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Fatty Acid & Protein
Metabolism

Master how the body breaks down fats and proteins for energy: from \(\beta\)-oxidation to amino acid catabolism and their connections to the citric acid cycle.

MCAT Biochemistry

Why Fat & Protein Metabolism Matters

Your body does not run on glucose alone. During fasting, prolonged exercise, and starvation, fatty acids become the primary fuel source. Amino acids from dietary protein are also catabolized to feed central metabolic pathways.

Fats
Yield \(\sim 9\) kcal/g: the most energy-dense macronutrient
Proteins
Yield \(\sim 4\) kcal/g and supply carbon skeletons to the TCA cycle
Integration
Both pathways converge at acetyl-CoA, linking to oxidative phosphorylation

The Central Hub: Acetyl-CoA

Glucose Fatty Acids Amino Acids Pyruvate Acetyl-CoA TCA Cycle ETC → ATP Glycolysis β-oxidation Transamination NADH, FADH₂

\(\beta\)-Oxidation: Breaking Down Fatty Acids

\(\beta\)-Oxidation is the repeating four-step spiral that shortens a fatty acyl-CoA chain by two carbons per cycle, producing acetyl-CoA, NADH, and FADH\(_2\). It occurs in the mitochondrial matrix.

1
Activation: Fatty acid + CoA → Fatty acyl-CoA (uses \(2\) ATP equivalents, cytoplasm)
2
Transport: Carnitine shuttle moves long-chain acyl-CoA into the mitochondrial matrix
3
Spiral: Each cycle: oxidation (FADH\(_2\)), hydration, oxidation (NADH), thiolysis (acetyl-CoA released)
4
Repeat: Cycle continues until the entire chain is cleaved into acetyl-CoA units

The Four Steps of Each Cycle

1. Oxidation
\(\text{FAD} \to \text{FADH}_2\)
2. Hydration
\(+ \text{H}_2\text{O}\)
3. Oxidation
\(\text{NAD}^+ \to \text{NADH}\)
4. Thiolysis
\(+ \text{CoA} \to \text{Acetyl-CoA}\)
Repeat
\((n/2) - 1\) cycles
Per cycle: \(1 \text{ FADH}_2, 1 \text{ NADH}, 1 \text{ Acetyl-CoA}\)

ATP Yield from Palmitate (\(C_{16}\))

1
8 Acetyl-CoA → 8 turns of TCA → \(8 \times 10 = \) 80 ATP
2
7 FADH\(_2\) → ETC → \(7 \times 1.5 = \) 10.5 ATP
3
7 NADH → ETC → \(7 \times 2.5 = \) 17.5 ATP
4
Activation cost: \(-2\) ATP equivalents (ATP → AMP + 2P\(_i\))

The Carnitine Shuttle: Gatekeeper

MITOCHONDRION Inner Membrane Cytoplasm Matrix CPT I Rate- limiting CPT II Fatty acyl-CoA Acyl-carnitine Fatty acyl-CoA β-Oxidation Spiral Malonyl-CoA ✕

Ketone Bodies: When Acetyl-CoA Overflows

During fasting or uncontrolled diabetes, excessive \(\beta\)-oxidation in the liver floods the mitochondria with acetyl-CoA. When oxaloacetate is depleted (used for gluconeogenesis), acetyl-CoA cannot enter the TCA cycle and is diverted to ketogenesis.

Acetoacetate
Primary ketone body; can be reduced to \(\beta\)-hydroxybutyrate
\(\beta\)-Hydroxybutyrate
Most abundant ketone in blood; used by brain during starvation
Acetone
Non-metabolizable; exhaled in breath (fruity odor in ketoacidosis)

Amino Acid Catabolism: The First Steps

Before their carbon skeletons can be oxidized for energy, amino acids must first lose their amino group. Two key reactions accomplish this:

Transamination
Aminotransferases transfer the \(-\text{NH}_2\) group to \(\alpha\)-ketoglutarate, forming glutamate. Requires PLP (vitamin B\(_6\)) as cofactor.
Oxidative Deamination
Glutamate dehydrogenase removes \(-\text{NH}_2\) as free \(\text{NH}_4^+\), regenerating \(\alpha\)-ketoglutarate. Occurs in liver mitochondria.

