Which of the following is a non-proteinaceous biocatalyst ?

Question

Accepted Answer

The correct option is 4: Ribozyme

Ribozyme is the correct answer because it is a molecule that possesses catalytic activity but is fundamentally made of Ribonucleic Acid (RNA), meaning it is non-proteinaceous. Traditionally, the concept of a "biocatalyst" was believed to be synonymous with "enzyme," and all known enzymes were thought to be exclusively proteins. This foundational understanding of cellular chemistry was overturned by the revolutionary discovery of ribozymes in the early 1980s. Ribozymes demonstrate that certain RNA molecules have the unique capacity to fold into complex three-dimensional structures that mimic the active site of a protein enzyme, enabling them to catalyze specific biochemical reactions. Examples of their catalytic roles include self-splicing of RNA introns and, most critically, the function of the ribosomal RNA in the large ribosomal subunit, which acts as the peptidyl transferase—the enzyme responsible for forming peptide bonds during protein synthesis. This discovery established that catalytic function is not exclusive to proteins and provided key evidence for the "RNA World" hypothesis in the study of the origin of life.

The incorrect option is 1: Virons

The term Viron refers to a complete, mature, and infectious virus particle that exists outside a host cell. Structurally, a virion is composed of a nucleic acid genome (either DNA or RNA) enclosed within a protective protein shell called a capsid, and sometimes a lipoprotein envelope. Its biological function is to serve as a transmission vehicle for the viral genome, not to catalyze metabolic reactions within a cell. Crucially, the virion's structure relies heavily on protein—the capsid and the envelope proteins—which immediately contradicts the requirement of the question for a non-proteinaceous biocatalyst. Therefore, a virion is primarily an infectious structure composed of both nucleic acid and protein.

The incorrect option is 2: Viroids

Viroids are the smallest known infectious agents, but their composition and function do not align with the definition of a biocatalyst. They are pathogens that consist of only a short, circular, and highly structured strand of naked RNA, completely lacking a protein coat. While they are indeed non-proteinaceous, their role is not to accelerate a chemical reaction but to act as a disease-causing agent, primarily in plants. Viroids rely entirely on the host cell's protein enzymes (like RNA polymerase) to replicate themselves and propagate the infection. Thus, they are passive infectious molecules that are not categorized as catalysts.

The incorrect option is 3: Prions

Prions represent another class of infectious particle, but they are fundamentally different from both virions and viroids in that they are composed exclusively of protein. A prion is an abnormally folded version of a normal host cellular protein (PrPC) which has undergone a conformational change to become PrPSc. This misfolded protein has the ability to cause other normal protein molecules to adopt the same pathological structure, leading to neurodegenerative diseases. Since prions are entirely protein in nature, they are intrinsically proteinaceous, failing to meet the "non-proteinaceous" requirement of the question. Their function is related to propagation of misfolding, not cellular catalysis.