Formation of sterols from squalene involves cyclization. First, a microsomal mixed-function oxidase (squalene epoxidase) forms squalene-2,3-oxide in the presence of NADPH, FAD and 02 (there is no requirement for cytochrome P450 in this reaction). The cyclization of the oxide to lanosterol then takes place by a concerted reaction without the formation of any stable intermediates. This conversion, which has been described as the most complex known enzyme-catalysed reaction, depends on a cyclase with a molecular mass of only 90 kDa. In plants and algae squalene-2,3-epoxide is cyclized to cycloar-tenol, which is the precursor of stigmasterol whereas lanosterol is the precursor of cholesterol and ergosterol (Fig. 7.21).
The conversion of lanosterol to cholesterol involves a 19-step reaction sequence catalysed by microsomal enzymes. The exact order of the reaction has not been delineated and, indeed, there may be more than one pathway. The main features of the transformation are the removal of three methyl groups, reduction of the 24(25)-double bond and isomerization of the 8(9)-double bond to position 5 in cholesterol.
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