What is a defining feature of functional proteins?

Functional proteins play crucial roles in various biological processes, primarily because they are involved in catalyzing biochemical reactions and enabling cellular functions. Enzymes, which are a type of functional protein, accelerate chemical reactions necessary for metabolism, DNA replication, and many other vital cell activities. Additionally, functional proteins are integral to processes such as signal transduction, transport of molecules across membranes, and immune responses, thereby supporting the overall functioning of organisms.

The other options focus on features that do not accurately describe the characteristics of functional proteins. Maintaining cell shape is more associated with structural proteins, and proteins can be found across a wide range of organisms, not limited to skeletal or multicellular forms. Thus, the defining feature of functional proteins is indeed their active participation in catalyzing reactions and other essential functions within cells.