Question about PPP on EK's FL exam

[HopefulMDclass2020]

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Which of the following is not true about the PPP cycle?

A. The PPP is constitutively active

B. The oxidative branch produces five carbon sugars for the cell

C. The PPP is regulated by NADPH

D. The oxidative branch produces NADPH

The answer is B. I do not know how I feel about that. The oxidative branch produces NADPH and also undergoes a decarboxylation step, making a 5 carbon sugar. Although this is not the final product, it is still a 5 carbon sugar. I choose A, because I knew D was correct, and if NADPH regulates it, it can't be constitutively active. The solution was not through on why A is correct.

I would appreciate it if someone could shed some light on this question. Thanks!

 

[65SpencerAve]

i understand what you're saying about "B" and the products of the oxidative branch (the production of ribulose-5-phosphate, a 5 carbon sugar, from decarboxylation of a Beta-keto acid.) i think this is kind of a "bad" question due to the answer choices - and also because it depends on where you read about PPP. according to EK: the oxidative branch generates NADPH (D=true), while the non-oxidative branch generates 5 carbon sugars, such as ribose-5-phosphate; also, the PPP is active at all times (A=true) because it is regulated by NADPH (C=true.) to better understand why A and C are true: basically, the rate of PPP is controlled by levels of NADP+. As NADPH is constantly used in reductive synthetic pathways, the concentration of NADP+ increases, stimulating PPP to replenish NADPH. also, PPP is active at all times because of the 4 different modes of the pathway. that's my understanding.

 

[HopefulMDclass2020]

i understand what you're saying about "B" and the products of the oxidative branch (the production of ribulose-5-phosphate, a 5 carbon sugar, from decarboxylation of a Beta-keto acid.) i think this is kind of a "bad" question due to the answer choices - and also because it depends on where you read about PPP. according to EK: the oxidative branch generates NADPH (D=true), while the non-oxidative branch generates 5 carbon sugars, such as ribose-5-phosphate; also, the PPP is active at all times (A=true) because it is regulated by NADPH (C=true.) to better understand why A and C are true: basically, the rate of PPP is controlled by levels of NADP+. As NADPH is constantly used in reductive synthetic pathways, the concentration of NADP+ increases, stimulating PPP to replenish NADPH. also, PPP is active at all times because of the 4 different modes of the pathway. that's my understanding.

Great explanation! I guess I am misunderstanding what constitutively active means. Do you mind elaborating on what that means? I was under the impression it was always active and not under control of feedback mechanisms.