Glucogenic vs. Ketogenic Amino Acids

Glucogenic Ketogenic
Carbon skeleton yields Pyruvate or TCA intermediates Acetyl-CoA or acetoacetyl-CoA
Can make glucose? Yes No
Purely ketogenic N/A Leucine, Lysine
Both Ile, Phe, Trp, Thr, Tyr
Mnemonic "Only Leucine and Lysine are purely ketogenic"

Carbon Skeleton Entry Points

Pyruvate Ala, Ser, Gly, Cys, Thr, Trp Acetyl-CoA Leu, Lys, Ile, Thr, Trp
\(\alpha\)-Ketoglutarate
 Glu, Gln, Pro, Arg, His Succinyl-CoA Val, Ile, Met, Thr Fumarate Phe, Tyr Oxaloacetate Asp, Asn TCA Cycle

\(\beta\)-Oxidation vs. Fatty Acid Synthesis

\(\beta\)-Oxidation Fatty Acid Synthesis
Location Mitochondrial matrix Cytoplasm
Acyl carrier CoA ACP (acyl carrier protein)
Carbon units Removes 2C as acetyl-CoA Adds 2C from malonyl-CoA
Redox coenzymes FAD, \(\text{NAD}^+\) (oxidized) NADPH (reduced)
Key enzyme Acyl-CoA dehydrogenase Fatty acid synthase
Regulation Inhibited by malonyl-CoA Activated by insulin/citrate

How to Solve Metabolism Problems

1
Identify the substrate: Is it a fatty acid, amino acid, or glucose? This determines the entry pathway.
2
Count carbons: For fatty acids, divide by 2 to find acetyl-CoA units. Cycles = \((n/2) - 1\).
3
Track coenzymes: Each \(\beta\)-oxidation cycle produces \(1 \text{ NADH} + 1 \text{ FADH}_2\). Each TCA turn produces \(3 \text{ NADH} + 1 \text{ FADH}_2 + 1 \text{ GTP}\).
4
Calculate ATP: \(\text{NADH} = 2.5 \text{ ATP}\), \(\text{FADH}_2 = 1.5 \text{ ATP}\). Subtract activation cost.
5
Check regulation: What hormones or metabolites control the rate-limiting step? (e.g., CPT I, malonyl-CoA)

Example 1: Lauric Acid (\(C_{12}\))

Question: How many total ATP molecules are produced from the complete oxidation of one molecule of lauric acid (\(C_{12}\))?

1
Acetyl-CoA units: \(12/2 = \) 6 acetyl-CoA
2
\(\beta\)-oxidation cycles: \(6 - 1 = \) 5 cycles → \(5 \text{ NADH} + 5 \text{ FADH}_2\)
3
TCA yield: 6 turns → \(6 \times 10 = 60\) ATP
4
\(\beta\)-oxidation yield: \(5 \times 2.5 + 5 \times 1.5 = 12.5 + 7.5 = 20\) ATP
5
Activation cost: \(-2\) ATP

Example 2: Alanine Catabolism

Question: Trace the metabolic fate of alanine during fasting. Which pathways are involved?

Alanine ALT Pyruvate Glucose (GNG) PDH Acetyl-CoA TCA Cycle
\(\text{NH}_4^+\)
Urea Cycle

Common Mistakes

✕ Forgetting Activation Cost
Fatty acid activation uses \(2\) ATP equivalents (ATP → AMP + 2Pi). Many students forget to subtract this from the total yield.
✕ Wrong Cycle Count
A \(C_n\) fatty acid undergoes \((n/2) - 1\) cycles of \(\beta\)-oxidation, not \(n/2\). The last cycle cleaves the final 4-carbon unit into two acetyl-CoAs.
✕ Confusing Locations
\(\beta\)-oxidation occurs in the mitochondrial matrix; fatty acid synthesis occurs in the cytoplasm. Mixing these up inverts the regulation logic.

Summary: Fatty Acid vs. Protein Catabolism

Fatty Acid Catabolism Protein Catabolism
Key pathway \(\beta\)-oxidation Transamination + oxidative deamination
Location Mitochondrial matrix Liver (primarily)
Entry to TCA Acetyl-CoA only Multiple: pyruvate, \(\alpha\)-KG, succinyl-CoA, fumarate, OAA
Nitrogen handling N/A Urea cycle (liver)
ATP yield ~106 per palmitate Varies by amino acid
Rate-limiting control CPT I (malonyl-CoA inhibition) Hormonal (cortisol, glucagon)

Question 1 of 5

How many cycles of \(\beta\)-oxidation are required to completely oxidize a \(C_{18}\) fatty acid?

Question 2 of 5

Which molecule inhibits CPT I, thereby preventing long-chain fatty acids from entering the mitochondria for \(\beta\)-oxidation?

Question 3 of 5

Which two amino acids are purely ketogenic (cannot be converted to glucose)?

Question 4 of 5

The liver produces ketone bodies during starvation but cannot oxidize them for its own energy. Why?

Question 5 of 5

During amino acid catabolism, the cofactor required by all aminotransferases for the transamination reaction is: