Amino Acid Metablism

Overview of AA metabolism Ex Biochem c8-AA metabolism Overview of AA metabolism Principal source of AA from food protein Broken down to free AA, absorbed into blood >50% dietary AA taken up by liver Essential AA: can not synthesized by body, arg, his, Isoleu, leu, lys, met, phenylala, threonine, tryptophan, val AA pool: AA in blood and extracellular fluids No ability to store AA, extra AA used as fuels Very small compared to total protein in body AA and protein turnover very quickly Liver responsible for much of AA metabolism Kidney in smaller extent Skeletal muscle the largest repository of free and protein-bound AA in body

Ex Biochem c8-AA metabolism

Overview of AA metabolism Ex Biochem c8-AA metabolism Overview of AA metabolism

Ex Biochem c8-AA metabolism AA transporters AA have charged groups, they need protein transporters to transfer between extracellular and intracellular compartments 2 broad categories of AA transporters Na-dependent: move into cell down Na concentration gradient, can be moved against AA concentration gradient Na-independent AA transporters may have broad specificity, recognizing several AA Some have narrow specificity, recognizing only 1-2 closely related AA Competition for the same transporters

Ex Biochem c8-AA metabolism Degradation of AA Balance among AA can be achieved by conversion reactions One AA changed into another by transfer of amino group 18 AA are glucogenic: provide all or part of their carbon atom for gluconeogenesis Ketogenic: leucine, lysine AA undergo constant oxidative degradation: Normal synthesis and degradation, not immediately used for protein synthesis Ingest more AA than body can use to make proteins Starvation Overtaining, imbalance in protein turnover (testosterone/cortisol ratio)

Transamination reactions Ex Biochem c8-AA metabolism Transamination reactions Transfer of amino groups in all AA except thr, lys Aminotransferase enzyme, transaminase Most transfer to a-ketoglutarate, making glutamate Freely reversible, net direction depend on relative concentration of 4 reactants Alanine aminotransferase (glutamate pyruvate transaminase, GPT) Alanine + a-KG < pyruvate + glutamate Aspartate aminotransferase (glutamate oxaloacetate transaminase, GOT) Asp + a-KG < oxaloacetate + glutamate

Transamination reactions Ex Biochem c8-AA metabolism Transamination reactions

Ex Biochem c8-AA metabolism

Deamination reactions Ex Biochem c8-AA metabolism Deamination reactions Nitrogen from amino groups in liver in the form of glutamate can be released as ammonia Glutamate + H2O + NAD+  a-KG + NADH + H+ + NH4+ Glutamate dehydrogenase Production of NH4 and its release from muscle proportional to exercise intensity Glutamate DHase reactions Deamination of AMP by AMP deaminase NH4+ play a role in peripheral and central fatigue Increased acidity in muscle Cross blood brain barrier, increased NH4+ uptake by brain

Ex Biochem c8-AA metabolism Glutamine Special AA even though not essential Important fuel for gut and immune system (macrophages, lymphocytes) Free glutamine concentration high in variety of cells and in blood ~60% AA pool Mostly synthesized from glutamate by glutamine synthetase Deamination of glutamine Glutaminase: glutamine + H2O  glutamate + NH4+

Ex Biochem c8-AA metabolism Glutamine synthesis

Branched-chain amino acids, BCAA Ex Biochem c8-AA metabolism Branched-chain amino acids, BCAA Most common essential AA in proteins Metabolized mainly in skeletal muscle Increased BCAA oxidation during exercise, used as fuel or provide carbon backbone for CAC intermediates BCAA aminotransferease, branched chain ketoacid dehydrogenase (BCKAD), acyl-CoA DHase BCKAD inhibited by BCKAD kinase, response to exercise Glucose-alanine cycle: transfer amino group from muscle to liver for urea synthesis Leucine can enhance protein synthesis by stimulating initiation of translation mTOR

Ex Biochem c8-AA metabolism

Ex Biochem c8-AA metabolism Metabolism of BCAA

Glucose-alanine cycle Ex Biochem c8-AA metabolism Glucose-alanine cycle

Ex Biochem c8-AA metabolism

Ex Biochem c8-AA metabolism Urea cycle NH4+ very toxic, especially to brain Temporary safe forms: glutamate, glutamine Converted to urea, secreted in urine Muscle release alanine, glutamine N from BCAA  glutamate  ala or gln Nitrogen in liver Ala  glu by alanine aminotransferease Gln  glu by glutaminase NH4+ taken up from blood The above 3 provide NH4+ for urea synthesis The other NH3 in urea from aspartate Regulation point: carbamoyl phosphate synthetase Metabolically expensive: 4 ATP for 1 urea

