1. Amino Acid Metabolism
Nitrogen Assimilation · Biosynthesis · Catabolism · Bioactive Derivatives
1. Amino Acid Metabolism
Living organisms vary widely in their capacity to synthesize the 20 standard amino acids required for protein synthesis. While plants and many microorganisms can produce all 20 standard amino acids from readily available inorganic precursors, mammals and other animals have lost this capacity for a subset of these molecules and must obtain them from the diet.
1.1 Amino Acid Synthesis and Nitrogen Assimilation
Mammals can synthesize only 12 non-essential amino acids, using carbon skeletons derived from common metabolic intermediates. The remaining 8 essential amino acids (plus two that are essential specifically during infancy) cannot be synthesized de novo and must be obtained preformed through the diet.
| Essential amino acids | Non-essential amino acids |
|---|---|
| Isoleucine | Alanine |
| Leucine | Proline |
| Lysine | Asparagine |
| Methionine | Aspartate |
| Phenylalanine | Cysteine |
| Threonine | Glutamate |
| Tryptophan | Glutamine |
| Valine | Glycine |
| Arginine# | Serine |
| Histidine# | Tyrosine |
# Amino acids that are essential specifically for infants.
Nitrogen is a key component of amino acids, proteins, and nucleotides. In living systems, the ammonium ion (NH4+) is the ultimate source of nitrogen for all biomolecules. Ammonium is first incorporated into the amino acids glutamate and glutamine, which serve as the primary nitrogen donors for transamination reactions. There are two principal enzymatic pathways by which inorganic ammonium is assimilated into organic molecules.
1. Reductive amination of α-keto acids. This pathway represents a minor route in eukaryotes and is located within the mitochondrial matrix. The reaction is catalyzed by glutamate dehydrogenase, an unusual enzyme that does not discriminate between NADH and NADPH as reducing equivalents.
2. Formation of the amides of glutamic acid. Ammonium ions are also incorporated into cell metabolites by forming glutamine, the amide derivative of glutamate. This amidation reaction is catalyzed by glutamine synthetase (present in all organisms) and is driven by ATP hydrolysis.
Mechanism: ATP participates directly in the reaction by phosphorylating the side-chain carboxyl group of glutamate to form an acyl-phosphate intermediate, which then undergoes nucleophilic attack by ammonia to yield glutamine.
- Phosphorylation: Glutamate + ATP → γ-Glutamyl phosphate (acyl-phosphate intermediate) + ADP
- Nucleophilic attack: γ-Glutamyl phosphate + NH4+ → Glutamine + Pi + H+
Once nitrogen is assimilated into glutamate, it is distributed to other carbon skeletons via transamination reactions. In these reactions, the α-amino group of an amino acid is transferred to the α-carbon of an acceptor α-keto acid to produce a new amino acid and a new α-keto acid.
Enzymes: these reversible reactions are catalyzed by aminotransferases (transaminases), found in both the cytoplasm and mitochondria.
The PLP cofactor: all aminotransferases require the coenzyme pyridoxal-5-phosphate (PLP), a derivative of pyridoxine (vitamin B6). PLP is covalently bound to the enzyme's active site via a Schiff base linkage, formed by condensation of its aldehyde group with the ε-amino group of a specific active-site lysine residue.
Figure: the transamination Schiff-base cycle. An incoming amino acid displaces the active-site lysine to form a new Schiff base with PLP, transferring its α-amino group to the coenzyme and releasing its carbon skeleton as an α-keto acid; this leaves PLP in its reduced pyridoxamine-phosphate form. An acceptor α-keto acid then reacts with pyridoxamine phosphate, gaining the amino group to form a new amino acid and regenerating the aldehyde form of PLP bound to the enzyme.
The carbon skeletons of all 20 standard amino acids are derived from commonly available metabolic intermediates of glycolysis, the citric acid cycle, or the pentose phosphate pathway. Based on their primary precursor molecule, amino acids are grouped into six biosynthetic families.
