What is meant by the following terms: (a) Substrate (b) Active site (c) Lock-and-key mechanism

A substrate is the specific reactant molecule that an enzyme acts upon. In biological systems, enzymes are proteins that catalyze, or speed up, specific biochemical reactions by binding to substrates. This binding allows the enzyme to reduce the activation energy needed for the reaction to occur and enhance the reaction rate. For example, in the process of converting glucose (substrate) to glucose-6-phosphate, an enzyme called hexokinase is responsible for this reaction. In this case, glucose is the substrate that hexokinase acts upon.

The active site is the specific region within an enzyme where the substrate binds. It is usually a cleft or pocket-shaped structure, formed by the three-dimensional arrangement of amino acid residues within the enzyme's polypeptide structure. The active site is complementary in shape to the substrate molecule, which ensures that the enzyme binds selectively and tightly to each substrate. For instance, in the enzyme-substrate complex formed between hexokinase and glucose previously mentioned, the active site within hexokinase would be the location where glucose binds and the reaction occurs.

The lock-and-key mechanism is a model that explains how enzymes recognize and bind to their specific substrates. In this model, the enzyme (lock) has a specific shape complementary to that of the substrate (key). Therefore, only the correct key (substrate) will fit into the lock (enzyme) and promote its catalytic action. This specific interaction ensures the high selectivity of enzymes for their substrates and contributes to the regulation of metabolic pathways. For example, in the hexokinase-glucose interaction, the active site of hexokinase acts as a lock that recognizes and selectively binds with the glucose molecule, which acts as a key. This forms an enzyme-substrate complex, allowing the reaction to proceed and produce glucose-6-phosphate.