Life depends on complex networks of chemical reactions. These are mediated by enzymes. Enzymes are catalysts of enormous power and high specificity. Consider a lump of sugar. It is combustible but quite difficult to set alight. A chemical catalyst would speed up its combustion, and we would end up with heat, a little light, carbon dioxide, and water. Swallowed and digested, the sucrose is broken down in many steps to carbon dioxide and water by the action of at least 22 different enzymes, and the energy released is used to drive other reactions in the body.
At a basic level, a reaction carried out by an enzyme can be expressed as
where E is the enzyme, which binds the substrate S to form the complex ES. The term substrate is used for a ligand that binds to an enzyme and that is then transformed to the product, P. The region of the protein where the substrate binds and the reaction occurs is called the active site. This binding is specific, often highly so. The enzyme j-galactosidase (page 109) is moderately specific and will split not only lactose but also any other dis-accharide that has a glycosidic bond to j-galactose. By contrast glycogen phosphorylase kinase (page 305) acts with absolute specificity on a single substrate, another enzyme called glycogen phosphorylase—none of the thousands of other proteins in the cell can substitute. In general, the specificity of an enzyme is conferred by the shape of the active site and by particular amino acid side chains that interact with the substrate. Binding of the substrate produces the enzyme-substrate complex ES; the catalytic function of the protein then converts the substrate to product, still bound to the enzyme in the complex EP. Finally the product dissociates from the enzyme. Chemical engineers use catalysts made of many materials, and within cells there are catalysts called ribozymes that are made of RNA. However, only proteins, with their enormous repertoire of different shapes can produce catalysts of high selectivity.
The catalytic rate constant kcat (also known as the turnover number) of an enzyme gives us an idea of the enormous catalytic power of most enzymes. It is defined as the number of molecules of substrate converted to product per molecule of enzyme per unit time (equally it is moles of substrate converted per mole of enzyme per unit time). Many enzymes have kcat values around 1000 to 10,000 per second. The reciprocal of kcat is the time taken for a single event. Thus if kcat is 10,000 s-1, one substrate molecule will be converted every tenth of a millisecond. Some enzymes achieve very much higher rates. Catalase, an enzyme found in peroxisomes, has a kcat of 4 x 107s-1, and so it takes only 25 ns to split a molecule of hydrogen peroxide into oxygen and water.
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