Gibberellins (GAs) are biosynthesised from the diterpene ent ‐kaurene by a series of oxidative reactions catalysed by two classes of enzymes. The early steps, involving transformations of highly hydrophobic substrates, are carried out by membrane‐associated monooxygenases, probably involving cytochrome P450, whereas the later reactions are catalysed by soluble 2‐oxoglutarate‐dependent dioxygenases. Some reactions involving substrates, such as GA 12 and GA 12 ‐aldehyde, that have intermediate polarity are catalysed by enzymes in both classes. The monooxygenases and dioxygenases catalyse the same types of reactions: hydroxylation, desaturation, alcohol and aldehyde oxidation. For both enzyme classes, the oxidant is thought to be an oxyferryl species, depicted as Fe lv =O, that is derived from molecular oxygen by different mechanisms, the reducing power being supplied by NADPH in the case of cytochrome P450 monooxygenases and by the decarboxylation of 2‐oxoglutarate to succinate for the dioxygenases. The recent availability of cDNA clones for several of the dioxygenases and the ability to prepare active enzymes by heterologous expression of cDNAs in Escherichia coli have provided new opportunities for examining the function of these enzymes. They have relatively low substrate specificity and, in many cases, are multifunctional. Consequently, fewer enzymes than expected are required to produce the large number of GA structures encountered in higher plants. In the present review, the major oxygenases of GA biosynthesis are described and their reactions are discussed in an attempt to rationalise this multifunctionality.
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Peter Hedden (1997) studied this question.