Quaternary Structure and Conformation of Lactic Dehydrogenase and Glyceraldehyde-3-Phosphate Dehydrogenase

Abstract

Since X-ray and sequence analyses have been successfully applied to lactic dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase⁺⁾ during the last few years, the quaternary structure of the two enzymes seems to be well established The three-dimensional analysis of dog-fish muscle LDH (1) and preliminary results on lobster G-PD (2), as well as the primary structures of different forms of GPD (3) unequivocally prove the tetrameric state for both enzymes. GPD in ail different forms investigated so far seems to represent a homogeneous quaternary structure while LDH from different sources forms isoenzymes with homogeneous or heterogeneous quaternary structures (4,5), the different isoenzymes being indistinguishable in their particle weights (6,7) and to a certain degree also in their gross structure (8). These observations are found to be true for the multiple forms as well as for LDH’s from different sources. Proceeding from crystallographic or chemical analysis to the physico-chemical approach to the native state in solution the following problems have to be discussed:

• 1. homogeneity or heterogeneity of the enzymes under specified solvent conditions,

• 2. correlation of quaternary structure and enzymic activity,

• 3. correlation of dissociation-association and conformational stability, and

• 4. conditions of dissociation as criteria of interprotomer interactions.