Ex Biochem c8-AA metabolism

Ex Biochem c8-AA metabolism

The Urea Cycle -Overview Ex Biochem c8-AA metabolism The Urea Cycle -Overview

Ex Biochem c8-AA metabolism The Urea Cycle

Ex Biochem c8-AA metabolism The Urea Cycle

Fate of AA carbon skeletons Ex Biochem c8-AA metabolism Fate of AA carbon skeletons 18 AA can be source for gluconeogenesis Leucine and lysine only form acetoacetyl CoA and acetyl CoA: ketogenic Many AA have carbon skeletons as CAC intermediates or substances directly related to CAC

Ex Biochem c8-AA metabolism

Ex Biochem c8-AA metabolism

AA metabolism during moderate-intensity exercise Ex Biochem c8-AA metabolism AA metabolism during moderate-intensity exercise Study of AA metabolism during exercise is complex Measurement of AA differences between arterial and venous blood Muscle biopsy Only see the equilibrium between AA synthesis and breakdown, unless use stable isotope During ex in postabsorptive state, skeletal muscle is in net protein catabolic state Most AA produced by net protein catabolism released into blood, except glutamate and alanine Net uptake of glutamate at rest and even more during exercise, glutamate used as precursor for glutamine Release of ala and gln far out of proportion to their content in skeletal muscle, synthesized in skeletal muscle at accelerated rate during exercise Ala release decline with exercise duration, less glucose  pyruvate

AA metabolism during moderate-intensity exercise Ex Biochem c8-AA metabolism AA metabolism during moderate-intensity exercise Exercise in low-glycogen, protein breakdown greater, corresponding increase in release of most AA from muscle Induce protein breakdown to release BCAA, use BCAA as fuel Decrease in total adenine nucleotide content during prolonged exercise Prevent AMP accumulation, AMP  IMP by adenylate deaminase Can reduce TAN by up to 50%, need to regenerate adenine purine nucleotide cycle, use aspartate, cost GTP Predominantly after exercise, activities of the enzymes involved too long to produce appreciable AMP during ex

Ex Biochem c8-AA metabolism

Purine nucleotide cycle Ex Biochem c8-AA metabolism Purine nucleotide cycle

Purine nucleotide cycle Ex Biochem c8-AA metabolism Purine nucleotide cycle

Purine nucleotide cycle Ex Biochem c8-AA metabolism Purine nucleotide cycle

AA metabolism during high-intensity exercise Ex Biochem c8-AA metabolism AA metabolism during high-intensity exercise Only modest increase in glutamine and alanine release from muscle, when exercise intensity is high Glutamine synthesis require ATP and glutamate Glutamate is source of a-KG, CAC intermediate Increased adenylate deaminase reaction during high-intensity exercise Recruitment of type II muscle fiber, in which adenylate deaminase activity is high Increase release of NH4+, IMP is trapped within muscle

Central fatigue theory Ex Biochem c8-AA metabolism Central fatigue theory voluntary maximal work of the muscle < the work when motor nerve was electrically stimulated Fatigue in central nervous system Increased production of serotonin in brain Increased ammonia entry to brain Tryptophan as precursor for serotonin synthesis Most tryptophan bind to albumin ↑ FFA during exercise compete for albumin, ↑free Trp BCAA compete with trp for blood brain barrier BCAA supplementation helpful? Endurance exercise? most animal studies support the theory, but most human studies failed to show benefit effect ↑ammonia, combination use with arginine?

Central fatigue theory Ex Biochem c8-AA metabolism Central fatigue theory

Central fatigue – supplementation of CHO and BCAA Ex Biochem c8-AA metabolism Central fatigue – supplementation of CHO and BCAA

Ex Biochem c8-AA metabolism

Additional roles for AA Ex Biochem c8-AA metabolism Additional roles for AA Precursors for many biologically active compounds Neurotransmitters Tyrosine: dopamine, norepinephrine, epinephrine Histidine: histamine Tryptophan: serotonin, role in central fatigue Maintain cellular redox state: glutathione

Ex Biochem c8-AA metabolism Glutathione

Ex Biochem c8-AA metabolism Some Small Peptides

Ex Biochem c8-AA metabolism

Ex Biochem c8-AA metabolism Tamaki et al, 1992 Aguiar et al, 2013