Lecture 12 - Proteins 2

Lecture 12 - Proteins 2 - Reactions of Protein...

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Reactions of Protein Catabolism Amino Acid (AA) Carbon skeleton (alpha-keto acid) glucose acetyl CoA glycogen energy triglyceride NH 3 - ammonia NH 4 + - ammonium Urea cycle Urine *Constant turnover between protein synthesis and breakdown *During protein breakdown very efficient at AA reutilization ~80% AA reused ~20% degraded
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Fate of NH 3 (NH 4 + ) from AA catabolism Muscle (or other non-hepatic tissue) urea glutamine glutamine & alanine 3 differences between fasted and fed state: 1. Fed state involves formation of both glutamine and alanine. 2. Fed state involves both liver and kidneys. 3. Fed state involves excretion of NH3 as urea, whereas the fasted state involves secretion of ammonium. FED FED STARVATION STARVATION
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Why the difference between fed and fast? Metabolizing alpha-ketoacid involves production of bicarbonate (HCO3-) Bicarbonate is a weak base that reacts with a H+ (if this happens, no change in pH) However….the fed state encourages alkalosis! High consumption of AA causes system overload (H+ used up) and pH increases a bit (to ~7.8) So why don’t we die when we eat a high protein diet? Alpha-ketoacids enter Kreb’s cycle, where CO2 produced…however, at physiological pH it is actually HCO3-
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Why the difference between fed and fast? - In the fasted state, little protein is catabolized - Primary source of energy triglycerides - Breakdown of TG causes a production of acidic ketone bodies. So…the fasted state encourages acidosis (pH of around 7.0-7.1)! also known as ketoacidosis Products of TG breakdown (long hydrocarbon chains) are not very water soluble…so the liver converts these long hydrocarbons into small soluble ketone bodies. So why don’t we die when we fast?
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Important AA in nitrogen metabolism Glutamate Source of free NH3 End product of transamination reactions Alpha-ketoacid is alpha-ketoglutarate Aspartate Carries nitrogen into urea cycle Alpha-ketoacid is oxaloacetate Alanine Inter-organ nitrogen carrier (goes to liver) Alpha-ketoacid is pyruvate Glutamine Inter-organ nitrogen carrier (goes to liver & kidney) Source of free NH3
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4 reactions move nitrogen from catabolized protein between organs for excretion 1) TRANSAMINATION : transfer of an amino group to an AA carbon skeleton (i.e. alpha ketoacid) catalyzed by “aminotransferases” all AA undergo transamination except lysine, histidine, and threonine Amino Acid - Substrate Alpha-Ketoacid - Product Aminotransferase 1 2 Pyridoxal phosphate* Pyridoxamine Phosphate *Pyridoxal phosphate (active form of Vit B6) coenzyme that hold NH3 group during transfer 1 2
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TRANSAMINATION, cont. Bidirectional reaction Active in all tissues Always produces an AA (typically glutamate) and alpha-ketoacid At least 1 transaminase for each AA, with each using glutamate/α- ketoglutarate as one of the pairings Most abundant aminotransferases: Glutamate pyruvate transaminase (GPT) (also known as ALT)
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