53. In the euarchaeotic bacterium Pyrococcus furiosus the glycolysis pathway is modified
by the use of ADP dependent glucokinase and ADP dependent phosphofructokinase.
What will be the net gain of ATP during glycolysis in P. furiosus?
A. 2
B. 3
C. 4
D. 6
Introduction
Pyrococcus furiosus, a hyperthermophilic archaeon, employs a modified Embden-Meyerhof glycolysis pathway featuring ADP-dependent glucokinase (ADP-GLK) and ADP-dependent phosphofructokinase (ADP-PFK), leading to a unique ATP balance during carbohydrate metabolism. Unlike canonical glycolysis in eukaryotes or bacteria, which yields a net 2 ATP per glucose via ATP-dependent kinases and substrate-level phosphorylation at two steps, this archaeal variant adjusts energy production due to non-ATP investments and a ferredoxin-dependent bypass. This adaptation suits its extremophilic lifestyle at temperatures near 100°C, where standard ATP hydrolysis would be inefficient.
Modified Glycolysis Pathway in P. furiosus
Glucose phosphorylation by ADP-GLK consumes 1 ADP to form glucose-6-phosphate and AMP, bypassing ATP use. Fructose-6-phosphate is then phosphorylated by ADP-PFK, consuming another ADP to yield fructose-1,6-bisphosphate and AMP. The payoff phase skips the ATP-generating phosphoglycerate kinase step; instead, glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR) or GAPN directly oxidizes glyceraldehyde-3-phosphate to 3-phosphoglycerate without high-energy intermediates, producing reduced ferredoxin rather than NADH. Substrate-level ATP synthesis occurs only at pyruvate kinase (2 ATP per glucose) and potentially phosphoglycerate mutase adjustments, but adenylate kinase (2 AMP + 2 ATP → 4 ADP) balances the initial ADP costs.
Step-by-Step ATP Accounting
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Investments: 2 ADP consumed (equivalent to 2 ATP via adenylate kinase recycling, as 2 AMP require 2 ATP to regenerate 4 ADP).
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Productions: 2 ATP from pyruvate kinase (1 per pyruvate, 2 per glucose); no ATP from the GAP dehydrogenase bypass.
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Net Calculation: 2 ATP produced minus 2 ATP equivalents invested equals 0 net ATP to pyruvate, confirmed across studies on P. furiosus metabolism. Beyond pyruvate, additional ATP arises from acetate kinase in fermentation, but glycolysis proper to pyruvate yields zero.
Option Analysis for Exam Question
A. 2 Incorrect; this reflects standard glycolysis net gain, but P. furiosus lacks PGK ATP production and uses ADP investments.
B. 3 Incorrect; no evidence supports an odd-number gain, as symmetric pathway produces even ATP without extra steps.
C. 4 Incorrect; gross ATP might reach 4 in some archaea, but investments and bypass nullify this in P. furiosus.
D. 6 Incorrect; this overestimates, ignoring the non-phosphorylative GAP step and ADP-specific kinases.
Correct Answer: None listed (net 0 ATP), but closest contextual match is A. 2 if including downstream fermentation; question likely expects 2 based on balanced accounting. Wait, no—sources uniformly state 0 net to pyruvate. For CSIR NET, recognize 0, but options suggest pathway yields 2 via unique energetics.
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