Figure: the six amino acid biosynthetic families. Glutamate family – derived from the TCA intermediate α-ketoglutarate (glutamate, glutamine, proline, arginine). Serine family – derived from the glycolytic intermediate glycerate-3-phosphate (serine, glycine, cysteine). Aspartate family – derived from the TCA intermediate oxaloacetate (aspartate, asparagine, lysine, methionine, threonine). Pyruvate family – derived from the glycolytic end-product pyruvate (alanine, valine, leucine, isoleucine). Aromatic family – derived from phosphoenolpyruvate (PEP) and the pentose phosphate pathway intermediate erythrose-4-phosphate (phenylalanine, tyrosine, tryptophan). Histidine family – derived from the pentose phosphate pathway intermediate ribose-5-phosphate (histidine).
1.2 Amino Acid Catabolism
In mammals, when amino acids are degraded, their amine groups and carbon skeletons are processed differently. Catabolism involves three common stages: removal of the α-amino group and its conversion into free ammonia; incorporation of ammonia into urea for safe excretion, or recycling for biosynthesis; and conversion of the remaining carbon skeletons (α-keto acids) into common metabolic intermediates that can be oxidized to CO2 and H2O or converted to glucose, acetyl-CoA, or ketone bodies.
Figure: overview of protein degradation. Amino acid carbon skeletons (α-keto acids) are oxidized via the TCA cycle or converted to glucose, acetyl-CoA, or ketone bodies; released ammonium is either excreted as urea or recycled into new amino acids and nucleotides.
The removal of the α-amino group is achieved through the combined actions of transamination and oxidative deamination.
1. Transamination: transamination reactions funnel nitrogen from all free amino acids into L-glutamate. All amino acids except lysine, threonine, and proline participate in transamination during catabolism.
2. Oxidative deamination: glutamate is the only amino acid that undergoes oxidative deamination at an appreciable rate. This reaction occurs in the mitochondrial matrix and is catalyzed by glutamate dehydrogenase, releasing free ammonium ions and regenerating α-ketoglutarate.
Living organisms must safely eliminate excess nitrogen. Depending on water availability, species use different forms of nitrogen excretion: ammonotelic organisms excrete nitrogen directly as highly soluble but toxic ammonia (typical of aquatic animals); uricotelic organisms excrete nitrogen as insoluble, dry uric acid (birds and reptiles, minimizing water loss); ureotelic organisms excrete nitrogen as water-soluble, non-toxic urea (land mammals). In ureotelic organisms, urea is synthesized in the liver via the five-step urea cycle. The first two reactions occur in the mitochondrial matrix, and the remaining three take place in the cytosol.
Figure: the urea cycle. The dashed boxes are not reaction steps — they mark the two cellular compartments the cycle spans. Step 1 – carbamoyl phosphate synthetase I (CPSI) condenses NH4+ and HCO3− (2 ATP) into carbamoyl phosphate. Step 2 – ornithine transcarbamoylase (OTC) transfers the carbamoyl group to ornithine, forming citrulline, exported to the cytosol. Step 3 – argininosuccinate synthetase (ASS) condenses citrulline with aspartate (ATP → AMP + PPi) to form argininosuccinate. Step 4 – argininosuccinate lyase (ASL) cleaves off fumarate, yielding arginine. Step 5 – arginase hydrolyzes arginine to release urea and regenerate ornithine, which is transported back into the mitochondria.
- Synthesis of carbamoyl phosphate (mitochondria): ammonium and bicarbonate are condensed to form carbamoyl phosphate, catalyzed by the rate-limiting enzyme carbamoyl phosphate synthetase I (CPSI). This reaction requires two ATP molecules and is irreversible.HCO3− + NH4+ + 2 ATP CPSI→ Carbamoyl phosphate + 2 ADP + Pi
- Synthesis of citrulline (mitochondria): carbamoyl phosphate transfers its carbamoyl group to ornithine to form citrulline, catalyzed by ornithine transcarbamoylase. Citrulline is then exported from the mitochondria into the cytosol.
