department of the medical chemistry and clinical

Amino acid metabolism I

Jana Novotná, Bruno Sopko

Department of the Medical Chemistry and Clinical Biochemistry

The 2nd Faculty of Medicine, Charles Univ.

Metabolic relationship of amino acids

Body proteins

Proteosynthesis Degradation

Amino acid

poolDietary

proteins

NONPROTEIN

DERIVATIVESPorphyrins

Purines

Pyrimidines

Neurotransmitters

Hormones

Komplex lipids

Aminosugars

UREA NH3

Carbon skeleton

conversion

250 – 300

g/day

Acetyl CoACarbohydrates

LipidsCO2

H2O

Glycolysis

Krebs cycle

Conversion to

Ketonbodies

http://pharmaxchange.info/press/2013/08/metabolism-of-amino-acids-%E2%80%93-bimolecular-ping-pong-mechanism-of-transamination/

Digestive tract:

Endopeptidases – hydrolysis of peptide bond inside a polypeptide chain:

pepsin (stomach), trypsin, chymotrypsin, elastase (pancreas)

Exopeptidases – split the peptide bond at the end of a protein molecule:

aminopeptidase, carboxypeptidases, dipeptidases (small intestine)

Hydrolysis of proteins polypeptides oligopeptides amino acids

intestinal lumen transport to target tissues

Pepsin (pH 1.5 – 2.5) – hydrolysis of peptide bond before Tyr, Phe and

between Leu and Glu.

Trypsin (pH 7.5 – 8.5) – peptide bond after Lys a Arg.

Chymotrypsin (pH 7.5 – 8.5) – peptide bond after Trp, Phe,Tyr, Met, Leu.

Pancreatic elastase (pH 7.5 – 8.5) - peptide bond after Ala, Gly and Ser

Degradation of amino acids intracellularly the first step is deamination,

transamination, oxidative decarboxylation

Enzymes cleaving the peptide bond

Absorption of amino acids

• Absorption from the lumen of small

intestine by transepitelial transport

• Semispecific Na+-dependent

transport system

• Na+-dependent carriers transport

both Na+ and an amino acid.

• At least six different Na+-dependent

carriers:

- neutral AA

- proline and hydroxyproline

- acidic AA

- basic AA (Lys, Arg) and cistine

Clinical note:

Genetically determined defect in the transport of amino acids across

the brush border membranes of cells in both small intestine and

renal tubules

Cystinuria – AR disease, caused by mutation in two genes for

transporter proteins in the kidney proper reabsorption of basic, or

positively charged, amino acids (Lys, Arg and ornithine) and

cysteine into bloodstream is prevent Cys is oxidized to insoluble

cystine formation of kidney stones renal colic.

Hartnup disease – relatively rare AR disease – defect in tranport of

neutral AA including essential (Ile, Leu, Val, Phe, Thr, Trp -

availability of essential AA may cause a variety clinical disorders

The urine of newborns is routinely screening.

g-Glutamyl cycle and amino acid transport

▪

▪ Gamma-glutamyl transferase

(gamma-glutamyl transpeptidase,

GGT)

▪ Found in many tissues, mainly in

the liver.

▪ Diagnostic marker for liver

disease - elevations in GGT in

patients with chronic viral hepatitis

infections.

▪Transport of AA across cell

membrane by reacting with

glutathion to for g-glutamyl amino

acid

▪ AA is released into the cell.

▪ Glutathion is resinthesized.

Transamination - exchange of NH2 group with C=O

General reactions of amino acid catabolism


Deamination


Decarboxylation

Decarboxylation of AA gives amines having a variety of functions.

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)

Active metabolic form of vitamin B6

Mechanism of transamination reaction: PLP complex with enzyme accept

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

group to an a-keto acid.

All amino acids except threonine, lysine, and

proline can be transaminated.

Transaminases are differ in their specificity for

individual L-a-amino acid.

The enzymes are named for the amino group donor.

