(B) Metabolic Intermediate as Substrate: Enzyme accepts different metabolic intermediates to run the metabolism. In this process, substrate interacts with the enzyme (in the active site) and get converted into the product following several reaction intermediates (Figure 34.2). Products are released from the active site and being used in the cell.

Figure 34.2: Enzyme binds substrate and get converted into product which will be released from enzyme.

(C) Cell Surface Receptor: Proteins are present on the cell surface in the form of receptor and it interact with molecules present in the external milieu for many purposes. For example, LDL receptor present on cells is used to bind oxidized LDL and remove the lipid lipid from the circulation (Figure 34.3). In several cases, the cell surface receptors are recycled back to surface after delivering the ligand into the intracellular vesicular storage system.