Question

The bacterial enzyme polynucleotide phosphorylase (PNPase) is a $3^{\text{'}}\to 5^{\text{'}}$ exoribonuclease that degrades mRNA. (a) The enzyme catalyzes a phosphorolysis reaction, as does glycogen phosphorylase (Section $16-1$), rather than hydrolysis. Write an equation for the mRNA phosphorolysis reaction. (b) In vitro, PNPase also catalyzes the reverse of the phosphorolysis reaction. What does this reaction accomplish and how does it differ from the reaction carried out by RNA polymerase? (c) PNPase includes a binding site for long ribonucleotides, which may promote the enzyme's processivity. Why would this be an advantage for the primary activity of PNPase in vivo?

Short answer

2. The reverse reaction of PNPase synthesizes polyribonucleotides. PNPase differs from RNA polymerase as it does not require a template.
3. In the cell, PNPase degrades mRNAs to nucleoside diphosphates which will be increased by the binding of large ribonucleotides.

Step-by-step solution

Define RNA polymerase

RNA polymerase, or more precisely DNA-directed/dependent RNA polymerase, is an enzyme that creates RNA from a DNA template in molecular biology.

Equation for mRNA phosphorolysis reaction

(a)

PNPase (polynucleotide phosphorylase) is a bacterial enzyme that uses phosphorolysis to generate nucleic acids. It degrades the mRNA in the 5' to 3' direction. The reaction of mRNA phosphorylysis is as follows:

Reaction being accomplished and its difference from reaction carried by the RNA polymerase

(b)

The PNPase can catalyze the reverse of mRNA phosphorylases. It involves the synthesis of polyribonucleotides. It differs from RNA polymerase as PNPase does not require a template, whereas RNA polymerase must. The DNA (deoxyribonucleic acid) serves as the template for RNA polymerase, while PNPase can act on both RNA and DNA.

Advantage for primary activity of PNPase in vivo

(C)

The PNPase has a binding site for large ribonucleotides, promoting enzyme efficiency. It is advantageous for the primary activity of PNPase that involves the breakdown of mRNA molecules as the binding of large ribonucleotides will allow the enzyme to break and convert them into ribonucleotide diphosphates.