Amino acid anabolic and catabolic pathways

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Amino Acid Anabolic and Catabolic Pathways

Amino acid metabolism sits at a crossroads linking protein turnover, energy metabolism, and biosynthesis of countless nitrogen-containing molecules. The same enzymatic machinery (transamination) serves both building up and breaking down, with direction determined by the cell's anabolic/catabolic state.

Overview

Overview of central metabolic integration - amino acids contribute to acetyl-CoA, the citric acid cycle, fatty acid synthesis, and ketone bodies. (Harper's Illustrated Biochemistry, 32nd ed.)

Amino acids either come from dietary protein (after digestion and absorption via portal vein) or from intracellular protein turnover. Their fate depends on nutritional state: in the absorptive state, surplus amino acids are deaminated and their carbon skeletons used for energy or fat synthesis; in the fasted/catabolic state, muscle protein is broken down to release BCAAs for energy and gluconeogenic precursors.

CATABOLIC PATHWAYS

Step 1 - Transamination (the universal first step)

Overall nitrogen flow: transamination → oxidative deamination → urea cycle

Overall flow of nitrogen in amino acid catabolism. (Harper's Illustrated Biochemistry, 32nd ed.)

Transamination is the entry point into catabolism for most amino acids. The α-amino group is transferred from the amino acid to α-ketoglutarate (α-KG), yielding:

Glutamate (from α-KG + amino group)
The corresponding α-keto acid (the de-aminated carbon skeleton)

Enzymes: Aminotransferases (transaminases). The two most clinically important are:

ALT (alanine aminotransferase) - Alanine + α-KG → Pyruvate + Glutamate
AST (aspartate aminotransferase) - Aspartate + α-KG → Oxaloacetate + Glutamate

Cofactor: Pyridoxal phosphate (PLP), derived from vitamin B6. PLP forms a Schiff base with the α-amino group and acts as an amino group "carrier" in a ping-pong mechanism.

Exceptions: Lysine and threonine do not undergo transamination (their α-amino groups are removed by other mechanisms).

All α-amino nitrogen is funneled into glutamate, because L-glutamate is the only amino acid that undergoes oxidative deamination at an appreciable rate in mammalian tissues. - Harper's Illustrated Biochemistry, 32nd ed., p. 294

Step 2 - Oxidative Deamination of Glutamate

Glutamate dehydrogenase (GDH) in the mitochondrial matrix:

Glutamate + NAD⁺ (or NADP⁺) → α-Ketoglutarate + NH₄⁺ + NADH

This liberates free ammonia (NH₄⁺). GDH is allosterically activated by ADP (energy deficit) and inhibited by GTP (energy surplus), coupling amino acid catabolism to cellular energy status.

Step 3 - Ammonia Transport

Free NH₃ is toxic (especially to the CNS). Two primary transport mechanisms carry nitrogen from peripheral tissues to the liver:

Alanine (glucose-alanine cycle): Muscle pyruvate (from glycolysis) transaminated with glutamate → alanine. Alanine travels to liver, where its amino group is removed → pyruvate (for gluconeogenesis) + ammonia.
Glutamine: In peripheral tissues with high amino acid catabolism, glutamate + NH₄⁺ (via glutamine synthetase) → glutamine. Glutamine is the main non-toxic ammonia carrier in blood. In the liver (and gut), glutaminase releases NH₄⁺ from glutamine for urea synthesis or gut fuel. - Basic Medical Biochemistry, 6th ed., p. 1316

Step 4 - The Urea Cycle (Krebs-Henseleit Cycle)

Nitrogen enters the urea cycle as NH₄⁺ and aspartate. The cycle runs partly in mitochondria, partly in the cytosol.

Urea cycle - 5 steps from carbamoyl phosphate to urea, showing ornithine, citrulline, argininosuccinate, arginine

The urea cycle. (Basic Medical Biochemistry, 6th ed.)

Step	Enzyme	Location	Reaction
1	Carbamoyl phosphate synthetase I (CPSI)	Mitochondria	NH₄⁺ + HCO₃⁻ + 2 ATP → Carbamoyl phosphate
2	Ornithine transcarbamoylase (OTC)	Mitochondria	Ornithine + Carbamoyl-P → Citrulline
3	Argininosuccinate synthetase	Cytosol	Citrulline + Aspartate + ATP → Argininosuccinate
4	Argininosuccinate lyase	Cytosol	Argininosuccinate → Arginine + Fumarate
5	Arginase	Cytosol	Arginine + H₂O → Urea + Ornithine

Key points:

Two nitrogens in urea: one from NH₄⁺ (via CPSI), one from aspartate (donated in step 3)
Ornithine is regenerated and re-enters mitochondria (like oxaloacetate in the TCA cycle)
Fumarate links the urea cycle to the TCA cycle (fumarate → malate → OAA → aspartate, completing the "aspartate-argininosuccinate shunt")
Net cost: 4 ATP equivalents per urea produced (3 ATP + 1 ATP hydrolyzed to AMP + PPi)
CPSI is the rate-limiting enzyme, allosterically activated by N-acetylglutamate

Step 5 - Fate of Carbon Skeletons

After deamination, the residual carbon skeletons are converted to amphibolic intermediates that feed into the TCA cycle, gluconeogenesis, or ketogenesis. Amino acids are classified accordingly:

