Protein aspartate carbamoyl-transferase

Fig. 38. The zinc-thiolate cluster of the regulatory subunit of aspartate carbamoyl-transferase (Honzatko et al 1982). Coordinates are from the Brookhaven Protein Data Bank.

The binding of PALA to the R conformation of aspartate carbamoyl transferase. Arg 105 and His 134 are provided by one domain of a c subunit, and Arg 167 and Arg 229 by the other domain. Ser 80 and Lys 84 are part of a loop of protein from a different c subunit. The PALA is indicated by red. The wavy green lines indicate polypeptide backbone structure. 

The classic oligomeric protein, hemoglobin, is a dimer of dimers (Liddington et al., 1992). Other examples are octomeric mandelate recanase (Neidhart et al, 1991) and OePe dodecameric aspartate carbamoyl transferase (Stevens et al, 1990). 

Birds appear unable to synthesise any arginine via the urea cycle (see Section 5.4), which may be because of lack of carbamoyl phosphate synthetase I in mitochondria (Baker, 1991), and, as a result, the dietary requirement for arginine is higher than in growing mammals. However, they do appear to have a carbamoyl phosphate synthetase II in the cytosol (Maresh, Kwan Kalman, 1969). This may be part of the multienzyme protein G D (carbamoyl phosphate synthase-aspartate carbamoyl transferase-dihydroorotase), responsible for the biosynthesis of 3ihydroorotate, a pyrimidine precursor, but this is bound on the multienzyme protein, and it seems unlikely that it would be available for arginine biosynthesis (see Price Stevens, 1989). 

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