Pathways to pyruvate: Which of the following pathways can directly catabolize glucose to pyruvate in bacteria?

Introduction / Context:
Microbes employ several routes to degrade glucose to pyruvate, the central node of carbon metabolism. Two well-known glycolytic routes are the Embden–Meyerhof–Parnas (EMP) pathway (classical glycolysis) and the Entner–Doudoroff (ED) pathway. The tricarboxylic acid (TCA) cycle does not convert glucose to pyruvate; it oxidizes acetyl-CoA derived from pyruvate.

Given Data / Assumptions:

• We focus on direct conversion of glucose to pyruvate.
• EMP is not listed among the options; ED is.
• TCA operates downstream of pyruvate formation.

Concept / Approach:
The ED pathway, present in many Gram-negative bacteria (e.g., Pseudomonas, Zymomonas), converts glucose to pyruvate (and glyceraldehyde-3-phosphate) with a distinct set of enzymes and yields fewer ATP than EMP. The TCA cycle, by contrast, is not a glycolytic route and requires acetyl-CoA input; thus it does not answer the question of converting glucose to pyruvate.

Step-by-Step Solution:
Identify which listed pathway generates pyruvate directly from glucose. Select ED pathway; exclude TCA as it follows pyruvate decarboxylation to acetyl-CoA. Reject combined option since TCA does not perform the stated step.

Verification / Alternative check:
Standard pathway maps show glucose → 6-phosphogluconate → KDPG → pyruvate (via ED). TCA begins after pyruvate dehydrogenase forms acetyl-CoA.

Why Other Options Are Wrong:

• TCA cycle / Electron transport: Neither converts glucose to pyruvate.
• Both / None: Incorrect because ED alone among the options fits the criterion.

Common Pitfalls:
Assuming all central metabolic cycles start with glucose; TCA is an oxidative hub after glycolysis, not a substitute for it.

Final Answer:
Entner–Doudoroff pathway directly degrades glucose to pyruvate.