Amino Acid Me Tab

PROTEIN/AMINO ACID

METABOLISM

Nitrogen balance

• Protein content of adult body remains

remarkably constant

– Protein constitutes 10-15% of diet

• Equivalent amount of amino acids must be lost each

day

Amino acids pool

• No storage facility for amino acids

– Amino acids incorporated into functional proteins

• Amino acids in blood and extracellular fluid

represent an ‘amino acid pool’

– Amino acids move through this pool

• Average 60 kg woman

– 10 kg protein

– 170 g free amino acids in pool

Fate of amino acids

• If not required for protein synthesis amino groups removed

– For most amino acids occurs primarily in liver

– For Branced Chain Amino Acids (leucine, isoleucine, valine)

occurs primarily in skeletal muscle

• amino groups transferred to alanine and taken to liver for disposal via

glucose-alanine cycle

– Carbon skeletons used for:

• Gluconeogenesis (in liver)

• Oxidised in Krebs Cycle

– Amino groups used for

• Synthesis of nonprotein nitrogen compounds

• disposed of via Urea Cycle

From: Summerlin LR (1981) Chemistry for the Life Sciences. New York: Random House p 563.

Figure Pool and Fate of Amino Acids in the Body

Metabolic relationship of amino acids

BODY PROTEINS

Proteosynthesis Degradation

AMINO ACIDS DIETARY PROTEINS GLYCOLYSIS KREBS CYCLE

NONPROTEIN

DERIVATIVES Porphyrins

Purines

Pyrimidines

Neurotransmitters

Hormones

Komplex lipids

Aminosugars

UREA NH3

Con

vers

ion

(Carb

on s

keleto

n)

250 – 300

g/day

ACETYL CoA GLUCOSE CO2 KETONBODIES

Amino acid structure

The 20 common amino acids of proteins

Endopeptidases – hydrolyse the peptide bond inside a

chain: pepsin, trypsin, chymotrypsin

Exopeptidases – split the peptide bond at the end of a

protein molecule: aminopeptidase, carboxypeptidases

Dipeptidases

Enzymes cleaving the peptide bond

pepsin (pH 1.5 – 2.5) – peptide bond derived from Tyr, Phe,

bonds between Leu and Glu

trypsin (pH 7.5 – 8.5) – bonds between Lys a Arg

chymotrypsin (pH 7.5 – 8.5) – bonds between Phe a Tyr

Essential amino acids in humans

Arginine*

Histidine*

Isoleucine

Leucine

Valine

Lysine

Methionine

Threonine

Phenylalanine

Tryptophan

*Required to some degree in young growing period and/or sometimes during illness.

Non-essential and nonessential

amino acids in humans

Alanine

Asparagine

Aspartate

Glutamate

Glutamine

Glycine

Proline

Serine

Cysteine (from Met*)

Tyrosine (from Phe*)

* Essential amino acids

Can be formed from a-keto acids by transamination and

subsequent reactions.

Amino acid metabolism

• Metabolism of amino acids differs, but 3

common reactions:

– Transamination

– Deamination

– Formation of urea

C

O

R COO-

+ NH4+

1. Deamination

2. Transamination C

O

R COO-

CH

NH2

R COO-

CH

NH2

R COO-

oxidative

decarboxylation

CH2

NH3+

R CO2 +

General reactions of amino acid catabolism

3. Urea Cycle

Transamination reactions

• Amino group removed from one amino acid and transferred to another

– Catalysed by aminotransferase enzymes

– Nearly all transaminations transfer amino group to a-ketoglutarate

• Forms new ketoacid and glutamate (amino acid)

– BCAAs transaminations in smooth muscles usually result in formation of alanine (via glutamate)

• Released from muscle

• Allows amino groups from BCAAs to move from smooth muscles to liver for disposal

BCAAs=Branced chain amino acids

From: Houston, ME. (2001) Biochemistry Primer for Exercise Science. Champaign: Human Kinetics. p151

Figure Diagram of transamination reactions of amino acids

Deamination reactions

• Amino group (and H) removed

– Forms ammonia (NH3)

– Carbon skeleton left can be

• Oxidised in Krebs Cycle

• used for gluconeogenesis

• converted to fatty acid

– 18 amino acids glucogenic/ketogenic

• Leucine and lysine purely ketogenic

From: Houston, ME. (2001) Biochemistry Primer for Exercise Science. Champaign: Human Kinetics. p148

Utilization of carbon skleton from amino acids catabolism (deamination)

Urea cycle

• Ammonia is toxic

– Readily ionises to ammonium ion NH4+

• NH4+ converted to urea in liver (urea cycle)

– Urea contains 2 x NH2

» One from NH4+

» One from aspartate

• Urea excreted in urine

Figure Urea Cycle

From: Stryer, LS (1988) Biochemistry (3rd Ed). New York: WH Freeman & Co. p500

The fate of the amino group during amino acid catabolism

Transamination reaction

The first step in the catabolism of most amino acids is

removal of a-amino groups by enzymes transaminases

or aminotransferases

All aminotransferases have the same prostethic group and

the same reaction mechanism.

