Fungi pyridoxal-5 -phosphate

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 jS-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-... 

In most bacteria, pyridoxal phosphate is synthesized by condensation between 4-hydroxythreonine and the pentose sugar 1-deoxy-xylulose phosphate, formed by condensation between pyruvate and glyceraldehyde 3-phosphate. The details of the pathway are not known. In plants, fungi, and some bacteria, the precursor is glutamine again, the pathway is unknown (Drewke and Leismer, 2001 Gupta et al., 2001). 

Despite the importance of pyridoxal phosphate (PLP) in cellular metabolism, the pathways by which this cofactor is synthesized have been characterized relatively recentlyTwo mutually exclusive pathways have been described, with eubacteria using deoxyxylulose-5-phosphate (DXP) as a key intermediate (Scheme 20). More recent work has identified a second, DXP-independent pathway (Scheme 21), which appears to be used by a wide range of organisms and is distinguished by the involvement ofPLP synthase. In B. subtilis, fungi,Plasmodium falciparum and plants, this enzyme is a heterodimer composed of two subunits encoded... 

The carbon skeleton of homocysteine is derived from the corresponding hydroxy amino acid, homoserine. The hydroxyl of homoserine is acylated with either a succinyl (bacteria) or acetyl (yeast, fungi, plants) group derived from the corresponding coenzyme A derivative. The O-acyl substituent is then displaced by the thiol group of cysteine producing a mixed thioether, cystathionine. This in turn undergoes a pyridoxal phosphate dependent -elimination to homocysteine, pyruvate and... 

A more recently discovered second pathway starts from ribulose 5-phosphate (49) that is condensed with ammonia and glyceraldehyde phosphate or its isomerization product dihydroxy-acetone phosphate (50), which affords pyridoxal 5 -phosphate in a single enzyme-catalyzed reaction step (Fig. 5B) (28, 29). This pathway seems to be widely distributed it is used in plants (30) and has also been shown to proceed in fungi, archaea, and most eubacteria. 

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