Amino acid metabolism · Nitrogen balance protein catabolism, synthesis biosynthesis normal N balance: N ingested = N excreted negative N balance: N ingested < N excreted positive N balance: N ingested > N excreted Dietary proteinamino acid poolN excretion (NH 4 +. urea)

Requirement for essential amino acids

Amino acid catabolism · accounts for ~ 10% of energy requirement of adults · When: · excess protein in diet (amino acids are not stored) · protein degradation exceeds demand for new protein · starvation when carbohydrates are not available · (protein storing seeds such as beans, peas, etc.) ·

Glucogenic vs ketogenic amino acids · ketogenic: yield AcCoA or AcAc as end products of catabolism - leu, lys · glucogenic: are degraded to pyruvate or a member of the TCA cycle (succinylCoA, OAA,  -ketoglutarate, fumarate). In absence of sugars, glucogenic amino acids permit continued oxidation of fatty acids by maintaining TCA cycle intermediates. Also source of carbons for gluconeogenesis in liver - ile, phe, tyr, trp · glucogenic and ketogenic: yield both ketogenic and glucogenic products. - all others

N catabolism General strategy:  removal of N from amino acid by transamination (generally first or second step of amino acid catabolic pathways) and  collection of N in glutamic acid  deamination of glutamic acid with release of NH glutamate dehydrogenase 3. Collection of N in glutamine or alanine for delivery to liver  removal of NH 4 + by : i. secretion; or ii. conversion to urea or other less toxic form. 214

PyridoxinePyridoxal Pyridoxamine Pyridoxal phosphate Vitamine B 6 family See Horton: page 212 section 7.7 pyridoxal phosphate to  -amino of lysine

NH Lys-protein R1R1 C-NH 3 + COO - H- + Schiff base with enzyme NH Lys-protein R1R1 H-C-COO - Schiff base with substrate  aminoacid-1 1. Transamination reaction see text p 537 and fig 17.7.

NH Lys-protein R1R1 H-C-COO - Schiff base with substrate NH 2 Lys-protein R1R1 C- COO - H- O +  ketoacid-1 Pyradoxamine phosphate

NH Lys-protein R2R2 H-C-COO - NH 2 Lys-protein R2R2 C- COO - H- O +  ketoacid-2

NH Lys-protein R2R2 H-C-COO - NH Lys-protein R2R2 C-NH 3 + COO - H- +  amino acid-2

Net reaction:  amino acid-1 +  ketoacid-2 PLP  amino acid-2 +  ketoacid-1 e.g. alanine +  -ketoglutarate pyruvate + glutamate

N catabolism General strategy:  removal of N from amino acid by transamination (generally first or second step of amino acid catabolic pathways) and  collection of N in glutamic acid  deamination of glutamic acid with release of NH glutamate dehydrogenase 3. Collection of N in glutamine or alanine for delivery to liver  removal of NH 4 + by : i. secretion; or ii. conversion to urea or other less toxic form. 214

2. glutamate dehydrogenase (see p 533 for reaction) - release or capture of NH 4 + · - located in mitochondria · - operates near equilibrium glutamate + H 2 O  -ketoglutarate + NH 4 + NAD NADH NADPH NADP amino acid +  -ketoglutar  keto acid + glutamate glutamate + NAD + H 2 O  -ketoglutar +NADH + H + + NH 4 + amino acid + NAD + H 2 O  -keto acid +NADH + H + + NH 4 +

3. transport of N to the liver - glutamine synthetase - glutaminase - alanine/glucose cycle 1. Glutamine synthetase glutamate + NH 4 + glutamine ADP + PiATP 2. Glutaminase glutamine glutamate + NH 4 + Note: glutamate can be used for glucose synthesis. How? 3.Formation of alanine by transamination: alanine/glucose cycle

Alanine-glucose cycle Muscle Liver glucose 2 pyruvate 2 alanine 2  -aa 2  -ka glucose 2 alanine glucose 2 pyruvate 2 alanine 2  -kG 2 Glu 2 NH 4 +

Glu  KG  aa  ka Pyr Ala Glu’NH 2 NH 4 +  KG Glu Ala Pyr NH 4 + Glu’NH 2 Glucose LIVER 2Glu’NH 2 2Glu  KG Glucose 2NH 4 + 4CO 2 CO 2 HCO 3 + H + H 2 CO 3 H2OH2O KIDNEY MUSCLE Urea protein energy Urea CO 2

Urea cycle Where: Liver: mito/cyto Why: disposal of N Immediate source of N: glutamate dehydrogenase glutaminase Fate of urea: liver kidney urine How much: ~ 30g urea / day

Reactions of urea cycle 1. Carbamyl phosphate synthetase I (mito) NH HCO ATP H 2 N-C-OPO Pi + 2 ADP O carbamyl phosphate committed step by N’Ac glutamate 2. Ornithine transcarbamylase (mito) Pi ornithine citrulline carbamyl phosphate

3. Arginosuccinate synthetase (cyto) ATP AMP + PPi + arginosuccinate 4. Arginosuccinate lyase (cyto) + arginine fumarate

5. Arginase (cyto) ornithine urea

ornithine fumarate asparate glutamate  KG    citrulline asparate glutamate NAD NADH + H + HCO 3 2ATP 2ADP +Pi ATP AMP + PPi MITO CYTO See fig 17.26

Glu’NH 2 cittruline Arginine creatine Arginine Arg Ornithine Urea cycle Glu’NH 2 creatine P-creatine creatinine Several steps Several steps Several steps Urea To urine Epithelial cells of intestine Kidney Muscle Liver glutamate 2 steps Adapted from Devlin, Biochemistry with Clinical Corrleation 4th ed. Interorgan relationships in N metabolism