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Conversion of Oxidation Energy to Reductive Power: Ill Hydrolytic Energy Conservation in Amino Acid. Metabolism. GASPAR BANFALVI. Institute of ...
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Conversion of Oxidation Energy to Reductive Power: Ill Hydrolytic Energy Conservation in Amino Acid Metabolism GASPAR BANFALVI

Institute of Biochemistry Department I Semmelweis University Medical School 1444 Budapest 8, Hungary Introduction A common feature in the energetic coupling of anaerobic oxidation and hydrolysis of water is that hydrolytic oxidation/reduction reactions share common carbon intermediates. The carbon core comes from one of the major types of fuel molecules, namely from carbohydrates, fats and proteins. Hydrolytic oxidations in the Citrate Cycle and in 13-oxidation of fatty acids were reviewed earlier in this Journal. 1,2 This paper deals with hydrolytic oxidations involved in the metabolism of amino acids. Two types of hydrolytic reactions with respect to the degradation of amino acids can be distinguished: (1) the removal of an aamino group in transamination reactions and its subsequent conversion to ammonia in the glutamine dehydrogenase reaction leading to the formation of a-keto acids, and (2) the oxidation of carbon skeleton of amino acids results in 13-keto acids. The transformation of carbon oxidation energy to reducing equivalents increases the reductive power. Hydrolytic a-oxidation of amino acids The a-amino group of amino acids can be transferred to oxaloacetate to yield aspartate. This transaminated group enters urea cycle by the synthesis of argininosuccinate and leaves the cycle as one of the two amino groups of urea. The other amino group of urea molecule is also of transamination origin where the a-amino group of amino acids is transferrred to a-ketoglutarate to form glutamate, which in turn is oxidatively deaminated in the form of ammonium ion. Ammonia enters the urea cycle after conversion to carbamoyl phosphate (Fig 1). In the formation of ammonium, two enzymatic reactions are involved the first catalyzed by transaminases: a-Amino acid

c~-Keto acid

~-Ketoglutarate

Glutamate

producing an a-keto acid and glutamate. Ammonium is then released in the glutamate dehydrogenase reaction and a-ketoglutarate is regenerated for the next cycle of transamination: Glutamate

ct-Ketoglutarate

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