Enzymes are specialized proteins, which accelerate or catalyze a biochemical reaction. Each enzyme catalyzes a specific reaction and is regulated by competitive and noncompetitive inhibitors and/or by allosteric molecules. Multiple enzymes can catalyze a series of consecutive reactions, known as pathways, to produce and/or break down complex biological molecules. Examples include amino acid synthesis and degradation, the coordinated reactions involved in protein synthesis, and the urea cycle. Problems with enzyme pathways can not only lead to disease but also offer the opportunity for disease treatment via medications, which target specific points in these pathways.
Enzymes bring together one or more molecules, called substrates, to form a resulting molecule called a product (Figure 5-1). Most enzymes catalyze one specific reaction. However, some multipart enzyme complexes catalyze a series of step-by-step reactions—the first enzyme passes its product, now a new substrate, to a second enzyme that is part of the complex, the second passes its product to a third and so on. Enzymes are responsible for many essential reactions in the human body; in fact, there are from 20,000 to 25,000 total human genes, with about 25% of them producing enzymes. It is not surprising that problems with enzymes are caused by or result in diseases.
Illustration of Simple One and Two Substrate Enzyme Reaction. Components include enzyme (E), substances (S1, S2, S3), and product (P). If the substance can bind to the enzyme’s substrate-binding site (e.g., S1, top figure, and S1 and S2, bottom figure), then it acts as a substrate. Molecular shape as determined by secondary, tertiary, and quaternary structure as well as the hydrophobic/hydrophilic and neutral or charged nature influences which substrate molecules can bind as substrates (represented graphically by differing shapes of S1, S2, and S3 and the triangular and circular binding pockets). The enzyme binds the substrate molecule(s), catalyzes the enzymatic reaction, and releases the product after which the enzyme is ready to catalyze the same reaction again. The rate of the overall reaction is influenced by each step in the process, including substrate binding, rate of the reaction, and product release as discussed in the text below. [Adapted with permission from Naik P: Biochemistry, 3rd edition, Jaypee Brothers Medical Publishers (P) Ltd., 2009.]
The concept of enzyme kinetics allows an exact description of the enzymatic reaction, including the influence of substrate and product molecules and how fast the enzyme catalyzes the reaction and the impact. More advanced enzyme kinetics allows the mathematical expression of how other molecules such as cofactors, inhibitors, and activators (see below) affect the enzyme reaction.
For example, as discussed in Chapter 1, specific amino ...