The prostethic group is pyridoxal phosphate (PPL),

the coenzyme form of pyridoxine (vitamin B6)

Biosynthesis of amino acid:

transamination reactions

amino acid1 +a-keto acid2 amino acid2 +a-keto acid1

NH3+

-O2CCH 2CH2CHCO 2-

Glutamate

O

R-CCO 2-+

O-O2CCH2CH2CCO 2

-

a-Ketoglutarate

NH2

R-CHCO 2-

+

Pyridoxal phosphate (PLP)-

dependent aminotransferase

Keto-acid

Amino acid

Active metabolic form of vitamin B6

Mechanism of transamination reaction: PPL complex with enzyme accept

an amino group to form pyridoxamine phosphate, which can donate its amio

group to an a-keto acid.

All amino acids except threonine, lysine, and

proline can be transaminated

Transaminases are differ in their specificity for L-amino

acids.

The enzymes are named for the amino group donor.

Clinicaly important transaminases

ALT

Alanine-a-ketoglutarate transferase ALT

(also called glutamate-pyruvate transaminase – GPT)

Aspartate-a-ketoglutarate transferase AST

(also called glutamate-oxalacetate transferase – GOT)

Important in the diagnosis of heart and liver damage caused by heart

attack, drug toxicity, or infection.

Glucose-alanine cycle

Ala is the carrier of ammonia and of the

carbon skeleton of pyruvate from muscle to

liver.

The ammonia is excreted and the pyruvate is

used to produce glucose, which is returned to

the muscle.

Alanine plays a special role in

transporting amino groups to liver.

According to D. L. Nelson, M. M. Cox :LEHNINGER. PRINCIPLES OF BIOCHEMISTRY Fifth edition

Glutamate releases its amino group as

ammonia in the liver

The amino groups from many of the a-amino acids are collected in the

liver in the form of the amino group of L-glutamate molecules.

Glutamate undergoes oxidative deamination catalyzed by L-glutamate

dehydrogenase.

Enzyme is present in mitochondrial matrix.

It is the only enzyme that can use either NAD+ or NADP+ as the acceptor of reducing

equivalents.

Combine action of an aminotransferase and glutamate dehydrogenase referred to as

transdeamination.

Ammonia transport in the form of glutamine

Glutamine synthetase

Excess ammonia is added to

glutamate to form glutamine.

Glutamine enters the liver and NH4+

is liberated in mitochondria by the

enzyme glutaminase.

Ammonia is remove by urea

synthesis.

Relationship between glutamate, glutamine

and a-ketoglutarate

a-ketoglutarate glutamate glutamine

NH3

NH3

NH3

NH3

glutamate + NAD+ + H2O a-ketoglutarate NH3 + + NADH

glutamate NH3 + glutamine

ATP ADP

glutamine H2O + glutamate NH3 +

A. Glutamate dehydrogenase

B. Glutamine synthetase (liver)

C. Glutaminase (kidney)

From transamination

reactions

To urea cycle

Oxidative deamination

Amino acids FMN H2O + +

a-keto acids FMNH2 NH3

L-amino acid oxidase

A. Oxidative deamination

FMN H2O2 H2O O2 +

+ +

O2 catalse

B. Nonoxidative deamination

serine

pyruvate

threonine

a-ketoglutate NH3 +

+

NH3

Serin-threonin dehydratase

•L-amino acid oxidase produces

ammonia and a-keto acid directly,

using FMN as cofactor.

•The reduced form of flavin must be

regenerated by O2 molecule.

•This reaction produces H2O2

molecule which is decompensated by

catalase.

Is possible only for hydroxy amino acids

Amino acid metabolism and central

metabolic pathways

20 amino acids are converted

to 7 products:

pyruvate

acetyl-CoA

acetoacetate

a-ketoglutarate

succynyl-CoA

oxalacetate

fumarate

Glucogenic Amino Acids

formed: a-ketoglutarate, pyruvate,

oxaloacetate, fumarate, or succinyl-CoA

Aspartate

Asparagine

Arginine

Phenylalanine

Tyrosine

Isoleucine

Methionine

Valine

Glutamine

Glutamate

Proline

Histidine

Alanine

Serine

Cysteine

Glycine

Threonine

Tryptophan

Ketogenic Amino Acids

formed acetyl CoA or acetoacetate

Lysine

Leucine

Both glucogenic and ketogenic amino

acids

formed: a-ketoglutarate, pyruvate,

oxaloacetate, fumarate, or succinyl-CoA in

addition to acetyl CoA or acetoacetate

Isoleucine

Threonine

Tryptophan

Phenylalanine

Tyrosine

Alanine

Serine

Cysteine

Threonine

The C3 family: alanine, serine, cysteine and

threonine are converted to pyruvate

Pyruvate

The C4 family: aspartate and asparagine are

converted into oxalacetate

Aspartic acid Asparagine

Oxalacetate

The C5 family: several amino acids are converted into

a-ketoglutarate through glutamate

Glutamine

Proline

Histidine

Arginine

a-ketoglutarate

Interconversion of amino acids and intermediates of

carbohydrate metabolism and Krebs cycle

Metabolism of some selected

amino acids

Serine biosynthesis from glycolytic

intermediate 3-phosphoglycerate

Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html

Glycine biosynthesis from serine

Reaction involves the transfer of the hydroxymethyl group from serine to the cofactor

tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF.


