enter glycolysis as glycerol-3-phosphate (→ DHAP), making it a gluconeogenic substrate — a critical point for understanding fasting physiology.

Quick check: A patient has a genetic deficiency of ApoC-II. What would you expect to see in their blood, and why?

Answer: Markedly elevated triglycerides (hypertriglyceridemia). ApoC-II activates lipoprotein lipase; without it, chylomicrons and VLDL cannot be cleared from circulation.

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Oxidation of Fatty Acids: β-Oxidation

Activation and the Carnitine Shuttle

Must know

Fatty acid oxidation occurs in the mitochondrial matrix, but fatty acids must first be activated in the cytoplasm. Fatty acyl-CoA synthetase (also called acyl-CoA ligase) on the outer mitochondrial membrane catalyzes:

$\ce{Fatty acid + CoA + ATP -> Fatty acyl-CoA + AMP + PP_i}$

Note carefully: ATP → AMP + PP$_i$, which is equivalent to consuming 2 ATP equivalents (the pyrophosphate is hydrolyzed by pyrophosphatase, which drives the reaction forward but costs an additional high-energy bond). This activation step is the "entry fee" for β-oxidation.

Long-chain fatty acyl-CoA cannot cross the inner mitochondrial membrane directly. It relies on the carnitine shuttle:

1. Carnitine acyltransferase I (CAT I / CPT-I) on the outer mitochondrial membrane transfers the acyl group from CoA to carnitine, forming acylcarnitine.
2. Acylcarnitine is transported across the inner mitochondrial membrane by a translocase (antiporter that simultaneously moves free carnitine back out).
3. CAT II (CPT-II) on the matrix side of the inner membrane reconjugates the acyl group to CoA, regenerating fatty acyl-CoA in the matrix.

Malonyl-CoA (the first committed intermediate of fatty acid synthesis) inhibits CPT-I — the key regulatory link that prevents synthesis and oxidation from running at the same time. Passage-level the translocase is an antiporter that returns free carnitine; medium-chain FAs skip the shuttle.

The Four Steps of β-Oxidation

Must know

Starting with a saturated fatty acyl-CoA, β-oxidation cleaves two-carbon units as acetyl-CoA in a four-step repeating cycle. The name "β-oxidation" refers to oxidation at the β carbon (C-3, counting from the carboxylate end).

Step	Enzyme	Reaction	Cofactor
1. Oxidation	Acyl-CoA dehydrogenase (FAD-linked)	Introduces trans double bond between Cα–Cβ	Produces FADH₂
2. Hydration	Enoyl-CoA hydratase	Adds $\ce{H2O}$ across the double bond → β-hydroxyacyl-CoA	—
3. Oxidation	L-3-Hydroxyacyl-CoA dehydrogenase	Oxidizes hydroxyl → ketone	Produces NADH
4. Thiolysis	β-Ketothiolase	Cleaves between α and β carbons; adds CoA	Releases acetyl-CoA

Each round of β-oxidation yields: 1 FADH₂, 1 NADH, 1 acetyl-CoA, and a shortened acyl-CoA (2 carbons shorter). The cycle repeats until the entire chain is converted to acetyl-CoA.

Counting Rounds and Products

Must know

For an even-chain saturated fatty acid with $n$ carbons, the number of β-oxidation rounds and the acetyl-CoA yield follow directly from the two-carbon cleavage:

$\text{Rounds} = \frac{n}{2} - 1 \qquad \text{Acetyl-CoA} = \frac{n}{2}$

So palmitate (16 C) undergoes 7 rounds, producing 8 acetyl-CoA plus 7 NADH and 7 FADH₂; the final round yields the last two acetyl-CoA simultaneously. Don't forget the 2 ATP-equivalent activation cost (ATP → AMP + PPᵢ) at the front.

You do not need to memorize an exact ATP total for a specific fatty acid. The MCAT-relevant takeaway is qualitative: because fatty acids are far more reduced (more C–H bonds) than carbohydrates, their complete oxidation yields roughly 2× more ATP per gram, which is why fat is the body's dense long-term fuel store.

Quick check (Optional reasoning): Why does β-oxidation use FAD rather than NAD⁺ in the first oxidation step?

Answer: Introducing a C2–C3 double bond is a relatively modest oxidation — not energetic enough to reduce NAD⁺, but enough to reduce FAD. The same logic explains why succinate dehydrogenase in the TCA cycle uses FAD.

