Premed · Premed · Biochemistry
Lecture 22: Amino Acid Biosynthesis
Biochemistry
Learning Objectives
By the end of this lecture, students will be able to:
- Distinguish essential from nonessential amino acids and explain the biochemical basis for this classification
- Describe the biosynthetic pathways for nonessential amino acids from common metabolic intermediates
- Explain the role of tetrahydrofolate and S-adenosylmethionine in one-carbon metabolism
- Describe the synthesis and physiological roles of important molecules derived from amino acids
- Outline the regulation of amino acid biosynthesis by feedback inhibition
- Discuss clinical disorders related to amino acid biosynthesis and one-carbon metabolism
Lecture Content
I. Essential vs. Nonessential Amino Acids
Nonessential amino acids (11 in total) can be synthesized by the body in sufficient quantities. They include alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, tyrosine, and arginine (the last being conditionally essential). Essential amino acids (9 in total) must be obtained from the diet because humans lack the enzymes required for their synthesis. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. A useful mnemonic is PVT TIM HALL (Phe, Val, Thr, Trp, Ile, Met, His, Arg*, Leu, Lys), where arginine is conditionally essential because it is synthesized in adults via the urea cycle but not always in sufficient amounts, and it is essential in growing children.
Several amino acids are classified as conditionally essential: cysteine requires methionine as its sulfur source, tyrosine requires phenylalanine as a precursor, arginine synthesis may be insufficient during growth or illness, and glutamine becomes essential in critical illness. The distinction between essential and nonessential amino acids ultimately rests on carbon skeleton complexity; humans simply cannot construct certain carbon frameworks from scratch.
II. Biosynthesis of Nonessential Amino Acids
A. From TCA Cycle Intermediates and Glycolytic Intermediates
Glutamate family amino acids derive from alpha-ketoglutarate. Glutamate itself is produced by reductive amination of alpha-ketoglutarate with NH4+ and NADPH, catalyzed by glutamate dehydrogenase. Glutamine is synthesized from glutamate, NH4+, and ATP by glutamine synthetase, a key regulatory enzyme; glutamine serves as the major nitrogen carrier in the blood and is important for ammonia detoxification in peripheral tissues. Proline is derived from glutamate through glutamate-5-semialdehyde, which undergoes spontaneous cyclization to delta-1-pyrroline-5-carboxylate and subsequent NADPH-dependent reduction to proline. Arginine is synthesized via the urea cycle (ornithine to citrulline to argininosuccinate to arginine), but because most arginine produced is immediately cleaved to urea and ornithine, net synthesis is limited.
Aspartate family amino acids derive from oxaloacetate. Aspartate is produced by transamination of OAA with glutamate (catalyzed by AST). Asparagine is formed when asparagine synthetase transfers an amide group from glutamine to aspartate, consuming ATP and releasing glutamate, AMP, and PPi.
From the pyruvate family, alanine is produced by transamination of pyruvate with glutamate, catalyzed by ALT.
The 3-phosphoglycerate family draws on the glycolytic intermediate 3-phosphoglycerate. Serine is synthesized through a three-step pathway: 3-phosphoglycerate dehydrogenase (3-PGDH, using NAD+) produces 3-phosphohydroxypyruvate, which undergoes transamination to 3-phosphoserine, and a phosphatase then liberates serine. 3-PGDH is feedback-inhibited by serine. Glycine is produced from serine by serine hydroxymethyltransferase, which also generates N5,N10-methylene-THF; this reaction requires THF and PLP. Cysteine synthesis requires both serine and methionine. Homocysteine (derived from methionine) condenses with serine to form cystathionine via the PLP-dependent enzyme cystathionine beta-synthase, and cystathionine gamma-lyase then cleaves cystathionine to yield cysteine, alpha-ketobutyrate, and NH4+. Methionine therefore serves as the sulfur donor for cysteine.
Tyrosine is produced from the essential amino acid phenylalanine by phenylalanine hydroxylase, which uses molecular oxygen and tetrahydrobiopterin (BH4) as cofactors. Dihydrobiopterin reductase regenerates BH4 using NADH. A deficiency of phenylalanine hydroxylase causes phenylketonuria (PKU).
