Human Biochemistry, Volume 1 - Murray R. 1993

Metabolism of Proteins and Amino Acids
Catabolism of Amino Acid Nitrogen
Biosynthesis of Urea

General Overview

A moderately active individual consuming approximately 300 g of CARBOHYDRATES, 100 g of fat, and 100 g of dietary protein daily must excrete about 16.5 g of nitrogen per day. Of this nitrogen, 95% is eliminated via the Kidneys and the remaining 5% in the feces. The primary pathway of nitrogen excretion in humans is as urea, which is synthesized in the Liver, subsequently enters the bloodstream, and is excreted by the kidneys. In humans following a typical Western diet, urea accounts for 80—90% of the excreted nitrogen.

Reactions of the Urea Cycle

The reactions of urea Biosynthesis and their corresponding intermediates are shown in Fig. 30.13; The formation of 1 mole of urea involves 1 mole of ammonium ions, 1 mole of carbon dioxide (activated by Mg²⁺ and ATP), and 1 mole of the alpha-amino nitrogen of aspartate. The synthesis consumes 3 moles of ATP (2 of which are converted to ADP and P_i, and the third to AMP and PP_i) and involves five sequential Enzymes that catalyze reactions 1—5 in Fig. 30.13. Of the 6 Amino Acids involved in urea synthesis, one (N-acetylglutamate) serves as an activator for one of the enzymes and does not participate in the chemical transformations. The remaining five—aspartate, Arginine, Ornithine, citrulline, and argininosuccinate—serve as carriers of the atoms that ultimately form the urea molecule. The first two of these Amino acids are protein constituents, whereas the other three (ornithine, citrulline, and argininosuccinate) are non-protein amino acids. The principal metabolic role of these latter Three amino acids in mammals is participation in urea synthesis. Note that urea formation is a partially cyclic process. Ornithine, consumed in reaction 2, is regenerated during reaction 5. Thus, there is neither a net loss nor accumulation of ornithine, citrulline, argininosuccinate, and arginine during urea synthesis; only the ammonium ion, CO₂, ATP, and aspartate are consumed.

Reaction 1: Synthesis of Carbamoyl Phosphate. The Condensation of an ammonium ion, carbon dioxide, and phosphate (derived from ATP) to yield carbamoyl phosphate is catalyzed by carbamoyl phosphate synthetase, an enzyme localized in the liver Cell/35.html">Mitochondria of all ureotelic organisms, including humans. The Hydrolysis of two ATP molecules during this reaction provides the energy required to form two covalent bonds: an amide bond and an anhydride bond in carbamoyl phosphate formed from carboxylic and phosphoric acids. This reaction requires Mg²⁺ ions as well as a dicarboxylate, preferably N-acetylglutamate. In the presence of these compounds, significant conformational changes occur in The Structure of carbamoyl phosphate synthetase, resulting in the exposure of certain sulfhydryl groups, the shielding of others, and an increased affinity of the enzyme for ATP.

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Fig. 30.13. Reactions and intermediates in urea biosynthesis. The amines directly involved in urea formation are highlighted. Mitochondrial enzymes are marked with an asterisk.

Reaction 2: Synthesis of citrulline. The transfer of the carbamoyl group from carbamoyl phosphate to ornithine, yielding citrulline and P_і, is catalyzed by hepatic mitochondrial L-ornithine transcarbamylase. This reaction is highly specific for ornithine, and its equilibrium lies far in the direction of citrulline synthesis.

Reaction 3: Synthesis of argininosuccinate. In the reaction catalyzed by argininosuccinate synthetase, the amino group of aspartate is added to citrulline. The reaction requires ATP, and its equilibrium heavily favors argininosuccinate synthesis.

Reaction 4: Cleavage of argininosuccinate into arginine and fumarate. The reversible cleavage of argininosuccinate into arginine and fumarate is catalyzed by argininosuccinase, an enzyme found in mammalian liver and kidneys. The reaction proceeds via a trans-elimination mechanism. The resulting fumarate can be converted into oxaloacetate through Reactions Catalyzed by fumarase and malate dehydrogenase; oxaloacetate is then converted to aspartate via Transamination.

Reaction 5: Cleavage of arginine into ornithine and urea. This reaction completes The urea cycle and regenerates ornithine, the substrate for Reaction 2. The hydrolytic Cleavage of the guanidino group of arginine is catalyzed by arginase, which is present in the liver of all ureotelic organisms. Small amounts of arginase are also found in the kidneys, Brain, Mammary Glands, Testes, and Skin. Mammalian liver arginase is activated by Ca²⁺ or Мn²⁺ ions. Ornithine and Lysine act as strong Competitive Inhibitors of the enzyme, competing with arginine.