Because non-biological catalysts are often very small compounds, it is reasonable to wonder why biological catalysts, namely, enzymes, are so large.
With the possible exception of enzymes that are secreted by cells, most of a cell’s enzymes are either permanently or cyclically associated with membranes and other particulate cell components.
Indeed, in a few cells (e.g., Euglena and Neurospora), there appear to be no “soluble” enzymes at all.
Therefore, it is apparent that one or more parts of an enzyme’s structure are involved in interactions that localize that enzyme in or on a specific cellular component and may not be involved in the catalysis itself.
Most of a cell’s enzymes, catalyze reactions that are part of a metabolic pathway, that is, each enzyme catalyzes a reaction that is one step in a series of ordered steps. It appears that in some (perhaps many) cases, the enzymes of a metabolic pathway occur in close proximity to one another and interact with each other.
Thus, still another portion of an enzyme’s total structure plays a role in this interaction. The localization of enzymes within a cell and the interactions between enzymes of a metabolic pathway.
Some parts of an enzyme may have no direct role in catalysis or in interaction with other enzymes or with intracellular membranes. For example, recent studies ‘ suggest that random, nonspecific binding of a substrate to sites on an enzyme’s surface other than the active site may enhance diffusion of the substrate to the active site.
Physicochemical studies indicate that an enzyme would have to have a surface area in excess of 1000 Å2 to exhibit even limited interactions with molecules other than the enzyme’s substrate. For proteins, this minimum surface area corresponds to a molecular weight of about 10,000. Therefore, it is not surprising that most enzymes have molecular weights that are greater than this.