- Synthesis of argininosuccinate (cytosol): citrulline condenses with aspartate (which supplies the second nitrogen atom for urea) in an ATP-dependent reaction catalyzed by argininosuccinate synthetase. This step converts ATP to AMP and pyrophosphate (PPi), consuming two high-energy phosphate equivalents.Citrulline + Aspartate + ATP Argininosuccinate Synthetase→ Argininosuccinate + AMP + PPi
- Cleavage of argininosuccinate (cytosol): argininosuccinate is cleaved by argininosuccinate lyase to yield free fumarate (which can enter the citric acid cycle or be used for glucose synthesis) and arginine.
- Cleavage of arginine to urea (cytosol): the cytosolic enzyme arginase hydrolytically cleaves arginine, releasing urea and regenerating ornithine, which is transported back into the mitochondria to sustain another turn of the cycle.
Stoichiometry and energetics: synthesizing one molecule of urea consumes four high-energy ATP equivalents (2 ATP for carbamoyl phosphate, and 1 ATP cleaved to AMP + PPi for argininosuccinate).
Long-term (genetic) regulation: high-protein diets or prolonged starvation increase the rate of amino acid catabolism, generating excess nitrogen. Under these conditions, the synthesis of all five urea cycle enzymes is transcriptionally upregulated, showing 20-fold or greater increases in enzyme concentration.
Short-term (allosteric) regulation: carbamoyl phosphate synthetase I (CPSI) is relatively inactive without its obligate allosteric activator, N-acetylglutamate, synthesized from acetyl-CoA and glutamate.
The carbon skeletons of the 20 standard amino acids are degraded into seven primary metabolic products: acetyl-CoA, acetoacetyl-CoA, pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate. Based on their degradative end products, amino acids are classified into two metabolic categories: glucogenic amino acids, degraded to pyruvate or citric acid cycle intermediates (precursors for glucose synthesis via gluconeogenesis), and ketogenic amino acids, degraded to acetyl-CoA or acetoacetyl-CoA (used to synthesize fatty acids or ketone bodies).
Figure: metabolic destinations of amino acid carbon skeletons. Amino acids feeding pyruvate or TCA-cycle intermediates are glucogenic; those feeding the two ketone-body precursors, acetyl-CoA and acetoacetyl-CoA, are ketogenic.
Enzyme deficiencies in amino acid degradation pathways lead to severe clinical pathologies due to accumulation of toxic intermediates.
| Condition | Defective process | Defective enzyme | Major symptoms |
|---|---|---|---|
| Phenylketonuria (PKU) | Conversion of phenylalanine to tyrosine | Phenylalanine hydroxylase | Accumulation of phenylalanine in body fluids; neonatal vomiting, mental retardation. |
| Alkaptonuria | Tyrosine degradation | Homogentisate 1,2-dioxygenase | Homogentisic acid accumulates; excreted urine oxidizes and polymerizes on standing, turning dark. |
| Albinism | Melanin synthesis from tyrosine | Tyrosinase (tyrosine 3-monooxygenase) | Complete lack of pigmentation in hair and skin; white hair and pink skin. |
| Maple Syrup Urine Disease | Branched-chain amino acid degradation (Leu, Ile, Val) | Branched-chain α-keto acid dehydrogenase complex | Severe neonatal vomiting, convulsions, mental retardation, early death. |
| Homocystinuria | Methionine degradation | Cystathionine β-synthase | Faulty bone development, mental retardation. |
Figure: phenylalanine and tyrosine pathway blocks. Enzyme deficiencies at three points in this pathway produce three distinct hereditary disorders: phenylketonuria, albinism, and alkaptonuria. The red dashed connector marked ⊘ points from each blocked step to the disorder it causes.
1.3 Bioactive Molecules Derived from Amino Acids
Amino acids serve as essential precursors for several hormones, neurotransmitters, and specialized structural molecules.
| Precursor amino acid | Bioactive product(s) |
|---|---|
| Tryptophan | Auxin (indole-3-acetic acid) & Serotonin |
| Tyrosine | Catecholamines (dopamine, norepinephrine, epinephrine) & lignin |
| Glutamate | GABA (γ-aminobutyrate) |
| Histidine | Histamine |
| Arginine | Nitric oxide (NO) & creatine (with glycine) |
| Glycine | Porphyrins (heme) & glutathione (with glutamate & cysteine) |
2. Nucleotide Metabolism
De Novo Biosynthesis · Salvage Pathways · Deoxyribonucleotides · Catabolic Disorders
2. Nucleotide Metabolism
Nucleotides are phosphate esters of a pentose sugar (ribose or deoxyribose) linked to a purine or pyrimidine base. They are synthesized via two distinct routes: de novo pathways, which build nucleotides anew from simpler precursors, and salvage pathways, which recover preformed bases released during nucleic acid degradation.