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Oxidation of Saturated Fats

Must know

The pathway described above — 7 rounds of β-oxidation, yielding 8 acetyl-CoA, 7 NADH, and 7 FADH₂ — is the standard, clean pathway for saturated fatty acids. Every round is identical, and no additional enzymatic steps are needed.

Key features of saturated fatty acid oxidation to know:

• Occurs exclusively in the mitochondrial matrix (and in peroxisomes for very-long-chain fatty acids >22 carbons — a detail the MCAT has tested)
• Requires oxygen (aerobic process, dependent on a functional electron transport chain to reoxidize NADH and FADH₂)
• Is highly regulated by the energy charge of the cell: high NADH/NAD⁺ ratio and high acetyl-CoA slow β-oxidation

Odd-chain fatty acids are less common but MCAT-testable because the final round yields propionyl-CoA (3 carbons) instead of two acetyl-CoA. Know the logic — propionyl-CoA becomes succinyl-CoA in three steps:

1. Propionyl-CoA carboxylase uses biotin to form methylmalonyl-CoA.
2. An epimerase rearranges methylmalonyl-CoA.
3. Methylmalonyl-CoA mutase uses vitamin B₁₂ to form succinyl-CoA.

Because succinyl-CoA enters the TCA cycle and can become oxaloacetate, odd-chain fatty acids are partly gluconeogenic — unlike even-chain fatty acids. This is an MCAT-tested distinction (note the two distinct cofactors: biotin for the carboxylase, B₁₂ for the mutase).

Quick check: A patient with a vitamin B₁₂ deficiency consumes a meal high in odd-chain fatty acids. Which enzyme will be impaired, and what will accumulate?

Answer: Methylmalonyl-CoA mutase (which requires adenosylcobalamin, a B₁₂ derivative) will be impaired, causing methylmalonyl-CoA and propionyl-CoA to accumulate. Methylmalonic acidemia results — a testable clinical vignette.

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Oxidation of Unsaturated Fats

Must know

Unsaturated fatty acids follow β-oxidation but require extra enzymatic steps to deal with their double bonds, and they yield less ATP than their saturated counterparts of the same chain length. This is the single most tested distinction between saturated and unsaturated fat oxidation.

Know the logic

β-oxidation needs a trans-Δ2 intermediate, but natural double bonds are cis and in the wrong position. Enoyl-CoA isomerase converts a cis double bond into the correct trans form. Polyunsaturated fatty acids also require an NADPH-dependent reductase (2,4-dienoyl-CoA reductase).

Because each pre-existing double bond lets the chain skip the FAD-linked first oxidation step, unsaturated fatty acids yield less ATP than saturated fatty acids of the same carbon count. Passage-level the exact positions (oleic acid reaching cis-Δ3 after 3 rounds, linoleic acid's full enzyme sequence, the precise FADH₂/NADPH tally) are detail you only need if a passage walks you through them — don't memorize them.

Quick check: Linoleic acid (18:2) produces fewer ATP than stearic acid (18:0), both being 18-carbon fatty acids. Name the two enzyme-level reasons this occurs.

Answer: (1) Enoyl-CoA isomerase handles one double bond, bypassing one FADH₂-producing step. (2) 2,4-dienoyl-CoA reductase handles the second double bond, which consumes 1 NADPH and again bypasses an FAD-linked step. Net result: 2 fewer FADH₂ and 1 NADPH consumed.

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Ketone Bodies

When and Why Ketones Are Made

Must know

When you fast, exercise vigorously, or (in pathology) when insulin is absent (Type 1 diabetes), your body mobilizes massive amounts of fat. The liver receives more fatty acids than it can fully oxidize or package as VLDL. The result is an excess of acetyl-CoA in hepatic mitochondria. Simultaneously, the TCA cycle intermediate oxaloacetate (OAA) is being diverted to gluconeogenesis, so acetyl-CoA cannot enter the TCA efficiently. The liver solves this by converting excess acetyl-CoA into ketone bodies — water-soluble, exportable fuel molecules.

