Christopher Walsh
professor
Harvard Medical School

Diana , RI, US

Why is it that a graph representing enzyme activity for pH is drawn in most textbooks as a perfect bell curve graph vs. a temperature graph, which tends to show a quicker denaturing of enzyme in small increments of temperature change? I would like to know what is happening to the enzyme at the molecular level.

Christopher Walsh
professor,
Department of Biological Chemistry and Molecular Pharmacology,
Harvard Medical School
In principle, the pH dependence need not be a bell graph if more than one acid/base side chain on an enzyme participates in generating the catalytically active form. For example, if each amino acid side chain that participates as proton donor or acceptor in the transformation under examination has a different pK_a then the pH curve could deviate from a bell curve.

For protein denaturation (e.g., on heating), I think of it as a (highly) cooperative transition from the folded to the less folded/unfolded state. As the protein gains more kinetic energy, key hydrogen bonds could get disrupted, and hydrophobic core van der Waals interactions could become disorganized. Departure from the active, folded conformation of a protein with enzymatic activity is, like an avalanche, likely to become irreversible rapidly as local parts of structure become disorganized and lead to more global unfolding of regions of secondary structure.

11/07/11 08:21