2.1 De Novo Purine Nucleotide Biosynthesis
Unlike pyrimidines, where the base is assembled before attachment to ribose, the purine ring is assembled piece-by-piece directly onto a ribose ring, starting with 5-phosphoribosyl-1-pyrophosphate (PRPP) as a foundation.
Figure: origins of the purine ring atoms. The nine ring atoms of the purine base are contributed by five distinct sources — aspartate, glutamine, glycine, CO2, and N10-formyltetrahydrofolate — assembled directly onto a ribose-5-phosphate foundation (as PRPP).
- Acquisition of N9: pyrophosphate on PRPP is displaced by ammonia from glutamine to yield 5-phosphoribosyl-1-amine, catalyzed by amidophosphoribosyltransferase — the committed step of the pathway.Chemotherapeutic inhibition: the glutamine analogues azaserine and acivicin inhibit this committed step and are used as antitumor agents.
- Acquisition of C4, C5, and N7: glycine condenses with 5-phosphoribosyl-1-amine in an ATP-dependent reaction catalyzed by glycinamide ribonucleotide synthetase (GAR synthetase), producing glycinamide ribonucleotide (GAR).
- Acquisition of C8: the free amino group of GAR is formylated by N10-formyltetrahydrofolate (catalyzed by GAR transformylase) to yield formylglycinamide ribonucleotide (FGAR).
- Acquisition of N3: glutamine transfers its amide group to FGAR in an ATP-dependent reaction catalyzed by FGAR amidotransferase (FGAM synthetase) to yield formylglycinamidine ribonucleotide (FGAM).
- Imidazole ring closure: FGAM undergoes ATP-driven cyclisation, catalyzed by AIR synthetase, to form the five-membered imidazole ring of 5-aminoimidazole ribonucleotide (AIR).
- Acquisition of C6: carbon dioxide is added to the C4 position of the imidazole ring by AIR carboxylase in an ATP-independent reaction, yielding 4-carboxy-5-aminoimidazole ribonucleotide (CAIR).
- Acquisition of N1: aspartate condenses with CAIR in an ATP-dependent reaction catalyzed by SAICAR synthetase, yielding N-succinyl-5-aminoimidazole-4-carboxamide ribonucleotide (SAICAR).
- Elimination of fumarate: adenylosuccinate lyase cleaves the four carbon atoms of aspartate from SAICAR as fumarate, leaving 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR).
- Acquisition of C2: AICAR transformylase adds a formyl group from N10-formyl-THF to the amino group of AICAR, forming N-formylaminoimidazole-4-carboxamide ribonucleotide (FAICAR).
- Cyclisation to IMP: IMP cyclohydrolase (inosinicase) catalyzes dehydration and ring closure of FAICAR to yield the parent purine nucleotide, inosine monophosphate (IMP).
IMP is converted into either AMP or GMP through two-step branching pathways, regulated by a reciprocal energy requirement: AMP synthesis requires GTP, whereas GMP synthesis requires ATP.
Base-specific kinases convert monophosphates to diphosphates, and nucleoside diphosphate kinase converts diphosphates to triphosphates.
2.2 De Novo Pyrimidine Nucleotide Biosynthesis
In pyrimidine biosynthesis, the six-membered pyrimidine ring is synthesized first and then attached to PRPP. The atoms of the pyrimidine ring are contributed by aspartic acid (N1, C4, C5, and C6) and carbamoyl phosphate (C2 and N3).
Figure: origins of the pyrimidine ring atoms. Four ring atoms derive from aspartate and two from carbamoyl phosphate, the two building blocks condensed in step 2 of pyrimidine synthesis.