The Three Ketone Bodies

Must know

Ketone Body	Structure	Key Note
Acetoacetate	β-keto acid	Primary product of ketogenesis; can be used directly
β-Hydroxybutyrate	Reduced form of acetoacetate	Most abundant in blood; not technically a ketone (has –OH group)
Acetone	Spontaneous decarboxylation of acetoacetate	Volatile; exhaled; causes fruity breath; not metabolically useful

Ketogenesis (in the liver)

Must know

The pathway occurs in hepatic mitochondria:

1. Two acetyl-CoA condense → acetoacetyl-CoA (thiolase, reverse of thiolysis)
2. Acetoacetyl-CoA + acetyl-CoA → HMG-CoA (HMG-CoA synthase) — this is the committed step
3. HMG-CoA → acetoacetate + acetyl-CoA (HMG-CoA lyase)
4. Acetoacetate + NADH → β-hydroxybutyrate (β-hydroxybutyrate dehydrogenase)

Note that HMG-CoA is also an intermediate in cholesterol biosynthesis (in the cytoplasm). The MCAT loves to test this intersection: statins inhibit HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis, which is a different enzyme from HMG-CoA lyase of ketogenesis.

Ketone Body Utilization (in peripheral tissues — NOT the liver)

Must know

The liver produces ketones but cannot use them — it lacks succinyl-CoA:acetoacetate CoA transferase (thiophorase). This enzyme is present in the brain, heart, skeletal muscle, and kidney. These tissues activate acetoacetate back to acetoacetyl-CoA, split it into two acetyl-CoA, and feed them into the TCA cycle.

This is elegant: the liver packages energy as ketones and ships them out for tissues to use, especially the brain, which normally cannot use fatty acids (they don't cross the blood-brain barrier) but can readily use ketone bodies during prolonged fasting.

Diabetic ketoacidosis (DKA): In uncontrolled Type 1 diabetes, insulin is absent → glucagon dominates → massive lipolysis + hepatic ketogenesis → ketone bodies accumulate faster than tissues can use them → blood pH drops (ketone bodies are acids) → life-threatening metabolic acidosis.

Quick check: Why can't the liver oxidize its own ketone bodies?

Answer: The liver lacks thiophorase (succinyl-CoA:acetoacetate CoA transferase), the enzyme needed to reactivate acetoacetate to acetoacetyl-CoA. This ensures that ketones produced by the liver are exported to fuel other tissues.

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Anabolism of Fats: Fatty Acid Synthesis

The Big Picture: Synthesis Is NOT Just Reverse β-Oxidation

Must know

This is one of the most important contrasts on the MCAT. Fatty acid synthesis is physically, biochemically, and regulatorily distinct from β-oxidation:

Feature	β-Oxidation	Fatty Acid Synthesis
Location	Mitochondrial matrix	Cytoplasm (cytosol)
Acyl carrier	CoA	Acyl Carrier Protein (ACP)
Two-carbon units	Removed as acetyl-CoA	Added as malonyl-CoA
Cofactors	FAD, NAD⁺ (oxidized)	NADPH (reduced)
Enzyme complex	Separate enzymes	Fatty acid synthase (FAS) — multienzyme complex
Key regulatory molecule	Inhibited by malonyl-CoA (via CPT-I)	Activated by citrate, insulin

Getting Acetyl-CoA into the Cytoplasm

Must know

Acetyl-CoA is produced in the mitochondria (from pyruvate dehydrogenase, β-oxidation, and TCA) but cannot cross the inner mitochondrial membrane directly. It is exported as citrate:

$\ce{Acetyl-CoA + OAA -> Citrate}$

Citrate exits via the citrate shuttle, and in the cytoplasm, ATP-citrate lyase regenerates acetyl-CoA and OAA.

The Committed Step: Acetyl-CoA Carboxylase

Must know

Acetyl-CoA carboxylase (ACC) converts acetyl-CoA to malonyl-CoA:

$\ce{Acetyl-CoA + CO2 + ATP -> Malonyl-CoA + ADP + P_i}$

This is the rate-limiting, committed step of fatty acid synthesis. ACC requires biotin as a cofactor (biotin carries the $\ce{CO2}$ group — the same role as in pyruvate carboxylase and propionyl-CoA carboxylase).

Regulation:

• Activated by: citrate (allosteric), insulin (activates via phosphatase)
• Inhibited by: palmitoyl-CoA (product feedback), glucagon/epinephrine (activate kinase that phosphorylates and inactivates ACC), and AMP-activated protein kinase (AMPK)

Fatty Acid Synthase (FAS)

Must know

In animals, FAS is a large multifunctional enzyme complex. It adds 2-carbon units from malonyl-CoA to a growing acyl chain carried by ACP. Know the logic (don't memorize the sub-enzyme names): each cycle is condensation → reduction → dehydration → reduction. The key MCAT ideas are that the two reductions use NADPH and the chain is carried by ACP.

