**Question:** PFK-1 is inhibited by:

**Core Concept:** PFK-1 stands for phosphofructokinase-1, which is an enzyme in the glycolysis pathway. It plays a crucial role in regulating glycolysis by converting fructose-6-phosphate into fructose-1,6-bisphosphate. PFK-1 is inhibited when its substrates or cofactors are not available, leading to decreased glycolysis rate.

**Why the Correct Answer is Right:** PFK-1 is inhibited by D-glyceraldehyde-3-phosphate (D-G3P) and ADP. When D-G3P and ADP bind to PFK-1, they compete with fructose-6-phosphate for the enzyme's active site, decreasing its affinity for fructose-6-phosphate. This results in lower conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, ultimately inhibiting glycolysis.

**Why Each Wrong Option is Incorrect:**

A. PFK-1 is not inhibited by ATP (adenosine triphosphate) or AMP (adenosine monophosphate), which are cofactors, not substrates.

B. PFK-1 is actually stimulated by NADH (nicotinamide adenine dinucleotide in its reduced form) and FADH2 (flavin adenine dinucleotide in its reduced form). These cofactors activate PFK-1, enhancing glycolysis.

C. PFK-1 is not inhibited by glucose-6-phosphate (G6P), another substrate in glycolysis; its presence actually activates PFK-1 and glycolysis.

D-glyceraldehyde-3-phosphate (D-G3P) and ADP are the correct answer, as they inhibit PFK-1 and subsequently glycolysis.

**Clinical Pearl:** Understanding PFK-1 regulation is essential for understanding glucose homeostasis and the adjustment of glycolysis rate in response to varying glucose availability in the cell. This is particularly relevant in situations such as hypoglycemia, where cells need to increase glycolysis to generate ATP, or hyperglycemia, where glycolysis should be suppressed to conserve glucose.