ABSTRACT
Hydrolysis reactions of phosphate esters in DNA and RNA are promoted by Mg2+- dependent endonucleases, polymerases, integrases, and ribozymes. These enzymes often contain more than one functional active size. DNA polymerase can act as a 3 ,5 -exonuclease, a 5 ,3 -DNA nuclease, and a DNA polymerase. In the DNA poly merase from T. aquaticus the 3',5 '-exonuclease active site is separated from the polymerase active site by 33 A and from the 5 ,3 -nuclease active site by 70 A [150]. It is likely that two Mg2+ ions are required at each active site, but this view is controversial [151,152]. These enzymes are discussed in Chapter 4 [153]. Structures of these enzymes with Mn2+ in the active site have been determined for the Klenow fragment of DNA polymerase I from E. coli [154], rat DNA polymerase P [155], bacteriophage T4 DNA polymerase [156], human DNA polymerase [157,158], for the exonucleases from T. aquaticus [150] and E. coli [159], endonuclease FEN-1 [160], BamHI [161], and EcoRV [162], an avian sarcoma virus integrase [163], and Maloney murine leukemia virus reverse transcriptase [164]. The role of Mn2+ in the mechanism and specificity of endonucleases has been reviewed by Baldwin et al. [165].
2.2.3.1. Phosphoenolpyruvate carboxylase (EC 4.1.1.31) Phosphoenolpyruvate carboxylase (PEPC) catalyzes the carboxylation of phosphoe nolpyruvate to produce oxaloacetate (Scheme 1) and requires Mn2+ or Mg2* for its activity. Mn2+ in E. coli PEPC is bound to Glu-506 and Asp-543, and so the bonding is similar to that of Mn2+ in pyruvate kinase [166,167].
