Which molecule is formed when glucose is phosphorylated in the first step of glycolysis?

• A. Glucose-6-phosphate
• B. Glucose-1-phosphate
• C. Fructose-6-phosphate
• D. Phosphoenolpyruvate

Answer: (A)

Phosphorylating glucose in the first step of glycolysis produces glucose-6-phosphate. This is done by the enzyme hexokinase (or glucokinase in liver), using ATP as the phosphate donor. Attaching a phosphate to glucose does two important things: it traps the sugar inside the cell (the charged molecule can’t easily cross the cell membrane) and it primes it for the next steps of glycolysis, starting with its isomerization to fructose-6-phosphate.

Glucose-1-phosphate comes from glycogen breakdown, not the first glycolysis step. Fructose-6-phosphate is formed only after glucose-6-phosphate is isomerized, so it is not the initial product. Phosphoenolpyruvate appears much later in glycolysis, near the end, just before pyruvate formation.