PROTEIN METABOLISM Prof.Dr.Fügen Aktan

Protein metabolism is a chemical procedure break down dietary or endogenous proteins into amino acids. peptide bond cleaving enzymes termed as proteinases and absorbed into the bloodstream. amino acids are then utilized to synthesize new N-structures or can be employed as an energy source.

DIGESTION AND ABSORPTION OF PROTEINS The proteases are secreted as inactive zymogens like pepsinogen, trypsinogen, chymotrypsinogen and proelatase There are two main classes of proteolytic digestive enzymes (proteases) Endopeptidases Exopeptidases carboxypeptidases, aminopeptidases, dipeptidases.

The digestion of proteins begins in the stomach. pepsin and hydrochloric acid pH of 1.5–3.5 denatures proteins

The food-gastric juice mixture (chyme) enters the small intestine, the pancreas releases sodium bicarbonate The small intestine releases secretin and CCK (Cholecystokinin),

The pancreas releases most of the digestive enzymes, Together, all of these enzymes break complex proteins into smaller individual amino acids then transported across the intestinal mucosa. These transporters bind sodium and then bind the amino acid to transport it across the membrane.

Nitrogen Pool The "nitrogen or amino acid pool" is a mixture of amino acids. Since proteins and amino acids are not stored in the body, there is a constant turnover of protein.

A nitrogen balance is very important for health. negative nitrogen balance positive nitrogen balance

General reactions of amino acids The general reactions of amino acids; deamination, transamination and decarboxylation The reactions of deamination and transamination the formation of keto acids

Transamination is catalyzed by transaminases or aminotransferases with pyridoxal phosphate functioning as coenzyme. transfer of an amino group from an amino acid to an a-keto acid, resulting in the formation of a new amino acid and keto acid Aspartate aminotransferase (AST) is also known as Glutamate - oxalo acetate transaminase (GOT) Alanine aminotransferase (ALT) is also known as Glutamate - pyruvate transaminase (GPT)

DEAMINATION OF AMINO ACIDS During the deamination, ammonia derived from the α-amino nitrogen of amino acids, that is highly toxic. deamination of glutamine in the liver releases ammonia, which is then converted to nontoxic urea. The deamination of amino acids leaves α-keto acid carbon skeletons. these α-keto acids are citric acid cycle intermediates. the ammonia is converted to urea.

The α -amino acids collected in the liver in the form of L-glutamate molecules. In hepatocytes, glutamate is transported from the cytosol into mitochondria, oxidative deamination catalyzed by L-glutamate dehydrogenase. The glutamate dehydrogenase is allosterically controlled by ATP and ADP. ATP acts as an inhibitor whereas ADP is an activator.

DECARBOXYATION OF AMINO ACIDS Decarboxylation is a chemical reaction that releases carbon dioxide (CO2). enzymes called decarboxylases or carboxy-lyases. the amino acids into the biogen amines and CO2.

Histidine Histamine Phenyl Alanine Dopamine Tryptophane Tryptamine Aspartate Beta Alanine Glutamate GABA

Transmethylation The transmission of methyl group from one compound to other is known as transmethylation by transmethylases. Transfer of methyl group generally includes methionine

Catabolism of the carbon skeleton of amino acids The carbon skeletons are recognized like α-keto acids. Synthesis of amino acids Glucogenic pathway Ketogenic pathway

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