<image>A metabolic map showing the biosynthesis of nonessential amino acids from central metabolic intermediates. The glycolytic pathway and TCA cycle are shown as a backbone. From 3-phosphoglycerate: arrows lead to serine, then to glycine (with THF involvement noted) and to cysteine (with methionine/homocysteine contributing sulfur). From pyruvate: an arrow leads to alanine. From alpha-ketoglutarate: arrows branch to glutamate, which further branches to glutamine, proline, and arginine (via the urea cycle). From oxaloacetate: arrows lead to aspartate, then to asparagine. Phenylalanine (essential, from diet) is shown converting to tyrosine via phenylalanine hydroxylase with BH4. Key cofactors (PLP, NAD+, NADPH, THF, BH4) are labeled at their respective reaction steps.</image>
III. One-Carbon Metabolism
A. Tetrahydrofolate (THF) — Folic Acid Derivatives
Folate (vitamin B9) is reduced first to dihydrofolate (DHF) and then to tetrahydrofolate (THF) by dihydrofolate reductase (DHFR). This enzyme is the target of important drugs: methotrexate (an anticancer agent) and trimethoprim (an antibacterial) both inhibit DHFR. THF carries one-carbon units at various oxidation states, including N5-methyl-THF (the most reduced form, carrying a methyl group), N5,N10-methylene-THF (carrying a hydroxymethyl group), N10-formyl-THF (the most oxidized form, carrying a formyl group), and N5-formimino-THF (from histidine degradation).
Each form of THF has specific biosynthetic roles. N5,N10-methylene-THF is used in thymidylate synthesis, where thymidylate synthase converts dUMP to dTMP. N10-formyl-THF donates carbon atoms C2 and C8 during purine ring synthesis. N5-methyl-THF is used to regenerate methionine from homocysteine by methionine synthase, a reaction that requires vitamin B12.
B. S-Adenosylmethionine (SAM) — The Universal Methyl Donor
Methionine is activated by reacting with ATP in a reaction catalyzed by methionine adenosyltransferase, producing SAM. SAM is the universal methyl donor of the cell, transferring its methyl group to a wide variety of acceptors: DNA (CpG methylation for epigenetic regulation), norepinephrine (producing epinephrine, via PNMT), guanidinoacetate (producing creatine), phosphatidylethanolamine (producing phosphatidylcholine, via PEMT), and acetylserotonin (producing melatonin).
After donating its methyl group, SAM becomes S-adenosylhomocysteine (SAH), which is hydrolyzed to homocysteine and adenosine. Homocysteine then faces two possible fates. In the remethylation pathway, homocysteine receives a methyl group from N5-methyl-THF via methionine synthase (requiring vitamin B12 as methylcobalamin), regenerating methionine. In the transsulfuration pathway, homocysteine condenses with serine to form cystathionine and eventually cysteine (requiring vitamin B6). The methionine cycle (SAM cycle) thus integrates folate metabolism, B12 function, and cellular methylation reactions into a unified network.
<image>A comprehensive diagram of one-carbon metabolism and the methionine cycle. Central panel: The methionine cycle shown as a circular pathway: Methionine -> SAM (ATP consumed) -> SAH (after methyl transfer to acceptors like DNA, creatine, phospholipids) -> Homocysteine -> Methionine (via methionine synthase with B12 and N5-methyl-THF). A branching pathway from homocysteine shows the transsulfuration pathway to cysteine (via cystathionine, requiring B6). Left panel: The folate cycle showing THF receiving a one-carbon unit from serine (forming glycine), producing N5,N10-methylene-THF, which can be reduced to N5-methyl-THF (by MTHFR) or used for dTMP synthesis. The connection between N5-methyl-THF and the methionine cycle is highlighted. Drug targets are marked: methotrexate inhibiting DHFR, and 5-fluorouracil inhibiting thymidylate synthase.</image>
IV. Important Molecules Derived from Amino Acids
A. Catecholamines (from Tyrosine)
The catecholamine pathway proceeds from tyrosine through a series of modifications: tyrosine hydroxylase (BH4-dependent and rate-limiting) produces DOPA, DOPA decarboxylase (PLP-dependent) produces dopamine, dopamine beta-hydroxylase (vitamin C-dependent) produces norepinephrine, and PNMT (SAM-dependent) produces epinephrine.
B. Serotonin and Melatonin (from Tryptophan)
Tryptophan is hydroxylated by tryptophan hydroxylase (BH4-dependent) to 5-hydroxytryptophan, which is decarboxylated (PLP-dependent) to serotonin (5-HT). Serotonin is acetylated to N-acetylserotonin and then methylated using SAM to produce melatonin.
C. Histamine (from Histidine)
Histidine decarboxylase (PLP-dependent) converts histidine directly to histamine.
