Coenzyme Reorientations in the Active Site

Reorientations of the coenzyme associated with the enzymatic catalytic cycle were first proposed by Ivanov and Karpeisky64 for aspartate transaminase on the basis of general topochemical considerations and studies of induced optical activity. Torchinsky and Koreneva65,66 found that induced optical activity of PLP bound to the transaminase is 

It has been found68-70 that the reaction of tryptophanase with the inhibitory amino acids, /3-phenyl-DL-serine (threo form), L-threonine and D-alanine, is accompanied by a manifold increase in the reduced LD, i.e. in the ratio of LD to absorbance (AA / A) in the 420-425 nm band (Fig. 9.12 Table 9.2). This band belongs to the protonated internal PLP-lysine 

The value of reduced LD depends on the degree of orientation (G) of the enzyme molecules in the gel slab and angle (a) between the direction of the transition dipole moment in the chromophore ring and the axis of orientation of the enzyme molecules in the gel67 7,)  

The value of reduced LD in the quinonoid band of the enzyme complexes with L-alanine and oxindolyl-L-alanine proved to be close to that in the absorption band of the external aldimine formed with L-threonine (Table 9.2). It does not follow from this coincidence that the external aldimine and quinonoid have a similar orientation in the active site. The two intermediates, in fact, differ in their orientation this is because the directions of the transition dipole moment in the pyridine ring of the aldimine and quinonoid are very different.72  

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