Pyridoxal phosphate—Electrophilic catalysis

Nucleophilic catalysis is a specific example of covalent catalysis the substrate is transiently modified by formation of a covalent bond with the catalyst to give a reactive intermediate. There are also many examples of electrophilic catalysis by covalent modification. It will be seen later that in the reactions of pyridoxal phosphate, Schiff base formation, and thiamine pyrophosphate, electrons are stabilized by delocalization. 

Effective concentration 65-72 entropy and 68-72 in general-acid-base catalysis 66 in nucleophilic catalysis 66 Elastase 26-30, 40 acylenzyme 27, 40 binding energies of subsites 356, 357 binding site 26-30 kinetic constants for peptide hydrolysis 357 specificity 27 Electrophiles 276 Electrophilic catalysis 61 metal ions 74-77 pyridoxal phosphate 79-82 Schiff bases 77-82 thiamine pyrophosphate 82-84 Electrostatic catalysis 61, 73, 74,498 Electrostatic effects on enzyme-substrate association rates 159-161... 

Fig. 8.13 Reactive sites of pyridoxal phosphate. The functional group of pyridoxal phosphate is a reactive aldehyde (shown in blue) that forms a covalent intermediate with amino groups of amino acids (a Schiff base). The positively charged pyridine ring is a strong electron-withdrawing group that can pull electrons into it (electrophilic catalysis).

Figure 8.51 Mechanisms of three types of bio-cataLyst that employ electrophilic catalysis as part of their mechanistic paths to successful bio-catalysis. All substrates are shown in red. Electrophilic catalysis is brought about by the use of pyridoxal phosphate, a natural cofactor for electrophilic catalysis operations.

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