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How does the amino acid sequence of an enzyme affect its activity?
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Enzyme
In the induced-fit theory of enzyme-substrate binding, a substrate approaches the surface of an enzyme (step 1 in box A, B, C) and causes a change in the enzyme shape that results in the correct alignment of the catalytic groups (triangles A and B; circles C and D represent substrate-binding groups on the enzyme that are essential for catalytic activity). The catalytic groups react with the substrate to form products (step 2). The products then separate from the enzyme, freeing it to repeat the sequence (step 3). Boxes D and E represent examples of molecules that are too large or too small for proper catalytic alignment. Boxes F and G demonstrate binding of an inhibitor molecule (I and I′) to an allosteric site, thereby preventing interaction of the enzyme with the substrate. Box H illustrates binding of an allosteric activator (X), a nonsubstrate molecule capable of reacting with the enzyme.
The amino acid sequence of an enzyme is fundamental to its biological function, as it dictates the enzyme's three-dimensional structure and its ability to catalyze specific chemical reactions.12
Structural Determination
An enzyme is composed of one or more polypeptide chains, and the specific order of amino acids determines how these chains fold into characteristic shapes. This folding process creates a specialized region known as the active site, a groove or pocket where the substrate binds. The precise arrangement of amino acids within and near this site provides the necessary chemical and electrical properties to recognize and bind only specific substrates, a concept known as enzyme specificity.132
Impact on Activity
- Substrate Binding: The amino acids at the active site facilitate binding through physical and chemical forces, such as electrostatic or hydrophobic bonds. If the sequence is altered, the resulting change in the active site's shape or charge can prevent the substrate from binding.24
- Catalytic Function: Modifications to amino acids at the active site often impair or abolish enzymatic activity because these residues are directly involved in the catalytic process.24
- Genetic Mutations: Inherited diseases often stem from mutations that change an amino acid within an enzyme. Such changes can render the enzyme defective by altering the active site's structure, which may prevent it from reacting with its substrate.4