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Chapter 26: Problem 7

(Integrates with Chapters \(18-20 \text { and } 22 .)\) By what pathway(s) does the ribose released upon nucleotide degradation enter intermediary metabolism and become converted to cellular energy? How many ATP equivalents can be recovered from one equivalent of ribose?

Short Answer

Expert verified
The ribose released from nucleotide degradation enters the metabolic pathway through the Pentose Phosphate Pathway and can be further oxidized to produce ATP. Around 15 ATP equivalents can be recovered from the complete degradation of one equivalent of ribose, depending on cellular conditions.

Step by step solution

01

Understanding Ribose Degradation Pathway

The ribose released from nucleotide degradation enters the intermediary metabolism through the Pentose Phosphate Pathway (PPP). After being released from nucleotides, ribose is converted to ribose 5-phosphate by nucleosidases and phosphopentomutase. This compound is an intermediate in the PPP.
02

Conversion of Ribose 5-phosphate

Within the PPP, ribose 5-phosphate can be converted into other sugars or oxidized further to produce NADPH. Ribose 5-phosphate is converted into glucose 6-phosphate, through a series of reactions in the PPP known as non-oxidative reactions.
03

Conversion to ATP

From glucose 6-phosphate it enters the glycolytic pathway where it is further broken down to form pyruvate. Pyruvate can then enter the citric acid cycle where it can be fully oxidized. In the process, reducing equivalents are produced that can be used to generate ATP in the electron transportchain.
04

Quantifying ATP Production

As for how much ATP can be generated from the equivalent of one ribose molecule, it depends on a number of variables. If we consider a single round of PPP which produces ribulose 5-phosphate, then after several transformations, it yields two ATP equivalents. However, full oxidation through glycolysis, the citric acid cycle plus electron transport and oxidative phosphorylation, generates approximately 15 ATP equivalents. Note that this general estimate assumes an efficient metabolism and may vary based on conditions within the cell.

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Most popular questions from this chapter

(Integrates with Chapter \(15 .\) ) Unlike its allosteric counterpart glycogen phosphorylase (an \(\alpha_{2}\) enzyme), \(E\) coli ATCase has a heteromeric \(\left(\alpha_{6} \beta_{6}\right)\) organization. The \(\alpha\) -subunits bind aspartate and are considered catalytic subunits, whereas the \(\beta\) -subunits bind CTP or ATP and are considered regulatory subunits. How would you describe the subunit organization of ATCase from a functional point of view?

Enzymes that bind phosphoribosyl-5-phosphate (PRPP) have a common structural fold, the PRT fold, which unites them as a structural family. PRT here refers to the phosphoribosyl transferase activity displayed by some family members. Typically, in such reactions, \(\mathrm{PP}_{\mathrm{i}}\) is displaced from PRPP by a nitrogen-containing nucleophile. Several such reactions occur in purine metabolism. Identify two such reactions where the enzyme involved is likely to be a PRT family member.

Write a balanced equation for the oxidation of uric acid to glyoxylic acid, \(\mathrm{CO}_{2},\) and \(\mathrm{NH}_{3}\), showing each step in the process and naming all of the enzymes involved.

Indicate which reactions of purine or pyrimidine metabolism are affected by the inhibitors (a) azaserine, (b) methotrexate, (c) sulfonamides, (d) allopurinol, and (e) 5 -fluorouracil.

Describe the underlying rationale for the regulatory effects exerted on ribonucleotide reductase by ATP, dATP, dTTP, and dGTP.
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