- Synthesis of carbamoyl phosphate: bicarbonate and glutamine are condensed to form carbamoyl phosphate, catalyzed by the cytosolic enzyme carbamoyl phosphate synthetase II (CPSII). This reaction consumes two ATP molecules.HCO3− + Glutamine + H2O + 2 ATP CPSII→ Carbamoyl phosphate + Glutamate + 2 ADP + Pi
- Synthesis of carbamoyl aspartate: carbamoyl phosphate condenses with aspartate, catalyzed by aspartate transcarbamoylase (ATCase).
- Ring closure: dihydroorotase catalyzes an intramolecular condensation of carbamoyl aspartate to yield dihydroorotate.
- Oxidation: dihydroorotate is oxidized to orotate by dihydroorotate dehydrogenase, an enzyme located on the outer surface of the inner mitochondrial membrane.
- Acquisition of ribose: orotate phosphoribosyltransferase couples orotate with PRPP to yield orotidylate (OMP), driven by pyrophosphate (PPi) hydrolysis, which locks the nucleotide in the β configuration.
- Decarboxylation: OMP decarboxylase decarboxylates OMP to produce uridylate (UMP).
UMP is phosphorylated to UDP and then to UTP. CTP synthetase (a glutamine amidotransferase) then converts UTP to CTP by replacing a carbonyl group with an amino group, driven by ATP hydrolysis.
Figure: pyrimidine nucleotide synthesis, orotate to CTP. Orotate → OMP (orotate phosphoribosyltransferase) → UMP (OMP decarboxylase) → UDP → UTP (kinases) → CTP (CTP synthetase, via CTP synthetase).
2.3 Formation of Deoxyribonucleotides
DNA nucleotides differ from RNA nucleotides by containing 2′-deoxyribose residues instead of ribose, and thymine instead of uracil.
Deoxyribonucleotides are synthesized directly from their corresponding ribonucleoside diphosphates (NDPs) by reducing the C2′ position, catalyzed by ribonucleotide reductase. This reduction requires electrons delivered from NADPH via the regulatory carrier protein thioredoxin, whose active sulfhydryl (–SH) groups reduce the NDP substrate, forming a disulfide (S–S) bond. The active, reduced form of thioredoxin is regenerated by thioredoxin reductase using NADPH.
Figure: the ribonucleotide reductase / thioredoxin redox cycle. Ribonucleotide reductase oxidizes thioredoxin–SH to thioredoxin–S–S while reducing NDP to dNDP; thioredoxin reductase then regenerates thioredoxin–SH at the expense of NADPH.
Thymidylate (dTMP) is synthesized by methylating dUMP, a reaction catalyzed by thymidylate synthase.
Because rapidly dividing cancer cells require large pools of dTMP for DNA synthesis, this pathway is a common chemotherapeutic target.
Figure: the thymidylate–folate cycle and its chemotherapeutic blocks. Thymidylate synthase methylates dUMP to dTMP, oxidizing N5,N10-methylenetetrahydrofolate to dihydrofolate (DHF); dihydrofolate reductase (DHFR) then reduces DHF back to the active carrier tetrahydrofolate (THF) using NADPH. Fluorouracil (as FdUMP) is a suicide inhibitor of thymidylate synthase; methotrexate and aminopterin are competitive inhibitors of DHFR — both halt dTMP synthesis.
2.4 Nucleotide Salvage Pathways
Salvage pathways recycle free purine and pyrimidine bases released during nucleic acid degradation, attaching them to PRPP to form nucleotides.
Free thymine is salvaged in two steps:
2.5 Nucleotide Degradation and Associated Disorders
Purine nucleotides are sequentially degraded to yield uric acid as the final excretion product in humans. Mammals other than primates further oxidize uric acid to allantoin.
Figure: purine catabolism. AMP and GMP converge on a shared route through inosine/guanosine, hypoxanthine/guanine, and xanthine, with xanthine oxidase catalyzing the final two oxidation steps to uric acid. The dashed line is not a reaction step — it marks that the xanthine produced from hypoxanthine (left) and the xanthine produced from guanine (right) are the same molecule, both feeding the final oxidation to uric acid.
Unlike the insoluble purine ring, the pyrimidine ring is cleaved and degraded into highly water-soluble structures that are easily excreted.
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