--- föreläsare: Martin Lidell tags: - biokemi - aminosyrametabolism - slides date: 2025-12-08 --- # LPG001 Martin Lidell Amino acid metabolism ## Lecture outline  Amino acids – a short introduction  How do we get access to amino acids?  Biosynthesis of non-essential amino acids  The origin of the a-amino group and the carbon skeleton  Degradation of amino acids  What happens with the amino group and the carbon skeleton?  The urea cycle  Transport of nitrogen to the liver (alanine/glutamine)  Examples of some defects in amino acid metabolism ## Amino acids Definition: An amino acid is a simple organic compound containing both a carboxyl and an amino group More than 500 different amino acids have been described in nature Twenty a-amino acids (21 if including selenocysteine) are commonly found in mammalian proteins. These proteinogenic amino acids are the only amino acids that are coded for by DNA ## Amino acids – examples of some important non-proteinogenic amino acids GABA (g-amino acid) g-aminobutyric acid (GABA) an inhibitory neurotransmitter Ornithine and Citrulline intermediates in the urea cycle Ornithine (a-amino acid) Citrulline (a-amino acid) ## Why are amino acids essential biomolecules? – some examples Building blocks in proteins Precursors of important biomolecules (neurotransmitters, hormones, etc. etc.) Dopamine Epinephrine Source of energy Acts as neurotransmitters (e.g. Glu and Gly) Involved in acid-base homeostasis (Gln) Transport ammonia in a nontoxic form (Gln and Ala) ## Overview of amino acid metabolism Endogenous proteins De novo synthesis of non-essential amino acids Dietary proteins Synthesis of other important biomolecules Degradation Amino group → Urea Carbon skeleton → Ketone bodies, Glucose/glycogen, Energy, CO2 + H2O, Fatty acids Refilling reactions Amino acids Urea cycle ## Digestion of dietary proteins in the gastrointestinal tract ## Amino acids, di- and tripeptides are absorbed by the enterocytes and released as amino acids into the blood The absorbed di- and tripeptides are digested by peptidases into free amino acids that are released into the blood ## Intracellular degradation of endogenous proteins – released amino acids can be reused Proteasomal degradation ## Biosynthesis of amino acids – the a-amino group and the carbon skeletons ## Biosynthesis of amino acids – the a-amino group The a-amino group is most often derived from glutamate ## Biosynthesis of amino acids – the carbon skeletons ## Most microorganisms can synthesize all of the common proteinogenic amino acids ## Biosynthesis of amino acids in humans – essential and nonessential amino acids Nonessential: Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Proline, Serine, Tyrosine Essential: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine Humans cannot make the essential amino acids; they must be supplied in the diet Some nonessential amino acids become essential under certain circumstances (“conditionally essential”) e.g. arginine for fetus/neonate; tyrosine in PKU ## Biosynthesis of nonessential amino acids in humans The carbon skeletons are derived from five precursors: • 3-Phosphoglycerate • Pyruvate • a-Ketoglutarate • Oxaloacetate • Phenylalanine ## Formation of glutamate from a-ketoglutarate Glutamate is primarily formed in transamination reactions catalyzed by different aminotransferases ## Aminotransferases / Transaminases Enzymes transferring amino groups from a-amino acids to a-keto acids a–amino acid-R1 + a–keto acid-R2 → a–keto acid-R1 + a–amino acid-R2 a-Ketoglutarate/Glutamate is the most common amino group acceptor/donor pair. The reactions are reversible. Essential for both synthesis and degradation of amino acids. ## ALT and AST – two important aminotransferases Amino acids: Alanine, Aspartate, Glutamate a-Keto acids: Pyruvate, Oxaloacetate, a-ketoglutarate ## Aminotransferases as indicators of tissue damage • Intracellular enzymes • Elevated plasma levels indicate cell damage • ALT mostly in liver • AST in liver, heart, skeletal muscle, kidney ## A second route of synthesis of glutamate from a-ketoglutarate Glutamate dehydrogenase (mitochondrial, liver-specific) ## Arginine and proline – synthesized from glutamate Arginine → part of urea cycle ## Glutamine and asparagine – formed by amidation Enzymes: glutamine synthetase, asparagine synthetase ## Tyrosine – synthesized from phenylalanine Reaction: Phenylalanine + O2 + NADPH + H+ → Tyrosine + NADP+ + H2O ## Phenylketonuria (PKU) Accumulation of phenylalanine, phenylpyruvate, phenyllactate, phenylacetate Deficiency of tyrosine and metabolites Autosomal recessive (PAH gene) Hundreds of mutations Insufficient phenylalanine hydroxylase activity ## PKU symptoms Intellectual disability, delayed development, seizures, musty odor, fair skin/blue eyes Treatment: dietary restriction, amino acid mix w/o Phe, tyrosine becomes essential, sapropterin may help ## “PKU-provet” – newborn screening since 1965 Blood sample after 48 hours Purpose: detect treatable congenital diseases early ## Diseases included today (25 total) Endocrine diseases (2) Fatty acid metabolism defects (3) Carnitine system defects (4) Organic acidurias (6) Urea cycle defects (3) Amino acid metabolism defects (4) Other metabolic diseases (2) SCID ## Summary of part 1 (Amino acids important, sources, essential vs nonessential, aminotransferases, PKU) ## Excess amino acids cannot be stored Amino acids not needed → degraded to intermediates that enter central metabolism ## How are amino acids degraded? • Remove a-amino group • Carbon skeleton becomes pyruvate, TCA intermediates, acetyl-CoA, acetoacetyl-CoA Occurs primarily in liver; skeletal muscle degrades branched-chain amino acids ## Challenge: ammonia toxicity Solution: liver → urea cycle Other tissues → transport as glutamine/alanine ## Glutamate as intermediate toward urea ## a-amino groups transfer to a-ketoglutarate → glutamate (ALT/AST) ## Oxidative deamination of glutamate Glutamate dehydrogenase (liver mitochondrial matrix) ## Serine and threonine can be directly deaminated (dehydratases) ## Side-chain nitrogen of glutamine and asparagine – release ammonia and form glutamate ## Ammonia is toxic to CNS Urea cycle detoxifies ammonia Only active in liver ## Urea cycle Carbamoyl phosphate synthetase I Ornithine transcarbamoylase Argininosuccinate synthetase Argininosuccinate lyase Arginase Urea contains 2 amino groups: one from NH4+, one from aspartate. Carbon from HCO3– ## Why is ammonia toxic? (theory) Glutamine synthetase in astrocytes → glutamine accumulation → osmotic swelling → edema ## Regulation of urea cycle N-acetylglutamate activates CPS I High glutamate + arginine → more N-acetylglutamate ## Defects in urea cycle – example: argininosuccinate lyase deficiency Autosomal recessive Symptoms: hyperammonemia, irregular breathing, hypotonia, vomiting, alkalosis, brain swelling, seizures Treatment: glucose infusion, drugs promoting nitrogen excretion, dialysis, low-protein diet, liver transplant ## Drug treatment: arginine and phenylbutyrate ## Nitrogen transport from extrahepatic tissues Extrahepatic tissues lack urea cycle Transport forms: glutamine and alanine Muscle uses BCAA ## Glutamine and alanine – nitrogen carriers ## Glucose-alanine cycle ## Where do carbon skeletons end up? ## Seven end-products of amino acid carbon skeleton degradation ## Citric acid cycle – source of building blocks Cycle must be refilled (anaplerosis) ## Anaplerotic reactions Pyruvate, amino acid skeletons refill TCA ## Glucogenic vs ketogenic amino acids Glucogenic → pyruvate or TCA intermediates → glucose Ketogenic → acetyl-CoA or acetoacetyl-CoA → ketone bodies 13 glucogenic 5 mixed (Phe, Iso, Thr, Trp, Tyr) 2 ketogenic only (Lys, Leu) ## Oxaloacetate as entry point for Asp/Asn ## a-Ketoglutarate as entry point for several amino acids Glutamate → a-ketoglutarate (via GDH) ## Degradation pathways generating acetyl-CoA ## Degradation of phenylalanine and tyrosine ## Degradation of branched-chain amino acids Occurs mainly in skeletal muscle ## Maple syrup urine disease (MSUD) Autosomal recessive Defect in branched-chain a-keto acid dehydrogenase complex Accumulation of Leu, Iso, Val and their keto acids Symptoms: poor feeding, vomiting, low energy, abnormal movements, delayed development; severe cases seizures/coma Treatment: protein-restricted diet lacking Leu/Iso/Val; controlled supplementation ## Summary of part 2 • Amino acid degradation → ammonia → toxic • Glutamate central • Liver → only site of urea production • Extrahepatic tissues use glutamine/alanine • Carbon skeletons used for refilling, glucose, ketone bodies, fatty acids, energy ## Some important enzymes ALT AST Glutamate dehydrogenase Glutamine synthetase Glutaminase Phenylalanine hydroxylase Carbamoyl phosphate synthetase I ## Läsanvisningar Biochemistry (Berg et al.) Chapter 23: 701–703, 708–731 Chapter 25: 766–790 Instuderingsfrågor på Canvas Amino acid metabolism