Glycine oxidation to CO2

Glycine produced from serine or from the diet can also be oxidized by glycine

decarboxylase (also referred to as the glycine cleavage complex, GCC) to yield a

second equivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and

CO2.


The sulfur for cysteine synthesis comes from the essential amino acid

methionine.

SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the

conversion of norepinephrine to epinenephrine).

Cysteine and methionine are metabolically

related

Condensation of ATP and methionine

yield S-adenosylmethionine (SAM)

SAM

Cysteine synthesis


1. Conversion of SAM to

homocysteine.

2. Condensation of

homocysteine with serine to

cystathione.

3. Cystathione is cleavaged to

cysteine.

Conversion of homocysteine back to Met. N5-

methyl-THF is donor of methyl group.

*

*folate + vit B12

Genetic defects for both the synthase and the lyase.

Missing or impaired cystathionine synthase leads to homocystinuria.

High concentration of homocysteine and methionine in the urine.

Homocysteine is highly reactive molecule.

Disease is often associated with mental retardation, multisystemic

disorder of connective tissue, muscle, CNS, and cardiovascular

system.

Homocystinuria

Biosynthesis of Tyrosine from Phenylalanine

Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is

incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the

tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced

state by the NADH-dependent enzyme dihydropteridine reductase

Hyperphenylalaninemia - complete deficiency of phenylalanine

hydroxylase (plasma level of Phe raises from normal 0.5 to 2 mg/dL to

more than 20 mg/dL).

The mental retardation is caused by the accumulation of

phenylalanine, which becomes a major donor of amino groups in

aminotransferase activity and depletes neural tissue of α-ketoglutarate.

Absence of α-ketoglutarate in the brain shuts down the TCA cycle and

the associated production of aerobic energy, which is essential to

normal brain development.

Newborns are routinelly tested for blood concentration of Phe.

The diet with low-phenylalanine diet.

Phenylketonuria

valine isoleucine leucine

a-ketoglutarate glutamate (transamination)

a-ketoisovalerate a-keto-b-methylbutyrate a-ketoisokaproate

oxidative decarboxylation

Dehydrogenase of a-keto acids* CO2

NAD+

NADH + H+

isobutyryl CoA a-methylbutyryl CoA isovaleryl CoA

Dehydrogenation etc., similar to fatty acid b-oxidation

propionyl CoA acetyl CoA

acetoacetate

acetyl CoA

propionyl CoA + +

Catabolism of branched amino acids

Branched-chain aminoaciduria

Disease also called Maple Syrup Urine Disease (MSUD) (because

of the characteristic odor of the urine in affected individuals).

Deficiency in an enzyme, branched-chain α-keto acid

dehydrogenase leads to an accumulation of three branched-

chain amino acids and their corresponding branched-chain α-keto

acids which are excreted in the urine.

There is only one dehydrogenase enzyme for all three amino

acids.

Mental retardation in these cases is extensive.

Histidine Metabolism:

Histamine Formation

N

NH

CH2CHCO2-

NH3

+

N

NH

CH2CH2NH2

Histidine Histamine

Histidine

decarboxylase

CO2

Histamine:

Synthesized in and released by mast cells

Mediator of allergic response: vasodilation,

bronchoconstriction

Tryptophan catabolism

Tryptophan has complex catabolic pathway:

1. the indol ring is ketogenic

2. the side chain forms the glucogenic products

Kynurenate and xanthurenate are excrete in the urine.

Enzymes which metabolised amino acides

containe vitamines as cofactors

THIAMINE B1 (thiamine diphosphate)

oxidative decarboxylation of a-ketoacids

RIBOFLAVIN B2 (flavin mononucleotide FMN, flavin adenine dinucleotide FAD)

oxidses of a-aminoacids

NIACIN B3 – nicotinic acid (nikotinamide adenine dinucleotide NAD+

nikotinamide adenine dinukleotide phosphate NADP+)

dehydrogenases, reductase

PYRIDOXIN B6 (pyridoxalphosphate)

transamination reaction and decarboxylation

FOLIC ACID (tetrahydropholate)

Meny enzymes of amino acid metabolism

http://themedicalbiochemistrypage.org/amino-acid-metabolism.html

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