Biosynthesis of Urea
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p282-283
2025-08-15
392
Urea biosynthesis occurs in four stages: (1) transamination, (2) oxidative deamination of glutamate, (3) ammonia trans port, and (4) reactions of the urea cycle (Figure 1). the expression in liver of the RNAs for all the enzymes of the urea cycle increases severalfold in starvation, probably secondary to enhanced protein degradation to provide energy.
Fig1. Overall flow of nitrogen in amino acid catabolism.
Transamination Transfers α-Amino Nitrogen to α-Ketoglutarate, Forming Glutamate
transamination reactions interconvert pairs of α-amino acids and α-keto acids (Figure 2). transamination reactions, which are freely reversible, also function in amino acid biosynthesis. All of the common amino acids except lysine, threonine, proline, and hydroxyproline participate in transamination. transamination is not restricted to α-amino groups. the δ-amino group of ornithine (but not the ε-amino group of lysine) readily undergoes transamination.
Fig2. Transamination. The reaction is freely reversible with an equilibrium constant close to unity.
Alanine-pyruvate aminotransferase (alanine aminotransferase, EC 2.6.1.2) and glutamate-α-ketoglutarate aminotransferase (glutamate aminotransferase, EC 2.6.1.1) catalyze the transfer of amino groups to pyruvate (forming alanine) or to α-ketoglutarate (forming glutamate).
Each aminotransferase is specific for one pair of substrates, but nonspecific for the other pair. Since alanine is also a substrate for glutamate aminotransferase, the α-amino nitrogen from all amino acids that undergo transamination can be concentrated in glutamate. this is important because l-glutamate is the only amino acid that undergoes oxidative deamination at an appreciable rate in mammalian tissues. the formation of ammonia from α-amino groups thus occurs mainly via the α-amino nitrogen of l-glutamate.
transamination occurs via a “ping-pong” mechanism characterized by the alternate addition of a substrate and release of a product (Figure 3). Following removal of its α-amino nitrogen by transamination, the remaining carbon “skeleton” of an amino acid is degraded by pathways discussed in Chapter 29.
Pyridoxal phosphate (PLP), a derivative of vitamin B6 , is present at the catalytic site of all aminotransferases, and plays a key role in catalysis. During transamination, PLP serves as a “carrier” of amino groups. An enzyme-bound Schiff base (Figure 28–11) is formed between the oxo group of enzyme bound PLP and the α-amino group of an α-amino acid. the Schiff base can rearrange in various ways. In transamination, rearrangement forms an α-keto acid and enzyme-bound pyridoxamine phosphate. As noted earlier, certain diseases are associated with elevated serum levels of aminotransferases.
Fig3. “Ping-pong” mechanism for transamination. E—CHO and E—CH2NH2 represent enzyme-bound pyridoxal phosphate and pyridoxamine phosphate, respectively. (Ala, alanine; Glu, glutamate; KG, α-ketoglutarate; Pyr, pyruvate.)
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