Proline-protein interactions: protection of structural and functional integrity of M4 lactate dehydrogenase.

A well defined labile isozyme, rabbit muscle M4-lactate dehydrogenase was denatured under freeze-thaw, heat and GuHCl treatment in the presence and absence of proline, and the corresponding structural changes of the enzyme were monitored through fluorescence and CD spectral studies. The data reveal that proline confers protection to the structural integrity of the enzyme, thereby protecting its activity. This was attributed to its property of forming hydrophilic colloids in aqueous media with a hydrophobic backbone interacting with protein. Unlike other osmolytes, proline is proposed to act on the enzyme stability not only by inducing preferential hydration of proteins but also through the interactions of its multimeric hydrophobic backbone with the solvent-accessible hydrophobic regions of the enzyme.