Protein Metabolism
To understand the importance of protein metabolism in the growth
and maintenance of the body, consider that most protein functions are a result
of protein anabolism (synthesis) in cells. Hormones have a major role in the
regulation of protein metabolism. Anabolism is enhanced by the effect of growth
hormone from the pituitary gland and the male hormone testosterone. Hormones
affecting the catabolism or break down of proteins are the glucocorticoids that
are enhanced by adrenocorticotropic hormone known as ACTH; these hormones are
secreted from the adrenal cortex. This process releases proteins in the cells to
break down to amino acids and then the amino acids travel in the blood stream,
contributing to an available pool of amino acids.
The liver cells begin the process of catabolism through deamination. Deamination
results in the amino acid (NH2) group breaking off from an amino acid molecule,
resulting in one molecule each of ammonia (NH3) and a keto acid. Liver cells
convert most of the ammonia to urea, which is later excreted in urine. The keto
acid may enter the TCA cycle to be used for energy or, through Gluconeogenesis
and lipogenesis, be converted to glucose and fat.
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