Clinicaly important transaminases

ALT

Alanine transaminase ALT(previously called serum glutamate-pyruvate transaminase – SGPT)

Predominantly found in the liver.

Important in the diagnosis of liver (viral hepatitis drug toxicity), ALT is a

more specific indicator of liver inflammation than AST.

Aspartate transaminase AST(previously called serum glutamate-oxaloacetate transaminase – SGOT).

- Found in the liver, heart, skeletal muscles, kidneys, brain, and red blood

cells.

- Elevated in liver diseases, myocardial infarction, acute pancreatitis, acute

hemolytic anemia, severe burns, acute renal diseases, musculoskeletal

diseases, and trauma (in 1954 defined as a biochemical marker for the

diagnosis of acute myocardial infarction)

Deamination

Amino acids FMN H2O+ +

a-keto acids FMNH2 NH3

L-a-amino acid oxidase

A. Oxidative deamination

FMN

H2O2 H2O + O2

+ +

O2

catalse

B. Nonoxidative deamination

serine

pyruvate

threonine

a-ketobutyrate

+ +

Serin-threonin dehydratase

• L-a-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.

Reaction is possible only for hydroxy amino acids

NH3 + H2O NH3 + H2O

Decarboxylation

• process is catalysed by enzymes decarboxylase – cofaktor is pyridoxalphosphate

• R-CHNH2-COOH R-CH2NH2 + CO2

• takes place only in small quantities

• primary amines

• biologically active amines

• hormones (neurotransmitters, coenzymes)

Synthesis of non-essential amino acids

Overview of the synthesis of non-

essential amino acids

The carbon of 10 AA may be

produced from glucose through

intermediates of glycolysis or the

TCA cycle.

Tyrosine from phenylalanine.

The sulphur of cysteine – from

methionine.

Amino acids derived from

intermediates of glycolysis

The major pathways for serine synthesis from

glucose and serine degradation

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.


Tetrahydrofolate acts as a carrier of reactive

single C units

Copy from: http://www.chembio.uoguelph.ca/educmat/chm452/lectur25.htm

Serine glycine – formation of N5,N10-methylen THF

Glycine CO2 - formation of N5,N10-methylen THF

Homocysteine methionine – donor of C is N5-methyl

THF

Histidine degradation – formation of N5-formiminoTHF;

N5,N10-metnhenyl a N10-formyl THF

Tryprophane degradation – formation of N10-formyl THF

Metabolism of glycine

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

Homocystinuria 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.

Clinical note

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

Amino acid degradation

Degradation of AA

20 amino acids are converted to

7 products:

➢ pyruvate

➢ acetyl-CoA

➢ acetoacetate

➢ a-ketoglutarate

➢ succinyl-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

Amino acids that form acetyl-CoA and

acetoacetate

Amino acids related through glutamate

Synthesis and degradation

of proline

Histidine degradation

Amino acids that form

succinyl-CoA

Amino acids related to oxalacetate

Aspartate and asparagine

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

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+ +

Degradation of branched amino acids

Branched-chain aminoaciduriaDisease 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.

Clinical note

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

Tetrahydrobiopterin as a cofactor of hydroxylases

Dihydrobiopterin

• Hyperphenylalaninemia, phenylketonuria -

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.

Clinical note

Tryptophan catabolism

Tryptophan has complex

catabolic pathway:

1. the indol ring is

ketogenic

2. the side chain

alanin

gluconeogenesis

Xanthurenic acid is

excrete in the urine.

Nicotinamide NAD and

NADP.

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-amino acids

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

Pictures were taken from textbooks:

Marks´ Basic Medical Biochemistry A Clinical Approach. Four edition M. Lieberman,

A.D. Marks ed., 2013.

Essentials of Medical Biochemistry With Clinical Cases. First edition. N.V. Bhagavan,

Chung-Eun Ha ed., 2011.

department of the medical chemistry and clinical

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