Dr. Samah Kotb Nasr Eldeen

C HAPTER 8

INTRODUCTION

 Amino acids are the building blocks of the proteins.  Twenty amino acids are commonly found in proteins.

5 Amino acids are essentially α-amino acids:

 They differ from each other in their side chains, or R groups, which vary in structure, size, and electric charge, and which influence the solubility of the amino acids in water.

 Arginine (Arg)  Threonine (Thr)  Lysine (Lys)  Valine (Val)  Phenylalanine (Phe)  Tryptophan (Trp)  Methionine (Met)  Histidine (His)  Leucine (Leu)  Isoleucine (Ile) 7

Catabolism of Amino Acids in Animal Tissue

 Only 10 % of the daily energy requirements of an average adult are provided by amino acid degradation.  This is not high when compared to monosaccharide (50%) and fatty acids (40%).

 Amino acids entering cells are primarily used by the cells for synthesize of various structurally or biologically important proteins such as enzymes and hormones.

 The remaining amounts of amino acids after sufficient proteins have been synthesized are now degraded to produce energy equal to 10 % of the daily requirements.

 20 amino acids catabolism give 10 % of daily energy requirements.

There are 2 stages of amino acid catabolism:-  STAGE 1: Removal of the primary amino groups by an enzymatically catalyzed reaction known as TRANSAMINATION catalyzed by enzymes known as TRANSAMINASES.

 STAGE 2: Degradation of the remaining keto-acids into acetyl-Co A or other TCA cycle intermediates.

The α amino group of the amino acid is enzymatically transferred to an acceptor molecule namely α-ketoglutarate leaving behind the corresponding keto acid. α-ketoglutarate becomes aminated and yields glutamate:-

α-ketoglutarate serves as the universal acceptor of amino groups during the transamination of all 20 amino acids. However a specific transaminase is required for each of the 20 amino acids. Eg:-

Glutamate undergoes oxidative deamination to remove the amino groups which appears as ammonia. The reaction is catalyzed by the enzyme Glutamate Dehydrogenase which uses NAD + as a coenzyme. (a mitochondrial enzyme).

 As a result of transamination 20 different ketoacids are formed.  Each of the 20 ketoacids is oxidized by a specific pathway and end up as either acetyl-CoA or one of the TCA cycle intermediates.  Some of these oxidative pathways are simple, whereas others are complex & multistep.

1.

2.

3.

4.