Why doesn't polynucleotide phosphorylase (Ochoa's enzyme) synthesize RNA in vivo?

Verified step by step guidance

1. Understand the function of polynucleotide phosphorylase (PNPase): it catalyzes the polymerization of ribonucleotides into RNA chains in vitro, but it primarily acts as a degradative enzyme in vivo.
2. Recognize that PNPase catalyzes a reversible reaction where it can both synthesize RNA from nucleoside diphosphates and degrade RNA into nucleoside diphosphates, depending on cellular conditions.
3. Consider the intracellular concentrations of substrates and products: in vivo, the concentration of nucleoside diphosphates (substrates for synthesis) is low, while inorganic phosphate (Pi) is abundant, favoring the degradative reaction.
4. Note that the cellular environment and regulatory mechanisms direct PNPase activity towards RNA degradation rather than synthesis, preventing it from synthesizing RNA in vivo.
5. Conclude that the physiological role of PNPase in the cell is RNA turnover and degradation, not RNA synthesis, which is carried out by RNA polymerases instead.

Key Concepts

Function of Polynucleotide Phosphorylase (PNPase)

PNPase is an enzyme that catalyzes the polymerization of nucleotides into RNA chains in vitro, but it primarily functions in RNA degradation in vivo. It adds nucleotides in a template-independent manner, which differs from the template-directed synthesis by RNA polymerases.

Template-Directed RNA Synthesis

In living cells, RNA synthesis requires a DNA template to ensure accurate and specific nucleotide sequences. RNA polymerases read the DNA template strand to produce complementary RNA, a process essential for proper gene expression and regulation.

Cellular Context and Enzyme Regulation

In vivo, PNPase activity is regulated and integrated into RNA degradation pathways rather than synthesis. Cellular conditions, cofactors, and competing enzymes prevent PNPase from synthesizing RNA, ensuring that RNA production is controlled by RNA polymerases.