Heat of PPi Hydrolysis Varies Depending on the Enzyme Used

With yeast-soluble inorganic pyrophosphatase, the heat released during PP(i) hydrolysis was -6.3 kcal/mol regardless of the KCl concentration in the medium. With the membrane-bound pyrophosphatase of corn vacuoles, the heat released varies between -23.5 and -7.5 kcal/mol depending on the KCl concentration in the medium and whether or not a H(+) gradient is formed across the vacuole membranes. The data support the proposal that enzymes are able to handle the energy derived from phosphate compound hydrolysis in such a way as to determine the parcel that is used for work and the fraction that is converted into heat.