DIRECT MEASUREMENT OF ENZYMATIC ACTIVITIES BY MICROCALORIMETRY: HIV PROTEASE

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A widely-applicable method for measuring enzymatic activity using micro-calorimetry is being developed. Chemical reactions are driven by a decrease in free energy, which is the sum of enthalpic and entropic contributions. The enthalpic contribution is characterized by heat. Using isothermal titration calorimetry, we measure the heat evolved during a reaction and express the results as reaction rates. Whereas classical enzyme assays calculate rate as a derivative of product appearance (or substrate depletion) with time, biocalorimetric assays measure reaction rates directly. Isothermal calorimetric analysis is a non-destructive (non-radioactive) technique that 1) is applicable to most biological reactions, 2) eliminates the requirement for coupling enzymes, 3) is useful in spectrophotometrically opaque solutions, and 4) permits rapid activity analysis using physiological substrates. We present data assaying enzymes from; E. Coli chaperonin GroEL, HIV-1 protease, F. Heparinium heparinase, and H. Pylorii urease.