Pyridoxal phosphate enzymes, atomic structures

An essential feature of such stabilization is that the atoms in the tt system are planar. The extended molecular orbital is constructed from atomic orbitals that are perpendicular to the plane. Thus, for the electrons involved in any bond making or breaking processes to be stabilized by delocalization, the bonds that are being made or broken must also be perpendicular to the plane. This criterion may be used by pyridoxal phosphate-utilizing enzymes in choosing which bond jp cleave, as may be seen when the intermediate 8.44 is redrawn so that it is perpendicular to the plane of the paper (structures 8.45 the pyridine ring is represented as a solid bar). In each case, the bond that is broken is the one at the top, so that the electrons may be fed into the tt system. 

Conformational changes have lost some of their mystical associations in recent years. From X-ray crystallography there is now an atomic-level resolution model for both the active, phosphorylated and inactive, dephosphorylated forms of the enzyme glycogen phosphorylase (11). From comparison of these structures, it is seen that the effect of phosphorylation of serine-14 of each subunit is to create an ordered helical conformation at each amino-terminus which in consequence binds more closely to the surface of the glycogen phosphorylase dimer. This produces rotation of each subunit about an axis perpendicular to the axis of symmetry of the dimer. This structural change clearly alters substrate binding at the catalytic site, even though the catalytic pyridoxal phosphates are located more than 30 A from the phosphoserine (12). 

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