Question

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

Short answer

In summary, Polynucleotide phosphorylase (Ochoa's enzyme) does not synthesize RNA in vivo because it lacks the ability to recognize and bind to DNA templates, uses different monomers (nucleotide diphosphates instead of triphosphates), and favors RNA degradation over synthesis due to the exergonic nature of its reaction. Instead, RNA synthesis is carried out by RNA polymerases, which form phosphodiester bonds between nucleotides using nucleotide triphosphates as substrates and following a DNA template.

Step-by-step solution

1. Role of Polynucleotide phosphorylase

Polynucleotide phosphorylase (Ochoa's enzyme) is an enzyme involved in the degradation and processing of RNA molecules. Its function is mostly related to breaking down RNA into its constituent nucleotides.

2. Mechanism of Polynucleotide phosphorylase and its difference from RNA synthesis

Polynucleotide phosphorylase catalyzes the phosphorolysis of the 3'-terminal phosphodiester bonds of RNA, leading to the sequential release of nucleoside diphosphates (NDPs). This process is the reverse of the one involved in the synthesis of RNA, which occurs through the formation of phosphodiester bonds between nucleotides. But, while the mechanism of Polynucleotide phosphorylase is reversible under certain conditions, it does not occur in living organisms for the reasons explained in the next steps.

3. Conditions required for RNA synthesis

The synthesis of RNA requires a DNA template, specific enzymes (RNA polymerases), and a supply of nucleotide triphosphates as precursors. RNA polymerases bind to the DNA template and synthesize RNA by extending the RNA chain according to the base-pairing rules between the DNA template and the incoming nucleotides.

4. Why Polynucleotide phosphorylase doesn't synthesize RNA in vivo

Polynucleotide phosphorylase doesn't synthesize RNA in vivo for the following reasons: - It lacks the ability to recognize and bind specific sequences of DNA to initiate RNA synthesis. - In living organisms, the monomers required for RNA synthesis are nucleotide triphosphates, not the nucleotide diphosphates generated by Polynucleotide phosphorylase. - Phosphorolysis, the reaction catalyzed by Polynucleotide phosphorylase, is highly exergonic, which means it will proceed spontaneously in the direction of RNA degradation and not synthesis under physiological conditions.

5. Enzymes responsible for RNA synthesis

The enzymes responsible for RNA synthesis in living organisms are called RNA polymerases. They catalyze the formation of phosphodiester bonds between nucleotides, using nucleotide triphosphates as substrates and following a DNA template. Different types of RNA polymerases are involved in the synthesis of specific RNA molecules such as mRNA, tRNA, and rRNA.