Protein metabolism & Protein denaturation PROTEINS • Proteins are polymers of amino acids which play a crucial role in biological processes. • Proteins have many important biological functions- Enzymes biological catalysts Antibodies defence proteins Transport proteins Regulatory proteins Structural proteins DIGESTION OF PROTEINS • The dietary proteins are denatured on cooking and therefore more easily to digested by a digestive enzymes. • All these enzymes are hydrolases in nature. • Proteolytic enzymes are secreted as inactive zymogens which are converted to their active form in the intestinal lumen. The proteolytic enzymes include: • Endopeptidases: They act on peptide bond inside the protein molecule, so that the protein becomes successively smaller and smaller units. This group includes pepsin, trypsin, chymotrypsin, and elastase. Exopeptidases: • This group acts at the peptide bond only at the end region of the chain. This includes carboxy peptidase acting on the peptide only at the carboxyl terminal end on the chain and amino peptidase, which acts on the peptide bond only at the amino terminal end of the chain. Absorption of amino acids The absorption of amino acids occurs mainly in the • small intestine. • It is an energy requiring process. • These transport systems are carrier mediated systems Over view of protein metabolism • The amino acids undergo certain common reactions like transamination followed by deamination for the liberation of ammonia. the amino group of the amino acids is’ utilized for the formation of urea which is an excretory end product of protein metabolism • The carbon skeleton of the amino acids is first converted to keto acids (by transamination) which meet one or more of the following fates. Utilized to generate energy Used for the synthesis of glucose Diverted for the formation of fat or ketone bodies. involved in the production of non-essential amino acids. General metabolism of amino acids: • Anabolic pathway • Catabolic pathway • Dietary proteins and body proteins are broken down to amino acids. This is called catabolic reactions • In transamination reaction, amino group of amino acid is removed to produce the carbon skeleton (keto acid). The amino group is excreted as urea . The carbon skeleton is used for synthesis of nonessential amino acids. It is also used for gluconeogenesis or for complete oxidation. Amino acids are used for synthesis of body proteins; this is anabolic reaction. Amino acid metabolism The break down of amino acid can incorporate three stage • Deamination – removal amino group which is either converted ammonia or transferred to become amino group of a glutamic acid molecule • Conversion of amino acid carbon skeleton (the alfa keto acid produced by the deamination) to citric acid cycle intermediate • Incorporation of ammonia to urea Transamination • transfer of an amino (- NH2) group from an amino acid to a keto acid is known as transamination. • This process involves the interconversion of a pair of amino acids and a pair of keto acids, catalyzed by a group of enzymes called transaminases (recently, aminotransferase. The predominant amino acid acceptor is the alfa ketoglutarate produced glutamic acid as the new amino acid Specific transaminases exist for each pair of amino and keto acids. However, only two namely, aspartate transaminase and alanine transaminase-make a significant contribution for transamination • Transamination is very important for the redistribution of amino groups and production of non’ essential amino acids, as per the requirement of the cell. lt involves both catabolism (degradation) and anabolism (synthesis) of amino acid • Transamination diverts the excess amino acids towards energy generation. • Each transaminase has specific amino acid substrate and most accept only alfa ketoglutarate or oxaloacetate as the keto acid substrate • Only glutamic acid and or aspartic acid is the major amino acid product of transamination • Glutamic acid and aspartic acid are converted by the reaction Transamination reaction does not result in any net deamination reaction Deamination • The removal of amino group from the amino acids as NH3 is deamination. • On the other hand, deamination results in the liberation of ammonia for urea synthesis. Simultaneously, the carbon skeleton of amino acids is converted to keto acids. Deamination may be either oxidative or non- oxidative. Oxidative deamination • Oxidative deamination is the liberation of free ammonia from the amino group of amino acids coupled with oxidation. This takes place mostly in liver and kidney. The purpose of oxidative deamination is to provide NH3 for urea synthesis and c-keto acids for a variety of reactions, including energy generation • Role of glutamate dehydrogenase : 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 glutamate dehydrogenase (GDH) to liberate ammonia Non oxidative deamination • Some of the amino acids can be deaminated to liberate NH3 without undergoing oxidation • Amino acid dehydrases : Serine, threonine and homoserine are the hydroxy amino acids. They undergo non-oxidative deamination catalysed by pyridoxal phosphate -dependent dehydrases (dehydratase Protein turn over – simultaneously protein synthesis and protein degradation is is known as protein turn over DECARBOXYLATION REACTION Removal of carboxylic group of amino acid with the formation of amines is called decarboxylation reaction Enzyme used for the reaction- decarboxylase pyridoxal phosphate example - Synthesis of dopamine and alfa gama amino butyric acid METABOLIC FATE OF AMINO ACID • Glucogenic – mostly glucogenic amino acid • Ketogenic- lysine and leucine • Glucogenic and ketogenic amino acid- phenyl alanine, tryptophan, isoleucine, tyrosine Functional properties of amino acid Optical isomerism – all amino acid except glycine (D and L isomerism) Acid and base or charge properties – amino acid act as acid and base (Cation at low Ph and anion at high Ph) Buffer action of amino acid- ampholyte act as buffer UV light absorption of amino acid - aromatic amino acid absorbed UV light Protein Denaturation MECHANISM OF PROTEIN DENATURATION CAUSES OF PROTEIN DENATURATION CHEMICAL AGENT detergents Acids and alkali Organic solvents Salts and heavy metals Alter Ph DENATURATION OF EGG PROTEINS .