Category	Intermediates Formed	Examples
Glucogenic only	Pyruvate, OAA, α-KG, succinyl-CoA, fumarate	Ala, Asp, Glu, Gly, Cys, Ser, Arg, His, Met, Pro, Thr, Val, Hyp
Ketogenic only	Acetyl-CoA or acetoacetyl-CoA	Leucine, Lysine
Both	Both of the above	Ile, Phe, Tyr, Trp

Table derived from Harper's Illustrated Biochemistry, 32nd ed., Table 29-1

Key catabolic pathways by amino acid group:

Asp, Asn → OAA (via asparaginase + transamination)
Glu, Gln → α-KG (via glutaminase + GDH)
Ala, Cys, Ser, Gly, Thr → Pyruvate
Val, Ile, Met → Succinyl-CoA (require B12)
Leu → Acetoacetyl-CoA + Acetyl-CoA (purely ketogenic)
Phe, Tyr → Fumarate + Acetoacetate
BCAAs (Leu, Ile, Val): The liver has limited capacity to degrade these - they pass to muscle for catabolism via BCAA aminotransferase and branched-chain α-keto acid dehydrogenase (BCKD). - Lippincott Biochemistry, 8th ed., p. 904

ANABOLIC PATHWAYS (Amino Acid Biosynthesis)

Nonessential vs. Essential Amino Acids

Essential (indispensable) amino acids must come from diet - humans lack the biosynthetic enzymes. Mnemonic: PVT TIM HaLL (Phe, Val, Thr, Trp, Ile, Met, His, Arg*, Leu, Lys). *Arg is conditionally essential (needed in growth/stress).

Nonessential amino acids are synthesized from metabolic intermediates, primarily by receiving amino groups via transamination from glutamate.

Core Anabolic Routes

Since transamination is reversible, it is equally a biosynthetic tool:

Nonessential AA	Precursor	Key Step
Glutamate	α-KG	GDH (reverse) or transamination
Glutamine	Glutamate + NH₄⁺	Glutamine synthetase (ATP-dependent)
Alanine	Pyruvate	ALT (transamination, reverse)
Aspartate	OAA	AST (transamination)
Asparagine	Aspartate + Gln	Asparagine synthetase (ATP)
Serine	3-Phosphoglycerate (glycolytic intermediate)	Oxidation → transamination → phosphatase
Glycine	Serine	Serine hydroxymethyltransferase (requires THF, B6)
Cysteine	Serine + Homocysteine (from Met)	Cystathionine β-synthase → cystathionine lyase
Tyrosine	Phenylalanine (essential)	Phenylalanine hydroxylase (PAH, requires BH4)
Proline	Glutamate → Glutamate-5-semialdehyde	Spontaneous ring closure
Arginine	Synthesized in urea cycle intermediates	OTC pathway (insufficient for adults - conditionally essential)

The general principle: α-keto acids derived from carbohydrate and lipid metabolism serve as carbon backbones; glutamate donates the amino group via transamination. - Harper's Illustrated Biochemistry, 32nd ed., p. 148

Other Anabolic Functions of Amino Acids

Amino acids are precursors for many bioactive molecules:

Amino Acid	Product
Tryptophan	Serotonin, melatonin, NAD⁺, niacin
Tyrosine	Dopamine, norepinephrine, epinephrine, T3/T4, melanin
Histidine	Histamine
Glutamate/Gln	Purines, pyrimidines, GABA (from Glu)
Glycine + Succinyl-CoA	Heme
Arginine	Nitric oxide (via NOS), creatine, polyamines
Methionine	S-adenosylmethionine (SAM) - universal methyl donor
Serine	Sphingolipids, phosphatidylserine
Cysteine	Glutathione (with Gly + Glu), taurine, coenzyme A

Integration Summary

DIET (protein)
    ↓ (digestion, absorption via portal vein)
AMINO ACID POOL
    ↙               ↘
CATABOLISM          ANABOLISM
• Transamination     • Protein synthesis
• Deamination (GDH)  • Nonessential AA synthesis (transamination, reverse)
• Urea cycle         • Biosynthesis of: purines, pyrimidines,
• C-skeleton:          neurotransmitters, heme, NO, creatine,
  → TCA cycle          SAM, glutathione, hormones
  → Gluconeogenesis
  → Ketogenesis/FA synthesis

Regulatory checkpoints:

GDH (mitochondria): ADP activates (catabolism), GTP inhibits (anabolism) - energy sensor
CPSI (urea cycle): activated by N-acetylglutamate (signals high amino acid load)
Glutamine synthetase: key anabolic enzyme; regulated by feedback inhibition from multiple end products
In starvation: urea cycle enzyme expression increases severalfold due to enhanced protein catabolism

Sources:

Harper's Illustrated Biochemistry, 32nd ed., Chapters 28, 29 (nitrogen metabolism, amino acid catabolism)
Basic Medical Biochemistry: A Clinical Approach, 6th ed., Chapter 36 (transamination, urea cycle)
Biochemistry (Lippincott Illustrated Reviews), 8th ed., Chapter 24 (absorptive state amino acid metabolism)

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