medical-notes/content/Biokemi/Metabolism/Aminosyrametabolism/Slides.md

föreläsare, tags, date

föreläsare	tags	date
Martin Lidell	biokemi aminosyrametabolism slides	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