The standard product is palmitate (16:0) — released as palmitoyl-ACP, which is then activated to palmitoyl-CoA or further elongated/desaturated by smooth-ER enzymes.

NADPH sources for fatty acid synthesis:

• Pentose phosphate pathway (primary source — this is why the PPP is important!)
• Malic enzyme (malate → pyruvate, generating NADPH)

Triglyceride Synthesis

Must know

Fatty acids are esterified to glycerol-3-phosphate (from DHAP or from glycerol via glycerol kinase) to form triglycerides. This occurs in the smooth ER and is the major form of fat storage. The liver packages TAG into VLDL for export; adipocytes store TAG in lipid droplets.

Quick check: A researcher adds insulin to fat cells in culture. Which key enzyme of fatty acid synthesis would you expect to be activated, and by what mechanism?

Answer: Acetyl-CoA carboxylase (ACC). Insulin activates a phosphatase that dephosphorylates ACC, shifting it to the active form. Simultaneously, insulin inhibits the lipolytic PKA pathway, pushing the cell toward storage and away from oxidation.

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Nontemplate Synthesis: Biosynthesis of Lipids and Polysaccharides

What "Nontemplate" Means

Must know

Unlike DNA replication, transcription, or translation — which all rely on a nucleic acid template — the biosynthesis of lipids and polysaccharides is enzymatically driven but not template-directed. The "sequence" of the final product is determined entirely by the specificity and availability of enzymes, not by reading a nucleic acid blueprint.

Lipid Biosynthesis (Key Additional Points)

Must know

Beyond fatty acid synthesis, you should know:

• Phospholipid synthesis occurs mainly in the smooth ER from fatty acids, a glycerol/DAG backbone, and activated head groups. (Optional the head groups are activated as CDP-derivatives, e.g., CDP-choline for phosphatidylcholine.)
• Cholesterol synthesis (the mevalonate pathway) also occurs in the cytoplasm/smooth ER. Acetyl-CoA → HMG-CoA → mevalonate (rate-limiting step: HMG-CoA reductase, the target of statins) → eventually cholesterol. Cholesterol is the precursor to steroid hormones, bile acids, and vitamin D.
• Know the logic Cholesterol feedback: when cellular cholesterol is low, cells increase cholesterol synthesis and LDL uptake; when cholesterol is high, both are downregulated. Defective LDL receptors cause familial hypercholesterolemia (high blood LDL, premature atherosclerosis), and HDL carries excess cholesterol back to the liver (reverse cholesterol transport). Optional this is run by the SREBP transcription factor and LDL-receptor-mediated endocytosis via clathrin-coated pits — mechanism you only need if a passage supplies it.
• Eicosanoid synthesis: Arachidonic acid (20:4) is released from membrane phospholipids by phospholipase A₂ and converted to prostaglandins (via COX, inhibited by aspirin/NSAIDs) and leukotrienes. Leukotrienes require lipoxygenase.

Polysaccharide Biosynthesis

Know the logic

Polysaccharides are synthesized by glycosyltransferases that add activated sugar units (typically UDP-glucose or other UDP-sugars) to a growing chain. Again, no template — the enzyme determines what gets added where.

• Glycogen synthesis: Glycogen synthase adds glucose (from UDP-glucose) in α-1,4 linkages to the non-reducing end of the chain. A separate branching enzyme creates α-1,6 branch points. Glycogen synthase is the rate-limiting enzyme, controlled by phosphorylation/dephosphorylation. (The hormonal coordination of glycogen synthesis vs. breakdown and gluconeogenesis—and how it tracks the fed/fasted state across tissues—is covered in the Metabolism (whole-body integration) guide.)

The MCAT won't ask you to memorize every enzyme, but it does expect you to understand the logic of these pathways — activated sugar donors, non-template directionality, and regulation by phosphorylation/dephosphorylation.

Quick check: What is the activated donor for glycogen synthesis, and what makes glycogen synthesis "nontemplate"?

Answer: UDP-glucose is the activated donor. It is nontemplate because the enzyme (glycogen synthase) directly determines the product structure — no nucleic acid sequence is read to direct which sugar goes where or how long the chain grows.

