Biosynthesis of Tetrapyrroles - download pdf or read online

By P.M. Jordan (Eds.)

ISBN-10: 0444803033

ISBN-13: 9780444803030

ISBN-10: 0444892850

ISBN-13: 9780444892850

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Protein chemistry has also been used to investigate the protein sequences derived from the E. coli hemB gene. The sequence of Echelard et al. [ 1551shows 9 cysteine and 2 tryptophan residues but the sequence of Li et al. [156] shows 6 cysteine and 1 tryptophan. Amino acid-specific cleavages of the purified E. coli 5-aminolaevulinic acid dehydratase at cysteine and tryptophan residues [ 1451 gave peptides which were consistent both in size and number with the latter sequence. 7. Order of binding the two substrates and the enzyme mechanism 5-Aminolaevulinic acid dehydratases are unusual in that they catalyse the condensation between two molecules of the same substrate and hence the order of substrate binding cannot be determined by classical steady state kinetics.

This is transformed into 5-aminolaevulinic acid by the transfer of the amino group at the C-4 position to the pyridoxal5’-phosphate to regenerate the pyridoxamine 5’-phosphate-enzyme complex. This mechanism explains why no amino donor other than the substrate is required. The likely involvement of the cyclic glutamate semialdehyde intermediate (HAT) as a substrate need not affect the above mechanism since the transaminase would catalyse ring opening as the first step in the mechanism. Such ring opening (mutarotation) occurs routinely in enzymic transformations involving glucose and other sugars.

The enzyme from R. sphaeroides has a M , of 240,000 Da and early reports suggested that it is made up of 6 identical subunits each of M, 39,500 Da [142]. More recent experiments have shown that this enzyme may in fact exist as an octamer [143]like the mammalian dehydratases. In the presence of potassium ions the R. sphaeriodes enzyme appears to aggregate to ‘octameric’ dimers, trimers and tetramers with an accompanying increase in the specific activity of the enzyme [144]. The dehydratase from R.

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Biosynthesis of Tetrapyrroles by P.M. Jordan (Eds.)

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