Which of the following enzyme catalyses the formation of AMP from two molecules of ADP?
First, I remember that ATP can be broken down into ADP and then AMP. The enzyme involved in ATP to ADP is ATPase, right? But the question is about the reverse: forming AMP from two ADPs. Wait, when you have two ADPs, combining them might form ATP and AMP? Wait, no. Let me think. The enzyme that can combine two ADP molecules into ATP and AMP is probably adenylate kinase. Because adenylate kinase is known to catalyze the reaction: 2 ADP → ATP + AMP. That reaction is important in cells for energy balance. So adenylate kinase would be the correct answer here.
Now, looking at the options. The correct answer is adenylate kinase. Let's check the other options. The other enzymes might be things like ATP synthase, which is involved in the production of ATP, but not in this specific reaction. Or maybe phosphofructokinase, which is in glycolysis. Or maybe something else like creatine kinase? Wait, creatine kinase is involved in the creatine phosphate system, which transfers a phosphate group to ADP to make ATP, but that's a different reaction. So those would be the distractors.
So the core concept here is the role of adenylate kinase in energy metabolism. The enzyme catalyzes the conversion of two ADP molecules into ATP and AMP. This helps maintain the balance of nucleotides in the cell. The clinical pearl here is that adenylate kinase is crucial in tissues with high energy demands, like muscle and neurons, because it helps manage ATP and ADP levels during high energy demand. Students often confuse this with ATP synthase, which is part of the electron transport chain, but adenylate kinase is more about direct nucleotide conversion. So the correct answer is adenylate kinase.
**Core Concept**
The question tests understanding of enzymes involved in nucleotide metabolism, specifically the interconversion of adenosine phosphates. Adenylate kinase is central to maintaining cellular energy homeostasis by balancing ATP, ADP, and AMP levels.
**Why the Correct Answer is Right**
Adenylate kinase catalyzes the reversible reaction: 2 ADP ⇌ ATP + AMP. This reaction is critical during high-energy demand (e.g., muscle contraction) to regenerate ATP and prevent ADP accumulation. The enzyme uses a phosphorylated histidine intermediate to transfer a phosphate group between ADP molecules, ensuring efficient energy redistribution.
**Why Each Wrong Option is Incorrect**
**Option A:** ATP synthase generates ATP from ADP and inorganic phosphate via oxidative phosphorylation, not from two ADP molecules.
**Option B:** Creatine kinase transfers a phosphate from creatine phosphate to ADP, forming ATP—distinct from the ADP-ADP reaction.
**Option C:** Phosphofructokinase-1 (PFK-1) is a glycolytic enzyme that phosphorylates fructose-6-phosphate, unrelated to adenosine phosphate interconversion.
**Clinical Pearl / High-Yield Fact**
Adenylate kinase