AMINO ACID METABOLISM

Jana NovotnáDepartment of the Medical Chemistry and Biochemistry

The 2nd Faculty of Medicine, Charles Univ.

Amino acid structure

The 20 common amino acids of proteins

Metabolic relationship of amino acids

BODY PROTEINS

Proteosynthesis Degradation

AMINO ACIDSDIETARYPROTEINS

GLYCOLYSISKREBS CYCLE

Digestion

Transa

mination

NONPROTEINDERIVATIVESPorphyrinsPurinesPyrimidinesNeurotransmittersHormonesKomplex lipidsAminosugars

UREA NH3Con

vers

ion

(Carb

on

skele

ton

)

250 – 300 g/day

ACETYL CoAGLUCOSE CO2 KETONBODIES

Endopeptidases – hydrolyse the peptide bond inside a chain: pepsin, trypsin, chymotrypsinExopeptidases – split the peptide bond at the end of a protein molecule: aminopeptidase, carboxypeptidasesDipeptidases

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 -keto acids by transamination and subsequent reactions.

C

O

R COO-

+ NH4+

deamination

transamination C

O

R COO-

CH

NH2

R COO-

CH

NH2

R COO-

oxidativedecarboxylation

CH2

NH3+

R CO2+

General reactions of amino acid catabolism

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 keto acid2 amino acid2 +-keto acid1

NH3+

-O2CCH 2CH2CHCO 2-

Glutamate

OR-CCO 2

-+

O-O2CCH 2CH2CCO 2

-

-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 -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--ketoglutarate transferase ALT(also called glutamate-pyruvate transaminase – GPT)

Aspartate--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 -ketoglutarate

-ketoglutarate glutamate glutamine

NH3

NH3

NH3

NH3

glutamate + NAD+ + H2O -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+ +

keto acids FMNH2 NH3

L-amino acid oxidase

A. Oxidative deamination

FMN H2O2

H2O O2+

+ +

O2

catalse

B. Nonoxidative deamination

serine

pyruvate

threonine

-ketoglutateNH3+

+

NH3

Serin-threonin dehydratase

•L-amino acid oxidase produces

ammonia and -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

-ketoglutarate

succynyl-CoA

oxalacetate

fumarate

Glucogenic Amino Acids

formed: -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: -ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-CoA in

addition to acetyl CoA or acetoacetate

IsoleucineThreonineTryptophanPhenylalanineTyrosine

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 -ketoglutarate through glutamate

Glutamine

Proline

Histidine

Arginine

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.

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

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.

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

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

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

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

-ketoglutarate glutamate (transamination)

-ketoisovalerate -keto--methylbutyrate -ketoisokaproate

oxidative decarboxylationDehydrogenase of -keto acids*

CO2

NAD+

NADH + H+

isobutyryl CoA -methylbutyryl CoA isovaleryl CoA

Dehydrogenation etc., similar to fatty acid -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

Histidinedecarboxylase

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 ketogenic2. the side chain forms the glucogenic productsKynurenate and xanthurenate are excrete in the urine.

Enzymes which metabolised amino acides containe vitamines as cofactors

THIAMINE B1 (thiamine diphosphate) oxidative decarboxylation of -ketoacids

RIBOFLAVIN B2 (flavin mononucleotide FMN, flavin adenine dinucleotide FAD)oxidses of -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

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