Bio Lab

Enzymes are biological catalysts capable of speeding up chemical reactions. One benefit of enzyme catalysts is that the cell can carry out complex chemical activities at a relatively low temperature.

Most enzymes are proteins and their 3-dimensional shape is important to their catalytic activity. Two specific regions on the enzyme structure play an important role in catalytic activity: the active site and the allosteric site. The active site is the area of the enzyme which binds to the substance(s) (substrate) and aids in the chemical reaction. The allosteric site is involved in forming the proper 3-dimensional shape when linked with specific cofactors. As a result of the unique characteristics of these sites, enzymes are highly specific in terms of the reactions they will catalyze and the condition under which they work best.

In biochemical reactions the enzyme, E, combines reversibly with its specific substrate, S, to form an enzyme-substrate complex, ES. One result of this temporary union is a reduction in the energy required to activate the reaction of the substrate molecule so that the products of the reaction, P, are formed.

This can be summarized in the equation:

E + S → ES → E + P

Note that the enzyme is not consumed in the reaction and can recycle to work with additional substrate molecules. Each enzyme is specific for a particular reaction because its amino acid sequence is unique which causes it to have a unique 3-dimensional structure. The active site is the portion of the enzyme that interacts with the substrate, so that any substance that blocks or changes the shape of the active site affects the activity of the enzyme.

In practice, this specificity permits one to mix a purified substrate with crude preparations of enzyme that might contain many other substances and obtain a quantitative assay (analysis) of the amount of enzyme present.

We will be working in this lab with a representative enzyme -- catalase. Catalase has a molecular weight of approximately 240,000 daltons and contains 4 polypeptide chains, each composed of more than 500 amino acid monomers. This enzyme occurs universally in aerobic organisms. One function of catalase within cells is to prevent the accumulation of toxic levels of hydrogen peroxide (H2O2) formed as a by-product of metabolic processes. Catalase might also take part in some of the many oxidation reactions going on in all cells.

The primary reaction catalyzed by catalase is the decomposition of H2O2 to form water and oxygen.

2 H2O2 → 2 H2O + O2 (gas)

In the absence of catalase, this reaction occurs spontaneously, but very slowly. Catalase speeds up the reaction considerably. In this experiment, a rate for this reaction will be determined.

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