Amino acid metabolism Content 1 2 3 4

Amino acid metabolism

Content 1) 2) 3) 4) Introduction General Metabolism of amino acids Transamination Deamination

Introduction Ø Amino acid catabolism is part of the whole body catabolism Ø Nitrogen enters the body in a variety of compounds present in the food, the most important being amino acids present in the dietary protein. Ø Nitrogen leaves the body as urea, ammonia, and other products derived from amino acid metabolism

Fate of amino acids Body protein Dietary protein Synthesis of non essential amino acids Amino acid Pool Synthesis of tissue protein Synthesis of Biological compounds Synthesis of glucose, ketone bodies or fatty acids Oxidation in TCA cycle to yield energy and CO 2

General Metabolism of amino acids 1) Anabolic reactions where proteins are synthesized 2) Catabolic reactions where dietary proteins & body proteins are broken down to amino acids 3) Synthesis of specialized products such as heme, creatine, purines and pyrimidines

4)Transamination: amino group is removed to produce carbon skeleton(keto acid), the amino group is excreted as urea 5) Carbon skeleton is used for the synthesis of non essential amino acids 6) Carbon skeleton is also used for gluconeogenesis or for complete oxidation 7) Amino acids are used for, other minor metabolic functions like conjugation, methylation, amidation etc. .

Biosynthesis of urea Urea biosynthesis occurs in stages: (1)Transdeamination (Removal of α-amino group) by –a coupled process of transamination and deamination o Transamination forms Glutamate in peripheral cells o Deamination of glutamate forms ammonia in liver (2) Minor pathway of oxidative and non oxidative deamination produce ammonia in peripheral cells (3)Ammonia transport-Ammonia is transported to liver as glutamate, glutamine or alanine (4) Detoxification of released ammonia – Ammonia is detoxified by specific reactions (Urea cycle) forming urea in liver

Transamination • Transamination interconverts pairs of α-amino acids and α -keto acids. • During Transamination, the amino group of an amino acid (amino acid R 1) is transferred to a keto acid (keto acid R 2), this produces a new keto acid while from the original keto acid, a new amino acid is formed

Aminoacid + Keto acid New Keto acid + New Amino acid

Role of B 6 Phosphate (PLP) in transamination • The transfer of α-amino group from donor amino acid to Pyridoxal phosphate forms Pyridoxamine phosphate, and a keto acid. • The α-amino group is finally passed on to acceptor α-keto acid to form a new amino acid.

Eg: (1) ALT- Serum glutamate Alanine transferase • Reaction catalyzed can be represented as follows-

Eg: (2) AST-Serum glutamateoxaloacetate-aminotransferase (SGOT) Reaction catalyzed can be represented as follows-

Characteristics of Transamination 1) The general process of transamination is reversible , occurs in all the tissues 2) It is catalyzed by Transaminases, also called amino transferases that require PLP(derived from Vit B 6) as coenzyme 3) Specific transaminases exist for each pair of amino &keto acids. however only two make significant contribution for transamination , aspartate transaminase (AST/SGOT) and alanine transaminase(ALT/SGPT) 4) Synthesis of non essential amino acids: all non essential aa can be synthesized by body from keto acids available from other sources. Eg: pyruvate to alanine, oxaloacetate to aspartate. .

5) Interconversion of aa , so that equalization of quantity of non essential aa can be achieved 6) It diverts excess aa towards energy generation 7) The resultant α-Keto acid can be completely oxidized to provide energy, glucose, fats or ketone bodies depending upon the cellular requirement. 8) Amino acid undergo transamination to finally concentrate nitrogen in glutamate , which is the only aa that undergoes oxidative deamination to a significant extent to liberate free NH 3 for urea synthesis

9) All aa except lysine, threonine, proline & hydroxyproline participate in transamination 10) Transamination is not restricted to α -amino groups. The δ -amino group of ornithine readily undergoes transamination. 11) Serum transaminases are important for diagnostic and prognostic purpose

Clinical significance of Transaminases Serum aminotransferases such as serum glutamate-oxaloacetate-aminotransferase (SGOT) (also called Aspartate aminotransferase, AST) and serum glutamate -pyruvate aminotransferase (SGPT) (also called alanine transaminase, ALT) have been used as clinical markers of tissue damage, with increasing serum levels indicating an increased extent of damage.

AST-Serum glutamate-oxaloacetateaminotransferase (SGOT) • AST is found in the liver, cardiac muscle, skeletal muscle, kidneys, brain, pancreas, lungs, leukocytes, and erythrocytes • The enzyme is both cytoplasmic as well as mitochondrial in nature • Normal levels 8 -20 U/L

AST… • Elevated in Myocardial Infarction • Significantly elevated in liver disease, primary hepatoma • In alcoholic hepatitis: AST> ALT • ↑ AST can also be seen in Muscle disorders like muscular dystrophies

ALT • ALT is found primarily in the liver. • The normal serum activity ranges between Levels: 13 -35 U/L in males, 10 -30 U/L in females

ALT. . • Very high levels in acute hepatitis of toxic /viral origin( 300 -1000 U/L) • ALT elevation> AST elevation • Moderate increase in cirrhosis, hepatitis C , NASH( 50 -100 U/L)

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Deamination • Removal of amino group from the amino acid as NH 3 is termed as deamination

Histidase

Oxidative deamination • Libration of free ammonia from the amino group of amino acids coupled with oxidation. • Takes place mainly in liver & kidney • Purpose: 1) To provide ammonia for urea synthesis. 2) To provide alpha keto acid for energy generation

(Enzyme)

• The nitrogen atom that is transferred to αketoglutarate in the transamination reaction forming Glutamate, is converted into free ammonium ion by oxidative deamination • Glutamate occupies a central place in the amino acid metabolism • Basically it acts as a collector of amino group of the amino acids.

• This reaction is catalyzed by glutamate dehydrogenase(GDH) • This enzyme is unusual in being able to utilize either NAD+ or NADP+ • Glutamate dehydrogenase is Zinc containing complex enzyme (six identical subunits), located in mitochondria, as are some of the other enzymes required for the production of urea.

Regulation of Glutamate dehydrogenase(GDH) o The activity of GDH is allosterically regulated o Guanosine triphosphate (GTP) and adenosine triphosphate (ATP) are allosteric inhibitors, whereas Guanosine diphosphate (GDP) and adenosine diphosphate (ADP) are allosteric activators o Hence, a lowering of the energy charge (more of ADP and GDP) accelerates the oxidation of amino acids favoring formation of alpha keto glutarate that can be channeled towards TCA cycle for complete oxidation to provide energy o Steroid & thyroid hormones inhibit GDH

• Thus Transamination and deamination are coupled processes though they occur at distant places • Often termed as "transdeamination. "

Minor pathways of oxidative deamination Via, 1) L-amino acid oxidase 2) D-amino acid oxidase

q. L-amino acid oxidase: • acts on all amino acids except hydroxy amino acids & dicarboxylic amino acids • Uses FMN as co-enzyme

+ Release of ammonia

q D-amino acid oxidase: • Acts on glycine & D amino acid that may be formed by bacterial metabolism • Uses FAD as co-enzyme

Minor pathways of Non oxidative deamination • Some of the amino acids can be deaminated to liberate NH 3 without undergoing oxidation Types of reactions: 1) Dehydratases: requires PLP as coenzyme 2) Desulfhydrases: requires PLP as coenzyme 3) Histidase