12 aminoacidmetab (9)

PRPTEIN METABOLISM

2

Dynamics of Protein

And Amino Acid Metabolism

Dietary Proteins Digestion to Amino Acids

Transport in Blood to Cells

Protein Synthesis Functional Proteins

Protein Degradation In

Proteasomes Following

Tagging With Ubiquitin

Amino Acids

Metabolites

3

Lumen

Amino Acids Oligopeptides

Intestinal Absorption

Oligopeptides

Amino Acids

Peptidases

Blood

Transport

Protein

Amino acid structure

The 20 common amino acids of proteins

AMINO ACID METABOLISM

BODY PROTEINS

Proteosynthesis Degradation

AMINO ACIDSDIETARYPROTEINS

GLYCOLYSISKREBS CYCLE

NONPROTEIN

DERIVATIVESPorphyrins

Purines

Pyrimidines

Neurotransmitters

Hormones

Komplex lipids

Aminosugars

UREA NH3

Con

vers

ion

(Carb

on s

keleto

n)

250 – 300

g/day

ACETYL CoAGLUCOSE CO2KETONBODIES

ENZYMES CLEAVING THE PEPTIDE CHAIN

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

Pepsin (pH 1.5 – 2.5) – peptide bond derived fromTyr, 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

Required in diet

Humans incapable of forming requisite

carbon skeleton

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

Not required in diet

Can be formed from a-keto acids by transamination and

subsequent reactions

Alanine

Asparagine

Aspartate

Glutamate

Glutamine

Glycine

Proline

Serine

Cysteine (from Met*)

Tyrosine (from Phe*)

* Essential amino acids

Glucogenic Amino Acids

Yield 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

Yield acetyl CoA or acetoacetate

Lysine

Leucine

R CH COOH

NH2

a-keto acidOxidativedeamination

Transaminatoin

Oxidativedecarboxylation

amin

a-keto acid

+ amino acid

General reactions of amino acids are transamination and deamination of a-amino group

Transamination – the transfer of the amino group to a suitable keto acid acceptor.

Oxidative deamination - the amino acid is converted into the corresponding keto acid by the

removal of the amine functional group as ammonia and the amine functional group is replaced by the

ketone group. The ammonia eventually goes into the urea cycle.

Oxidative decarboxylation – the formation of biogenic amines.

Transamination reaction

The first step in the catabolism of most amino acids is removal

of a-amino groups by enzymes aminotransferases or

transaminases

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 acids: transamination reactions

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

NH3+

-O2CCH 2CH2CHCO 2-

Glutamate

O

R-CCO 2-+

O-O2CCH 2CH2CCO 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

Schiff

base

(Aldimine)

(Ketimine)

(Aldimin)

(Ketimin)

Pyridoxal

phosphate

Pyridoxamine phosphate

Clinicaly important transaminases

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.

Aminotransferases are differ in their specificity for L-

amino acids.

The enzymes are named for the amino group donor

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

C. Glutaminase

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-ketoglutateNH3+

+

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

Both glucogenic and ketogenic amino acids

Yield a-ketoglutarate, pyruvate, oxaloacetate,

fumarate, or succinyl-CoA in addition to acetyl

CoA or acetoacetate

Isoleucine

Threonine

Tryptophan

Phenylalanine

Tyrosine

Metabolism of some amino acids

Glycine biosynthesis

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.

Serine biosynthesis from glycine

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

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

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 biosynthesis

Condensation of ATP and methionine yield

S-adenosylmethionine (SAM)

SAM

Utilization of methionine in the synthesis of cysteine

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

Missing or deficient phenylalanine hydroxylase results in

hyperphenylalaninemia.

Phenylketonuria is the most widely recognized

hyperphenylalaninemia (and most severe) It is the genetic

disease.

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.

Phenylketonuria

Enzymes which metabolised amino acides containe vitamines as cofactors

Vater soluble vitamins B

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

Nitrogenous derivatives of amino acids

Glycine

heme, purine, creatine, conjugation of bile acids

Histidine

histamine

Ornithine a arginin

creatine, polyamines (spermidine, spermine)

Tryptophan

serotonine (melatonine)

Tyrosine

Epinephrine, norepinephrine

Glutamic acid

g-aminobutyric acid (GABA)