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Protein Metabolism. Catabolism. Synthesis. Amino acids. Protein. Gut. Degradation. Deposition. Protein turnover. Protein synthesis. On-going, semicontinuous activity in all cells but rate varies greatly between tissues. Rate of protein synthesis. Protein synthesis.
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Catabolism Synthesis Amino acids Protein Gut Degradation Deposition Protein turnover
Protein synthesis • On-going, semicontinuous activity in all cells but rate varies greatly between tissues
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 • Also controlled by hormones and energy status • Method to assist in metabolic control • turns off enzymes
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
Catabolism Synthesis Amino acids Protein Gut Degradation Deposition Protein turnover
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 • 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 • 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 LIVER Amino acids keto acids NH3 CO2 Urea Blood KIDNEY Urea Urine
Protein catabolism • Occurs when • dietary protein exceeds requirements • composition of absorbed amino acids is unbalanced • gluconeogenesis is increased
Amino acid metabolism • Biosynthesis of nonessential amino acids • Catabolism • Conversion to glucose or fat • Urea cycle
Biosynthesis of nonessential amino acids • Transamination reactions • allow extensive interconversion between nonessential amino acids • requires vitamin B6 as a coenzyme
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 • 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 • From intermediates of TCA • From oxaloacetate • aspartate • amino donor in urea synthesis • pyrimidine and purine synthesis • asparagine
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 • Oxidative deamination • Released NH3 converted to urea • Carbon skeleton can be • oxidized (TCA cycle) • used for glucose synthesis (gluconeogenesis) • used for fat synthesis
Ketogenic amino acids • Leucine and isoleucine • converted to acetoacetate or acetyl CoA in liver • fuel for other tissues