amino acid metablism 1. ex biochem c8-aa metabolism 2 overview of aa metabolism principal source of...

Amino Acid Metablism

1

Ex Biochem c8-AA metabolism 2

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


Overview of 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


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 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


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


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+


Glutamine synthesis


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 15Metabolism of BCAA


Glucose-alanine cycle


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


The Urea Cycle -Overview

Carbamoyl phosphate

CO2

2ATP

2ADP + 2H2O

+ NH4+

H3N-CHCH2COO-COO-

Aspartate

+

H2N-C-NH2

Ureacycle

Urea

C C

H

COO-H

-OOCFumarate

H2N-C-OPO32-

O

O


The Urea Cycle

H2N-C-OPO32-

O

(CH2)3

NH3+

CH-NH3+

COO-

Ornithine

Aspartate

(CH2)3

NH

CH-NH3+

COO-

C

NH2

O

Citrulline (CH2)3

NH

CH-NH3+

COO-

C

NH2

Argininosuccinate

N-CHCH2COO-

COO-

H3N-CHCH2COO-COO-

+


The Urea Cycle

(CH2)3

NH3+

CH-NH3+

COO-

Ornithine

(CH2)3

NH

CH-NH3+

COO-

C

NH2

NH2+

Arginine

H2N-C-NH2

O

Urea

C CH

COO-H

-OOCFumarate

(CH2)3

NH

CH-NH3+

COO-

C

NH2

Argininosuccinate

N-CHCH2COO-

COO-


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 27AA metabolism duringmoderate-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

Ex Biochem c8-AA metabolism 28AA metabolism duringmoderate-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


Purine nucleotide cycle

Ex Biochem c8-AA metabolism 33AA metabolism duringhigh-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 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 36Central fatigue – supplementation of CHO and BCAA


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


Glutathione

Glutathione, GSH(reduced form)

Glutathione, GS-SG(oxidized form)

2e- oxidation

2e- reduction

A disulfide bond

O O

HN

NH3+

-O

SH

NH O

OO-

O O

HN

NH3+

-O

S

NH O

OO-

O O

NHNH3

+

-O

SHN

O

OO-


Some Small Peptides

-Alanyl-L-histidine (Carnosine)

L-Aspartyl-L-phenylalanine methyl ester(Aspartame)

H3N-CH-C-NH-CH-C-OCH3

O

CH2CH2

C6H5COO-

O+H3N-CH2-CH2-C-NH-CH-COO-

O

CH2

N

NH

+


Tamaki et al, 1992

Aguiar et al, 2013

amino acid metablism 1. ex biochem c8-aa metabolism 2 overview of aa metabolism principal source of...

Documents

free aa

glutaminespecial aa

aa metabolismkidney

extra aa

aa poolmostly

dietary aa

aa transportersaa

liveressential aa