FUNDAMENTALS OF FOOD TECHNOLOGY (1+1)

Protein metabolism

The metabolism of proteins can be conveniently discussed under the following heads.

Breakdown and synthesis of tissue proteins

• Nitrogen balance
• Oxidation of amino acids

1)Breakdown and synthesis of tissue proteins
Recent studies have shown that breakdown and synthesis of tissue proteins proceed simultaneously. A part of the tissue protein broken down continuously is replaced by the formation of new tissue proteins from the amino acids supplied by the diet. The breakdown of tissue protein is ‘catabolism’ and the formation of new tissue proteins is called ‘anabolism’

2)Nitrogen balance
When a subject is on a protein free diet, the tissue proteins are broken down and the resulting amino acids are oxidized and the nitrogen is excreted in urine. The nitrogen lost in urine and feces on a protein free diet is called “endogenous nitrogen”, it is derived from the body. When protein is taken in the diet, new tissue proteins are formed. When the nitrogen intake equals the nitrogen lost from the body through urine and faces, the body is said to be in a state of nitrogen equilibrium. When the nitrogen lost from the body is less than that of the nitrogen intake, the body is in ‘positive’ nitrogen balance. In this case the nitrogen is retained to form new tissues, as for example during growth of the body. If the nitrogen lost from the body is higher than the nitrogen intake, the body is said to be in the ‘negative nitrogen balance’. Under the above conditions the body proteins are slowly depleted. Negative nitrogen balances occur in under nutrition, fever, starvation etc.

3)Oxidation of amino acids
The amino acids not utilised for the formation of tissue proteins are oxidised by enzymes in the liver as indicated below


Amino acid -----------> keto acids + NH3
oxidation

The ammonia thus formed is converted into urea in the liver and excreted in the urine. The keto acids are oxidized to yield energy.