Dehydroquinate synthetase

The interaction of phosphonate and homophos-phonate analogues of 3-deoxy-D-arabino heptulosonate 7-phosphate with 3-dehydroquinate synthetase from Escherichia coli. 

Dehydroquinate synthetase, the enzyme responsible for the cyclisation of DAHP (9) to give 3-dehydroquinate (10), the first cyclic intermediate in the shikimate pathway, was obtmned in partially purified form from Escherichia coli. The enzyme required Co and NAD" " (but not NADP" ) for full activity. No intermediates were isolable when these cofactors were removed but it was observed in a kinetic analysis of the enzymic transformation that the release of orthophosphate, the disappearance of DAHP and the formation of 3-dehydroquinate aU proceeded at the same rate. These observations indicated, it was suggested, that one enzyme was responsible for the whole sequence of reactions necessary for the convosion. On the basis of these observations, Sprinson and his collaborators formulated a working hypothesis for the steps involved in the cyclisation of the substrate DAHP and this is discussed in more detail later. 

Figure 13 Proposed reaction mechanism for S-dehydroquinate synthetase (D.B. Sprinson et al.)...

Occasionally, 100-200-fold purification of 5-dehydroquinate synthetase was achieved, but extracts having this or an even lower degree of purity were unstable, making a detailed study of the reaction difficult. With a prepara-... 

A working hypothesis to explain the activity of 5-dehydroquinate synthetase may be formulated as in Fig. 7. The hydroxyl group on C5 of 3-deoxy-D-orai tno-heptulosonic acid 7-phosphate is first oxidized by diphos-phop3uidine nucleotide, to facilitate the elimination of phosphate in the next step. The carbonyl group on C5 is then reduced (by the reduced di-phosphopjTidine nucleotide formed in the first reaction) to a hydroxyl group... 

In B. subtilis the single DAHPS enzyme is repressed most strongly by tyrosine, but maximal repression required the presence of both tyrosine and phenylalanine [121]. Tryptophan appeared to play no role in repression eontrol of synthesis of the enzyme. Nester et al. [121] also reported that a mixture of the three aromatic amino acids repressed synthesis of DAHPS, dehydroquinate synthetase, and shikimate kinase noncoordinately, but had no effect on the level of dehydroquinase. 

TDP-streptose Biosynthesis and 5-Dehydroquinic Acid Synthetase The question mark in the above title should indicate that the next two examples are only a tempting, though hopefully constructive, speculation to demonstrate that TDP-streptose biosynthesis as well as 5-dehydro-quinic synthetase are possible candidates for this group of enzymes with identical initiation of enzyme catalysis by hydrogen transfer to form enzyme-NADH. 

Dehydroquinic acid synthetase ° Predicted, but not experimentally verified. 5 5 47... 

The reactions of the shikimate pathway pose a number of interesting questions of both a mechanistic and stereochemical nature. Detailed studies have been made of the DAHP synthetase, 3-dehydroquinate dehydratase, 5-enolpyruvylshikimate-3-phosphate synthetase and chorismate synthetase reactions which have added further important knowledge to this area of molecular biology. Whilst these investigations have done nothing to detract from the important dictum that cells obey the laws of chemistry , they have nevertheless revealed some of the distinctive facets of enzyme chemistry and have highlighted some of the important differences between enzyme and relat chemically catalysed reactions. 

---