Which substance is required for trapping fructose in metabolic pathways?

Fructokinase is the enzyme that plays a critical role in the metabolism of fructose by phosphorylating fructose to fructose-1-phosphate. This phosphorylation is essential for trapping fructose within the cell and allowing it to be further metabolized. By converting fructose to fructose-1-phosphate, fructokinase prevents fructose from diffusing back out of the cell, effectively 'trapping' it in the metabolic pathway.

This reaction is especially important in the liver, where fructose is metabolized, allowing for its utilization in various biosynthetic processes or its conversion into glucose through gluconeogenesis. The action of fructokinase is specifically relevant because it links the presence of fructose in the cell to subsequent metabolic processes.

Other enzymes mentioned in the options have important roles in carbohydrate metabolism, but none of them specifically phosphorylate fructose. For instance, hexokinase is involved in phosphorylating glucose and can also phosphorylate other hexoses, but its role does not specifically pertain to fructose and does not effectively trap it in the same way fructokinase does. Similarly, phosphofructokinase-1 (PFK-1) is a key regulatory enzyme in glycolysis acting on fructose