D. GABA (from Glutamate)
Glutamate decarboxylase (PLP-dependent) converts glutamate to gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter.
E. Nitric Oxide (from Arginine)
Nitric oxide synthase converts arginine, O2, and NADPH into citrulline and NO (nitric oxide), using BH4 as a cofactor. Nitric oxide functions as a vasodilator, neurotransmitter, and immune defense molecule.
F. Creatine (from Arginine, Glycine, and SAM)
Creatine synthesis begins in the kidney, where arginine and glycine combine to form guanidinoacetate and ornithine. In the liver, guanidinoacetate is methylated by SAM to produce creatine. Creatine is phosphorylated by creatine kinase to phosphocreatine, which serves as a high-energy phosphate reserve in muscle and brain. Creatine and phosphocreatine spontaneously cyclize to creatinine, which is excreted by the kidneys and used clinically as a marker of glomerular filtration rate.
G. Heme (from Glycine and Succinyl-CoA)
Heme synthesis begins with the condensation of glycine and succinyl-CoA by ALA synthase (PLP-dependent and rate-limiting) to form delta-aminolevulinic acid (ALA). Through a series of intermediates including porphobilinogen and protoporphyrin IX, the pathway culminates in heme when ferrochelatase inserts Fe2+ into the ring. Heme is a component of hemoglobin, myoglobin, cytochromes, and catalase. Defects in heme synthesis cause the porphyrias, a group of disorders characterized by accumulation of pathway intermediates and presenting with photosensitivity, abdominal pain, and neuropsychiatric symptoms.
V. Regulation of Amino Acid Biosynthesis
The primary regulatory mechanism is feedback inhibition, in which the end product of a biosynthetic pathway inhibits the first committed step. For example, serine inhibits 3-phosphoglycerate dehydrogenase, and glutamine synthetase is subject to cumulative feedback inhibition by multiple end products in bacteria. Transcriptional regulation also plays a role: in mammals, the kinase GCN2 senses uncharged tRNAs (indicating amino acid scarcity) and phosphorylates eIF2-alpha, activating the transcription factor ATF4, which upregulates amino acid biosynthetic genes. Substrate availability further modulates flux: transamination reactions depend on the availability of both carbon skeletons and amino group donors.
VI. Clinical Correlations
A. Homocysteinemia
Elevated homocysteine in the blood is an independent risk factor for cardiovascular disease and thrombosis. Causes include deficiency of vitamins B6, B12, or folate, as well as genetic defects in cystathionine beta-synthase or MTHFR. Homocystinuria, caused by autosomal recessive defects in cystathionine beta-synthase, presents with elevated homocysteine and methionine in the blood, lens subluxation (characteristically downward, distinguishing it from Marfan syndrome), marfanoid habitus, intellectual disability, and thromboembolism. Treatment includes high-dose B6 (some patients are B6-responsive), folate, B12, and betaine.
B. Folate Deficiency
Folate deficiency causes megaloblastic anemia because of impaired DNA synthesis resulting from decreased dTMP production. In pregnancy, folate deficiency increases the risk of neural tube defects in the developing fetus, which is why folate supplementation is recommended. Common causes include dietary deficiency, alcoholism, and drugs such as methotrexate, trimethoprim, and phenytoin.
C. Vitamin B12 Deficiency
Vitamin B12 deficiency also causes megaloblastic anemia through the "methyl-folate trap": N5-methyl-THF accumulates because B12 is needed to recycle it back to THF. Importantly, B12 deficiency also causes neurological symptoms (subacute combined degeneration of the spinal cord), which distinguishes it from isolated folate deficiency. Causes include pernicious anemia (lack of intrinsic factor), dietary deficiency in strict vegans, and malabsorption.
<image>A summary diagram of amino acid-derived bioactive molecules. A central hub shows key amino acids (tyrosine, tryptophan, histidine, glutamate, arginine, glycine) with branching arrows to their derivatives. Tyrosine branches to DOPA, dopamine, norepinephrine, and epinephrine (catecholamine pathway) and to thyroid hormones and melanin. Tryptophan branches to serotonin and melatonin, and to NAD+/NADP+ (via the kynurenine pathway with niacin). Histidine branches to histamine. Glutamate branches to GABA. Arginine branches to NO (via NOS) and to creatine (with glycine and SAM). Glycine + succinyl-CoA branches to heme. Key cofactors (BH4, PLP, SAM, vitamin C) are annotated at each step.</image>