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Metabolism of Proteins

Overview: Proteins as Fuel

Must know

Proteins are not a primary fuel source, but during prolonged fasting, severe illness, or after heavy exercise, amino acids from muscle protein catabolism contribute significantly to ATP production and gluconeogenesis. The first step is always removing the nitrogen, which cannot be oxidized for energy by mammals.

Protein Digestion

Must know

Before amino acids can be metabolized, dietary protein must be hydrolyzed. Digestion uses zymogens (inactive precursors) activated by proteolytic cascades, protecting the secreting cells from self-digestion:

• Stomach: HCl from parietal cells denatures protein and converts pepsinogen → pepsin (autocatalytically and via low pH). Pepsin is an endopeptidase that cleaves internal peptide bonds.
• Small intestine: The pancreas secretes zymogens. Enteropeptidase (enterokinase) from the brush border activates trypsinogen → trypsin, and trypsin then activates the rest of the cascade (chymotrypsinogen → chymotrypsin, proelastase → elastase, procarboxypeptidase → carboxypeptidase). Trypsin, chymotrypsin, and elastase are endopeptidases; carboxypeptidases are exopeptidases that remove C-terminal residues.
• Brush border: Aminopeptidases and dipeptidases complete digestion to free amino acids, which are absorbed by secondary active transport (Na⁺-coupled).

Transamination

Must know

Most amino acids first undergo transamination, transferring their amino group ($\ce{-NH2}$) to α-ketoglutarate (α-KG), producing glutamate and the corresponding α-keto acid (carbon skeleton of the original amino acid).

$\ce{Amino acid + \alpha-ketoglutarate <=> \alpha-keto acid + Glutamate}$

The enzyme is an aminotransferase (transaminase), requiring pyridoxal phosphate (PLP), the active form of vitamin B₆, as a cofactor. Two clinically important aminotransferases:

• ALT (alanine aminotransferase) — abundant in the liver; elevated in hepatocellular damage
• AST (aspartate aminotransferase) — found in liver, heart, muscle; elevated in liver disease and myocardial infarction

Elevated AST and ALT are classic MCAT biomarkers for hepatocellular damage.

Oxidative Deamination

Must know

Glutamate (the amino group collector) is deaminated in the mitochondria by glutamate dehydrogenase:

$\ce{Glutamate + NAD+ (or NADP+) -> \alpha-ketoglutarate + NH4+ + NADH}$

This releases free ammonium ($\ce{NH4+}$), which is toxic to the brain (cerebral edema, encephalopathy). The ammonium must be disposed of promptly — enter the urea cycle.

The Urea Cycle

Must know

The urea cycle occurs in hepatocytes (steps split between mitochondria and cytoplasm) and is the primary mechanism for nitrogen excretion in humans (ureotelic organisms).

Key steps and intermediates (MCAT-tested):

Step	Enzyme	Location	Key Input/Output
1	Carbamoyl phosphate synthetase I (CPS-I)	Mitochondria	$\ce{NH4+ + CO2 + 2ATP}$ → carbamoyl phosphate
2	Ornithine transcarbamylase (OTC)	Mitochondria	Ornithine + carbamoyl phosphate → citrulline
3	Argininosuccinate synthetase	Cytoplasm	Citrulline + aspartate + ATP → argininosuccinate
4	Argininosuccinate lyase	Cytoplasm	Argininosuccinate → arginine + fumarate
5	Arginase	Cytoplasm	Arginine + $\ce{H2O}$ → ornithine + urea

One molecule of urea contains 2 nitrogens: one from $\ce{NH4+}$ (via carbamoyl phosphate), one from aspartate. This is a classic MCAT question.

Urea is released into the blood, filtered by the kidneys, and excreted in urine. BUN (blood urea nitrogen) is a standard lab measure of kidney function and protein catabolism.

Regulation: N-acetylglutamate allosterically activates CPS-I (the committed step). When amino acid catabolism is high, N-acetylglutamate levels rise, revving up the urea cycle.

Hyperammonemia (from urea cycle enzyme deficiencies or severe liver disease) presents with neurological symptoms, vomiting, and altered consciousness — a classic MCAT clinical scenario.

