Abstract
Determination of enzyme activity is crucial for discovery, research, and development in life sciences. The activity of enzymes is routinely determined using spectrophotometric assays that measure rates of substrate consumption or product formation. Though colorimetric-based detection systems are simple, rapid, and economical to perform, the majority of enzymes are unsuitable for this technique as their substrates/products do not absorb in the UV or visible range. This limitation can be addressed by the use of coupled-enzyme assays or artificial chromogenic substrates; however these approaches have their own drawbacks. Here, we describe a method based on the use of an isothermal titration calorimeter (ITC) to measure the heat produced or absorbed during any enzyme-catalyzed reaction. The concept of calorimetric enzyme assays was demonstrated for the determination of enzyme hexokinase activity, which cannot be monitored colorimetrically without first coupling it to another enzymatic reaction. The assay is suitable for incorporation into undergraduate laboratory classes, providing students with an appreciation for; the versatility and ease of use of ITC assays; ITC as a flexible generic method for exploring the functional characteristics of uncharacterized enzymes; an activity detection parameter suitable for enzymes that either have no straightforward colorimetric methods available or require the use of nonartificial chromogenic substrates.