N H

Tryptophan

The second part of the reaction requires pyridoxal phosphate (Fig. 22-18). Indole formed in the first part is not released by the enzyme, but instead moves through a channel from the a-subunit active site to the ^-subunit active site, where it condenses with a Schiff base intermediate derived from serine and PLP. Intermediate channeling of this type may be a feature of the entire pathway from chorismate to tryptophan. Enzyme active sites catalyzing different steps (sometimes not sequential steps) of the pathway to tryptophan are found on single polypeptides in some species of fungi and bacte ria, but are separate proteins in others. In addition, the activity of some of these enzymes requires a noncova-lent association with other enzymes of the pathway. These observations suggest that all the pathway enzymes are components of a large, multienzyme complex in both prokaryotes and eukaryotes. Such complexes are generally not preserved intact when the enzymes are isolated using traditional biochemical methods, but evidence for the existence of multienzyme complexes is accumulating for this and a number of other metabolic pathways (p. 605).

COO"

COO"

ch2 II 2

HO H Chorismate

2 II

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