Carbon Skeletons: Glucogenic vs. Ketogenic Amino Acids

Must know

After the nitrogen is removed, the carbon skeleton (α-keto acid) enters central metabolism. Where it enters determines whether the amino acid is glucogenic, ketogenic, or both:

• Glucogenic amino acids: carbon skeletons feed into gluconeogenic intermediates (pyruvate, OAA, α-KG, succinyl-CoA, fumarate). They can be converted to glucose. Most amino acids are glucogenic.
• Ketogenic amino acids: carbon skeletons yield acetyl-CoA or acetoacetyl-CoA directly — they can produce ketone bodies but cannot be used for net glucose synthesis.
• Purely ketogenic: leucine and lysine (the only two).
• Both glucogenic and ketogenic: isoleucine, phenylalanine, tyrosine, threonine, and tryptophan.

The key purely ketogenic amino acids: Leucine and Lysine (memory: "LeucKine and LysKine" — both L's and K sounds). Everything else is at least partially glucogenic.

Essential vs. nonessential amino acids: Just as linoleic and α-linolenic acids are essential fatty acids, nine amino acids cannot be synthesized by humans and must come from the diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (mnemonic "PVT TIM HaLL"). The rest are nonessential (synthesized from common intermediates) or conditionally essential (e.g., arginine, cysteine, tyrosine, glutamine — needed from the diet during growth, illness, or when a precursor is lacking, such as tyrosine in PKU).

Optional

Reference table — useful for interpreting a passage, but don't memorize every entry; the high-yield minimum is the leucine/lysine and "both" lists above.

Entry Point	Amino Acids
Pyruvate	Ala, Cys, Gly, Ser, Thr
α-Ketoglutarate	Glu, Gln, His, Pro, Arg
Succinyl-CoA	Met, Ile, Val, Thr
Fumarate	Phe, Tyr
OAA	Asp, Asn
Acetyl-CoA / Acetoacetyl-CoA	Leu, Lys, Ile, Phe, Tyr, Trp, Thr

Amino Acids as Biosynthetic Precursors

Must know

The MCAT also tests the biosynthetic roles of amino acids:

• Tryptophan → serotonin, melatonin, niacin (vitamin B₃)
• Phenylalanine/Tyrosine → dopamine, norepinephrine, epinephrine, thyroid hormones, melanin
• Histidine → histamine
• Glutamate → GABA
• Arginine → nitric oxide (via nitric oxide synthase)
• Glycine + succinyl-CoA → heme

Inborn Errors of Amino Acid Metabolism

Must know

Defects in the enzymes that handle specific amino acid carbon skeletons cause classic, high-yield genetic diseases:

• Phenylketonuria (PKU): deficiency of phenylalanine hydroxylase (or its $\ce{BH4}$ cofactor), which normally converts phenylalanine → tyrosine. Phenylalanine accumulates and is shunted to phenylketones; untreated, it causes intellectual disability, hypopigmentation, and a "musty" odor. Treatment is dietary phenylalanine restriction (and tyrosine becomes essential). This is the single most-tested amino acid inborn error.
• Maple syrup urine disease: deficient branched-chain α-keto acid dehydrogenase, blocking degradation of leucine, isoleucine, and valine → sweet-smelling urine, neurotoxicity.
• Alkaptonuria: deficient homogentisate oxidase in the tyrosine pathway → homogentisic acid accumulates, darkening urine on standing; usually benign.

Passage-level

MCAT clinical distinction:

• OTC deficiency: hyperammonemia + elevated orotic acid (carbamoyl phosphate backs up into pyrimidine synthesis).
• CPS-I or NAGS deficiency: hyperammonemia without elevated orotic acid.
• Liver failure/cirrhosis: global reduction in urea-cycle capacity → systemic hyperammonemia.

Quick check: A neonate has hyperammonemia and elevated urinary orotic acid. Which urea-cycle enzyme is most likely deficient?

Answer: Ornithine transcarbamylase (OTC). The carbamoyl phosphate that OTC can't use spills into the pyrimidine pathway, raising orotic acid — distinguishing OTC deficiency from CPS-I/NAGS deficiency, which lack the orotic acid rise.

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Common Confusions & Tricks

1. "β-oxidation is just fatty acid synthesis in reverse." This is false and a critical MCAT trap. They occur in different compartments (mitochondria vs. cytoplasm), use different acyl carriers (CoA vs. ACP), different cofactors (FAD/NAD⁺ vs. NADPH), different activators, and are regulated oppositely. Malonyl-CoA — the first intermediate of synthesis — actively inhibits β-oxidation by blocking CPT-I.

