Q.Proteins perform many physiological functions. For example, some functions as enzymes. Which of the following represents an additional function that some proteins discharge?

• Proteins carry out a wide range of jobs beyond acting as enzymes - they transport nutrients, defend against infection, act as receptors, and act as hormones, among other roles.
• Insulin is a clear example of a protein that functions as a hormone.
• Antibiotics are not, in this scheme, listed as a protein function - they belong to the separate category of secondary metabolites made by plants, fungi and microbes. …

Proteins are the most versatile class of biomolecule, taking on a wide variety of physiological jobs in a living organism. Alongside enzymes such as trypsin, proteins also transport nutrients across membranes (as GLUT-4 does for glucose), fight infectious agents (as antibodies do), handle sensory reception (as receptors do), provide structural support (as collagen does in the intercellular ground substance), and act as hormones (as insulin does). …

Method: Sort the Options by Molecule Class, Not Just Function

The fastest way to solve this MCQ is to classify each option by WHAT KIND of molecule performs it, before worrying about whether it sounds like a plausible "protein job." The question is specifically asking for an additional protein function, so any option whose real-world example is not itself a protein can be ruled out immediately, regardless of how biologically important that function is.

Go option by option with that lens: antibiotics (like penicillin) are, as established elsewhere in this chapter, secondary metabolites — small organic compounds, not proteins — so this option is out. The pigments that colour skin and those that colour flowers (melanin-type compounds, carotenoids, anthocyanins) are likewise not proteins; they are separate classes of biomolecule entirely. Hormones, however, include many genuine proteins — insulin being the standard textbook example — so …