Catabolism of amino acid

Catabolism of amino acid



       At the end of this lecture, student will be able to

      Explain the catabolism of amino acid

      Explain Transamination

      Describe Deamination

      Describe Decarboxylation

      Explain ammonia toxicity

Amino acid pool

       Protein turnover is the balance between protein synthesis and protein degradation

        More synthesis than breakdown indicates an anabolic state –positive nitrogen balance, more breakdown than synthesis indicates a catabolic state negative nitrogen balance


Catabolism of amino acid

Ø  Amino group of aminoacid – utilized for formation of urea – excretory end product of protein metabolism

Ø  Carbon skeleton of amino acid converted to keto acid which meet one of the following

       Utilized for energy

       Used for synthesis of glucose

       Formation of fat or ketone bodies

       Production of non-essential aminoacids



Ø  Transfer of amino group from amino acid to keto acid catalyzed by a group of enzymes transaminases or aminotransferases to form a new amino acid and keto acid – transamination

Ø  For this process pair of amino acids and a pair of keto acids are involved

Ø  It involves interconversion of a pair of amino acids and a pair of keto acids


Salient features of Transamination

       All transaminases requires pyridoxal phosphate (PLP) co-enzyme, which is obtained from vit-B6

       Specific transaminases exist for each pair of amino and keto acids

       No free NH3 liberated, only transfer of amino group occurs

       It is a reversible process

       Important for production of Non-essential amino acids

       Helps in energy generation

       All amino acids except lysine, threonine, proline and hydroxy proline participate in transamination

Mechanism of Transamination

       It occurs in two steps

  1. Transfer of amino group to coenzyme pyridoxal phosphate to form pyridoxamine phosphate

The amino group of pyridoxamine phosphate is then transfer to a keto acid to produce new amino acid and enzyme with PLP is regenerated

Glutamate + oxaloacetate   ———– > α-ketoglutarate + aspartate

pyridoxal phosphate



       The removal of amino group from amino acid as NH3Deamination

       Liberation of ammonia for urea cycle

       It is classified in to two types

                1. Oxidative deamination

                2. Non oxidative deamination

Transamination and deamination occurs simultaneously, involving glutamate as central molecule

Oxidative Deamination

       Liberation of free ammonia from the amino group of amino acids coupled with oxidation

       Takes place mostly in liver and kidney

       Purpose of oxidative deamination is to provide NH3 for urea synthesis and α-keto acids for a variety of reactions, including energy generation

Role of Glutamate Dehydrogenase (GDH):

       In the process of transamination, the amino groups of most amino acids are transferred to a-ketoglutarate to produce glutamate

       Thus, glutamate serves as a collection centre for amino groups in the biological system

       Glutamate rapidly undergoes oxidative deamination, catalysed by GDH to liberate ammonia

       Enzyme is unique and utilize either NAD+ or NADP+ as a co-enzyme

       Conversion of glutamate to α-ketoglutarate occurs through the formation of an intermediate, α-iminoglutarate

       GDH is involved in both catabolic and anabolic reactions

Regulation of GDH activity:

       GDH is a zinc containing mitochondrial enzyme

       GDH is controlled by allosteric regulation

       GTP and ATP inhibit GDH

       GDP and ADP activate GDH

       Steroid and thyroid hormones inhibit GDH

       After ingestion of a protein rich meal, liver glutamate level is elevated


Oxidative deamination by amino acid oxidase


       L-Amino acid oxidase and D-amino acid oxidase are flavoproteins, possessing FMN and FAD, respectively

       Act on corresponding amino acids( L or D) to produce α-keto acids and NH3

       In this reaction, oxygen is reduced to H2O2, which is later decomposed by catalase

       Activity of L-amino acid oxidase is much low while that of D-amino acid oxidase is high in tissues(mostly liver and kidney).

       L –Amino acid oxidase does not act on glycine and dicarboxylic acids

       This enzyme, due to its very low activity, does not appear to play any significant role in the amino acid metabolism

Non oxidative deamination

       Some of the amino acids can be deaminated to liberate NH3 without undergoing oxidation

a. Amino acid dehydrases:

       Serine, threonine and homoserine are the hydroxy amino acids

       They undergo non-oxidative deamination, catalysed by PLP-dependent dehydrases (dehydratase)


b. Amino acid desulfhydrases:

       The sulfur amino acids, namely cysteine and homocysteine undergo deamination coupled with desulfhydrationto give keto acids

c. Deamination of histidine :

       The enzyme histidase acts on histidine to liberate NH3 by a non-oxidative deamination process


       Tissues like liver and microorganisms of the intestinal tract contain enzymes called decarboxylase which require pyridoxal phosphate as coenzyme

       They remove CO2 from carboxylic group and convert aminoacid to its corresponding amine

       The physiologically active amines epinephrine, nor-epinephrine, dopamine, serotonin, α-amino butyrate and histamine are formed through decarboxylation of the corresponding precursor amino acids

Function of ammonia

       Ammonia is not just a waste product of nitrogen metabolism. lt is involved (directly or via glutamine) for the synthesis of many compounds in the body. These include nonessential amino acids, purines, pyrimidines, amino sugars, asparagine etc

        Ammonium ions (NHa*) are very important to maintain acid-base balance of the body

Disposal of ammonia

       The organisms, during the course of evolution, have developed different mechanisms for the disposal of ammonia from the body. The animals in this regard are of three different types

a. Ammoniotelic: The aquatic animals dispose of NH3 into the surrounding water

b. Uricotelic: Ammonia is converted mostly to uric acid e.g. reptiles and birds.

c. Ureotelic: The mammals including man convert NH3 to urea. Urea is a non-toxic and soluble compound, hence easily excreted

Toxicity of ammonia

       Even a marginal elevation in the blood ammonia concentration is harmful to the brain

       Only traces 10-20 mg/dl is present in blood

       When accumulates in the body, results in slurring of speech and blurring of the vision and causes tremors

       lt may lead to coma and finally death, if not corrected

       Hyperammonemia : Elevation in blood NH3 level

       lmpairment in urea synthesis due to a defect in any one of the five enzymes is described in urea synthesis

       Hyperammonemia leads to mental retardation.

       Acquired hyperammonemia may be due to hepatitis, alcoholism etc.


       Protein turnover is the balance between protein synthesis and protein degradation

       Transfer of amino group from amino acid to keto acid is known as transamination

       Removal of amino group from amino acid as NH3 is known as deamination

       All transamination required PLP co-enzyme

       Marginal elevation in blood ammonia concentration is harmful to the brain

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