2. Activation cost is 2 ATP, not 1. The reaction produces AMP + PP$_i$, not ADP + P$_i$. The hydrolysis of PP$_i$ is what drives the reaction forward, but it means you've consumed the equivalent of two phosphoanhydride bonds — net cost = 2 ATP equivalents. Always subtract 2 when calculating fatty acid ATP yield.

3. The liver makes ketones; the liver cannot use them. The liver lacks thiophorase. Conversely, the brain cannot directly use fatty acids but CAN use ketone bodies after fasting adaptation (~3 days).

4. Odd-chain fatty acids are gluconeogenic; even-chain are not. Even-chain fatty acids yield only acetyl-CoA, which cannot produce net glucose (it enters the TCA but the carbons are lost as CO₂). Odd-chain yields propionyl-CoA → succinyl-CoA → OAA → glucose. This is the textbook exception.

5. HMG-CoA appears in TWO different pathways — different locations. Mitochondrial HMG-CoA is an intermediate in ketogenesis (cleaved by HMG-CoA lyase). Cytoplasmic HMG-CoA is the substrate for HMG-CoA reductase in cholesterol synthesis (inhibited by statins). Same molecule, different compartment, different fate.

6. Unsaturated fats yield LESS ATP than saturated fats of the same length. Each pre-existing double bond bypasses one FAD-linked oxidation step (losing 1 FADH₂), and polyunsaturated fats also consume NADPH via 2,4-dienoyl-CoA reductase.

7. Two nitrogens in urea. One comes from ammonium (NH₄⁺, via carbamoyl phosphate), one from aspartate. Students frequently forget the aspartate nitrogen. The fumarate released from the urea cycle connects it to the TCA cycle — a link the MCAT has tested.

8. ALT vs. AST. ALT is more liver-specific; AST is also found in heart and muscle. Elevated ALT/AST suggests tissue damage, especially hepatocellular injury.

9. Carnitine shuttle is only needed for long-chain fatty acids. Medium-chain fatty acids (8–12 carbons) cross the inner mitochondrial membrane without carnitine; very-long-chain fatty acids are first shortened in peroxisomes.

10. Acetone is a ketone body but is metabolically useless. Acetone is exhaled (causing the fruity breath of DKA) and cannot be metabolized further. Don't confuse it with acetoacetate or β-hydroxybutyrate, which are actual metabolic fuels.

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Key Equations

Equation	Variables / When to Use
$\ce{Fatty acid + CoA + ATP -> Fatty acyl-CoA + AMP + PP_i}$	Fatty acid activation; note AMP (not ADP) → costs 2 ATP equivalents
$\text{Rounds of }\beta\text{-oxidation} = \frac{n}{2} - 1$	$n$ = number of carbons; gives number of β-oxidation cycles for even-chain saturated FA
$\text{Acetyl-CoA from even chain} = \frac{n}{2}$	Total acetyl-CoA molecules from complete β-oxidation
$\ce{Acetyl-CoA + CO2 + ATP ->[ACC][biotin] Malonyl-CoA + ADP + P_i}$	Acetyl-CoA carboxylase; rate-limiting step of fatty acid synthesis; requires biotin
$\ce{Amino acid + \alpha-KG <=>[transaminase][PLP] \alpha-keto acid + Glutamate}$	Transamination; PLP (vitamin B₆) required; collects nitrogen into glutamate
$\ce{Glutamate + NAD+ ->[GDH] \alpha-KG + NH4+ + NADH}$	Oxidative deamination; releases ammonium for urea cycle; occurs in mitochondria
$\ce{NH4+ + CO2 + 2ATP ->[CPS-I] Carbamoyl phosphate + 2ADP + P_i}$	First (committed) step of urea cycle; mitochondria; requires N-acetylglutamate activator
$\text{Urea} = 2\,\text{N}\,(\text{one from NH}_4^+,\text{ one from Asp})$	Conceptual equation for MCAT: each urea molecule carries 2 nitrogens
$\ce{HMG-CoA ->[HMG-CoA reductase][statin target] Mevalonate}$	Rate-limiting step of cholesterol synthesis in cytoplasm; distinct from mitochondrial HMG-CoA lyase in ketogenesis