Metabolism of Amino Acid TUMS Metabolism of Amino Acid (Carbon Skeletons) Part 2 Dr. Azin Nowrouzi Tehran University of Medical Sciences

Fate of the C-Skeleton of Amino Acids 2

Outline of catabolism of 20 amino acids Products # enzymatic steps Cofactors Glycogenic or Lipogenic Alanine Pyruvate 1 PLP G Glycine 2 N5,N10 CH2 THF Serine Cysteine PLP, NADH Threonine 3 Aspartic acid Oxaloacetate Asparagine Histidine α-ketoglutarate 5 THFA, PLP Glutamic acid Glutamine Arginine 4 PLP, NAD Proline O2, PLP 4

Amino acid Products enzymatic steps Cofactors Glycogenic or Lipogenic Methionine Succinyl CoA 9 ATP, CoA, NAD, biotin, Vit B12 G Valine 10 PLP, NAD, CoA, Vit B12 Isoleucine PLP, NAD, CoA, FAD, biotin, Vit B12 G, L Leucine Succinyl CoA, Acetoacetyl coA 6 Thiamin PP, lipoic acid, PLP, CoA, NAD, FAD L Phenylalanine Succinyl CoA, fumarate 7 O2, NADPH, tetrahydrobiopterin Tyrosine Succinyl CoA, Fumerate Tryptophan Succinyl CoA, Alanine O2, NADPH, NAD Lysine Acetoacetyl CoA, NADPH, NAD, NADP, PLP, CoA, FAD 5

Degradation of Carbon Skeletons Seven products result from the catabolism of amino acid carbon skeletons: oxaloacetate, α-ketoglutarate, pyruvate, fumarate, acetyl coA, acetoacetyl coA, succinyl coA Glycogenic Their catabolism produces pyruvate or one of the intermediates of the Crebs cycle. These are substrates for gluconeogenesis So they can produce glycogen in liver and muscle. Lipogenic (or ketogenic) Their catabolism produces acetoacetate or its precursors acetyl coA or acetoacetyl coA 6

Amino Acids that produce Oxaloacetate

Amino Acids that produce α-ketoglutarate

Amino Acids that produce Pyruvate

Amino Acids that produce Fumarate

Phenylketonuria (PKU) Disease Deficiency of Phe hydroxylase Occurs in 1:20,000 live births in U.S. Seizures, mental retardation, brain damage Treatment: limit phenylalanine intake Screening of all newborns mandated in all states 11

Amino Acids that produce Acetyl CoA or Acetoacetyl CoA

Amino Acids that produce Succinyl CoA

Catabolism of Branched Chain Amino Acids

Transfer of nitrogen components from tissues to the liver for urea synthesis

Fed state 16

Fasting (starvation) (i) For the first 7 days, maintain blood glucose (brain use 65% of glucose  400 - 600 Cal) (ii) > 7 days: Protein proteolysis decreases (protect essential proteins) therefore use over a prolonged period compromises organism. (iii) → Switch to Ketone bodies 17

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Where does this Ala & Gln come from? AA are released from muscle during the post- absorptive state (O/N fast). Of the AA released by muscle Ala= 30% & Gln= 25% (total> 50%) But output (Ala+Gln) > abundance in muscle proteins which contain 7-10% Ala & 6% Gln Where does this Ala & Gln come from? 19

Sources of Alanine (from Muscle) (i)Muscle: Protein → Ala + aa aa→ NH4+ + α keto acids α keto acids → Ala (“simplest” aa). Therefore total Ala released > Ala derived from proteins (ii) Liver: Ala → NH4+ + α keto acids NH4+ → urea (iii) As well Glucose → Pyruvate (no N) → Ala (with N) Therefore Ala serves as a vehicle for transport of NH4+ from muscle to liver (NH4+ is generated through breakdown of aa  → energy). (iv) Because free NH4+ is very toxic even at low levels therefore Pyruvate + NH4+ → Ala (non-toxic) (v) In liver: NH4+ → urea for excretion Sources of Alanine (from Muscle) 20

Specialized Amino Acid Roles 1. Certain NEAA continue being synthesized even when adequate levels are supplied in diet because of a specialized role 2. ARG → urea synthesis ASP → urea synthesis GLU → conduit for disposal of N 3. ALA & GLN → key role in exchange between tissues (liver & skeletal muscle) 4. Liver: major site gluconeogenesis (AA → Glucose) major site urea synthesis (kidneys to a lesser extent) 5. Skeletal Muscle: 60% total body protein, 50% total body AA pool and is the major source to provide AA carbons → hepatic gluconeogenesis 21

Amino Acid Degradation Removal of alpha-amino groups Nitrogen excretion Fate of carbon skeletons 22

Removal of alpha-amino groups Mechanisms of –NH2 removal Transamination Oxidative deamination Amino acid oxidases Threonine or Serine dehydratase 23

A. Transamination Removal of Nitrogen by aminotransferase 24

B. Oxidative Deamination 25

L- and D- Amino Acid Oxidases They are present in liver and kidneys. They have low activity Their physiologic value is not clear. Amino Acid + H2O α-ketoacid + NH3

D. Amino acid Dehydratase Threonine Urea Serine and Threonine can be Directly Deaminated 27

Fate of Nitrogen in Different Organisms Other excretion products creatinine uric acid 28

Disposal of amino group Urea cycle (Krebs-Henseleit cycle) Provides 25-30 g of urea daily for urine formation in the kidneys Carbamoyl Phosphate Synthetase Ornitine Carbamoyl Transferase Argininosuccinate Synthetase Argininosuccinate Lyase Arginase Excretion of free ammonia Glutamine synthetase Glutaminase 29

Ammonium Ion is Converted into Urea Urea cycle 31

The Urea Cycle is Linked to the Citric Acid Cycle NH4+ 32

Amino Acid Metabolism 33