Protein metabolism

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Protein Metabolism:

Protein Metabolism

Protein turnover:

Protein turnover Amino acids Protein Deposition Catabolism Gut Synthesis Degradation

Rate of protein synthesis:

Rate of protein synthesis K s (%/d) Tissue Pig Steer Liver Gut Muscle 23 45 5 21 39 2 K s = fraction of tissue protein synthesized per day

Protein synthesis:

Protein synthesis On-going, semicontinuous activity in all cells but rate varies greatly between tissues Rate is regulated by hormones and supply of amino acids and energy Energetically expensive requires about 5 ATP per one peptide bond Accounts for about 20% of whole-body energy expenditure

Protein degradation:

Protein degradation Also controlled by hormones and energy status Method to assist in metabolic control turns off enzymes

Changes in deposition:

Changes in deposition Synthesis Degradation Deposition No change No change No change

Protein synthesis and degradation:

Protein synthesis and degradation Synthesis must exceed degradation for net protein deposition or secretion Changes in deposition can be achieved by different combinations of changes in synthesis and degradation Allows for fine control of protein deposition

Protein synthesis and degradation:

Protein synthesis and degradation Other possible reasons for evolution of protein turnover include Allows post-translational conversion of inactive peptides to active forms (e.g., pepsinogen to pepsin) Minimizes possible negative consequences of translation errors

Protein catabolism:

Protein catabolism Some net catabolism of body proteins occurs at all times Expressed as urinary nitrogen excretion yields urea Minimal nitrogen excretion is termed endogenous urinary nitrogen (EUN)

Urinary nitrogen excretion:

Urinary nitrogen excretion Urine KIDNEY LIVER Urea Urea CO 2 Amino acids keto acids NH 3 Blood

Protein catabolism:

Protein catabolism Occurs when dietary protein exceeds requirements composition of absorbed amino acids is unbalanced gluconeogenesis is increased

Amino acid metabolism:

Amino acid metabolism Biosynthesis of nonessential amino acids Catabolism Conversion to glucose or fat Urea cycle

Biosynthesis of nonessential amino acids:

Biosynthesis of nonessential amino acids Transamination reactions allow extensive interconversion between nonessential amino acids requires vitamin B 6 as a coenzyme

Biosynthesis of nonessential amino acids:

Biosynthesis of nonessential amino acids Tyrosine From phenylalanine (PKU; 1 in 15,000) hydroxylation of phenylalanine important in adrenaline, noradrenaline, thyroxine and melanin synthesis

Biosynthesis of nonessential amino acids:

Biosynthesis of nonessential amino acids From intermediates of glycolysis from 3-phosphoglycerate and glycine serine phosphatidylserine from serine glycine high demand (10-50x greater than dietary intake) synthesis of purines, collagen, bile salts and glutathione cysteine S from methionine glutathione

Biosynthesis of nonessential amino acids:

Biosynthesis of nonessential amino acids From intermediates of TCA From oxaloacetate aspartate amino donor in urea synthesis pyrimidine and purine synthesis asparagine

Biosynthesis of nonessential amino acids:

Biosynthesis of nonessential amino acids From intermediates of TCA From α-ketoglutarate glutamate/glutamine purine and pyrimidine synthesis proline arginine intermediate in the urea cycle source of vasodilator, nitric oxide

Catabolism:

Catabolism Oxidative deamination Released NH 3 converted to urea Carbon skeleton can be oxidized (TCA cycle) used for glucose synthesis (gluconeogenesis) used for fat synthesis

Ketogenic amino acids:

Ketogenic amino acids Leucine and isoleucine converted to acetoacetate or acetyl CoA in liver fuel for other tissues

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