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 protein amino acid pool N excretion (NH4+. 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 (generallyfirst or second step of amino acid catabolic pathways) and

        collection of N in glutamic acid

        deamination of glutamic acid with release of NH4+

-glutamate dehydrogenase

3. Collection of N in glutamine or alanine for delivery to liver

      removal of NH4+ by : i. secretion; or ii. conversion to

urea or other less toxic form.

2

1

4

Pyridoxine Pyridoxal Pyridoxamine

Pyridoxal phosphate

Vitamine B6 family

See Horton: page 212 section 7.7 pyridoxal phosphate

to -amino of lysine

NH

Lys-protein

R1

C-NH3+

COO-

H- +

Schiff base with enzyme

NH Lys-protein

R1

H-C-COO-

Schiff base with substrate

aminoacid-1

1. Transamination reaction see text p 537 and fig 17.7.

NH Lys-protein

R1

H-C-COO-

Schiff base with substrate

NH2Lys-protein

R1

C-

COO-

H- O+

ketoacid-1

Pyradoxamine phosphate

NH Lys-protein

R2

H-C-COO-

NH2Lys-protein

R2

C-

COO-

H- O+ketoacid-2

NH Lys-protein

R2

H-C-COO-

NH

Lys-protein

R2

C-NH3+

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 (generallyfirst or second step of amino acid catabolic pathways) and

        collection of N in glutamic acid

        deamination of glutamic acid with release of NH4+

-glutamate dehydrogenase

3. Collection of N in glutamine or alanine for delivery to liver

      removal of NH4+ by : i. secretion; or ii. conversion to

urea or other less toxic form.

2

1

4

2. glutamate dehydrogenase (see p 533 for reaction)

• - release or capture of NH4+

·        - located in mitochondria

·        - operates near equilibrium

glutamate + H2O -ketoglutarate + NH4+

NAD NADH

NADPHNADP

amino acid + -ketoglutar keto acid + glutamate

glutamate + NAD + H2O -ketoglutar +NADH + H+ + NH4+

amino acid + NAD + H2O -keto acid +NADH + H+ + NH4+

3. transport of N to the liver- glutamine synthetase- glutaminase- alanine/glucose cycle

1. Glutamine synthetase

glutamate + NH4+ glutamine

ADP + PiATP

2. Glutaminaseglutamine glutamate + NH4

+

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 -aa2 -ka

glucose 2 alanine

glucose

2 pyruvate

2 alanine2 -kG2 Glu

2 NH4+

Glu

KGaa

ka Pyr

Ala

Glu’NH2NH4+

KG Glu

Ala Pyr

NH4+

Glu’NH2

Glucose

Glucose

LIVER

2Glu’NH22GluKG

Glucose

2NH4+ 2NH4

+

4CO2

CO2HCO3 + H+H2CO3

H2O

KIDNEY

MUSCLE

Urea

protein

energy

Urea

CO2

Urea cycle

Where: Liver: mito/cyto

Why: disposal of N

Immediate source of N: glutamate dehydrogenaseglutaminase

Fate of urea:liver kidney urine

How much: ~ 30g urea / day

Reactions of urea cycle

1. Carbamyl phosphate synthetase I (mito)

NH4+ + HCO3

- + 2 ATP H2N-C-OPO3-2 + Pi + 2 ADP

O

carbamyl phosphate

• committed step

• by N’Ac glutamate

2. Ornithine transcarbamylase (mito)

NH2CH2

CH2CH2

CH

NH3+

COO-

HNCH2

CH2CH2

CH

NH3+

COO-

CNH2

ONH2

COPO3

-2O+

Pi

ornithinecitrulline

carbamylphosphate

3. Arginosuccinate synthetase (cyto)

HNCH2

CH2CH2

CH

NH3+

COO-

CNH2

OCOO-

CH2CH

COO-

NH3+

HNCH2

CH2CH2

CH

NH3+

COO-

C NH

COO-

CHCH2

COO-

H2N

ATP AMP+

PPi

+

arginosuccinate

4. Arginosuccinate lyase (cyto)

HNCH2

CH2CH2

CH

NH3+

COO-

C NH

COO-

CHCH2

COO-

H2N

HNCH2

CH2CH2

CH

NH3+

COO-

C NH2 COO-

CHCH

COO-

H2N

+

arginine

fumarate

5. Arginase (cyto)

HNCH2

CH2CH2

CH

NH3+

COO-

C NH2

H2NNH2CH2

CH2CH2

CH

NH3+

COO-

NH2

C

NH2

O+

ornithine

urea

NH2CH2

CH2CH2

CH

NH3+

COO-

+

NH2

COPO3

-2O

HNCH2

CH2CH2

CH

NH3+

COO-

CNH2

O

HNCH2

CH2CH2

CH

NH3+

COO-

C NH

COO-

CHCH2

COO-

H2N

HNCH2

CH2CH2

CH

NH3+

COO-

C NH2

H2N

NH2

C

NH2

O

ornithine

ornithine

fumarate

asparate

glutamate

KG

citrulline

asparate

glutamate

NAD

NADH + H+

HCO3

2ATP2ADP +Pi

ATP

AMP + PPi

MITO

CYTO

See fig 17.26

Glu’NH2Glu’NH2

cittruline

cittruline

Arginine

creatine

ArginineArg

Ornithine

Ureacycle

Glu’NH2

creatine

P-creatine

creatinine Severalsteps

Severalsteps

Severalsteps

Urea

To urine

Epithelial cells of intestine

Kidney

Muscle

Liver

glutamate

2steps

Adapted from Devlin, Biochemistry with Clinical Corrleation4th ed.

Interorgan relationships in N metabolism