Indole-3-glycerolphosphate

Fig. 8.16 Biosynthesis of L-Trp. Compounds ANT, anthrani-late CDRP, l-(o-carboxyphenylamino)-l-deoxyribulose-5-phosphate I3GP, indole-3-glycerolphosphate IND, indole PRAA, phosphoribosyl anthranilate PRPP, 5-phosphoribosyl-a-pyrophosphate.

Indole-3-glycerolphosphate + L-serine - L-tryptophan + D-glyceraldehyde-3-phosphate... 

Indole-3-glycerolphosphate - indole -I- 3-glyceraldehyde-3-phosphate (5) Indole + L-serine -> L-tryptophan (6)... 

In the presence of both the substrates, indole-3-glycerolphosphate and L-serine, tryptophan synthetase, 02)82 catalyses two parallel reactions. The a subunits in the complex carry out the cleavage of indole-3-glycerolphosphate releasing glyceralde-hyde-3-phosphate in the medium but the indole remains bound to the complex. Concomitantly, the /Sj subunits promote the conversion of serine into the aminoacrylate-pyridoxal-P species (Fig. 27, 1) which acts as a Michael acceptor for the indole producing the L-tryptophan-pyridoxal-P complex from which the amino acid is released by hydrolysis (Fig. 27). That the aforementioned molecular events can be described in such vivid detail is due to an ingenious series of partial reactions... 

Finally, the three-carbon side chain of the indole-3-glycerolphosphate is removed to produce indole itself and glyceraldehyde-3-phosphate (Scheme 12.25). The indole moves from one subunit of the enzyme, tryptophan synthase (EC 4.2.1.20), to another where it encounters the result of dehydration of a pyridoxal-bound serine (Ser, S) to which it adds. Then, hydrolytic cleavage of the resulting pyridoxal derivative of tryptophan (Trp, W) yields pyridoxal, ready for another reaction with, for example, serine (Ser, S) and tryptophan (Trp, W). 

Scheme 12.24. Chorismate is converted to anthranilate (and glutamate [Gin, E] and pyruvate) using anthranilate synthase (EC 4.1.3.27). Anthranilate and ribose-1,5-diphosphate, in the presence of anthranilate phosphoribosyltransferase (EC 2.4.2.18), produces N- 5-phospho-P-D-ribosyl) anthranilate. Finally, indole-3-glycerolphosphate is formed via l-(2-carboxyphenyl-amino)-l-deoxy-D-ribulose-5-phosphate with loss of carbon dioxide (CO2).

Scheme 13.59. A repetition of Scheme 12.24. A representation of a pathway from chorismate to indole-3-glycerolphosphate (on the way to tryptophan [Trp, W]).

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