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