Question

Why is MgATP2− , instead of ATP4− , a substrate for the hexokinase reaction? Select two correct phrases. Mg2+ prevents ATP hydrolysis. Hexokinase cannot bind ATP when it is not complexed with Mg2+ . Mg2+ makes the ???? (terminal) phosphorus atom of ATP more accessible to electrophilic attack by a glucose –OH group. Mg2+ shields the negative charges of two of the phosphate oxygen atoms of ATP. Mg2+ makes the ???? (terminal) phosphorus atom of ATP more accessible to nucleophilic attack by a glucose –OH group.

Answer 1

Mg²+ is crucial in the hexokinase reaction for shielding negative charges on ATP's phosphate groups and increasing the electrophilicity of the terminal phosphorus atom, enabling its efficient phosphorylation of glucose.

Step-by-step explanation:

In the hexokinase reaction, MgATP²⁻ is the substrate, rather than ATP⁴⁻, because the presence of Mg²⁻ is critical for the activity of the enzyme. Firstly, Mg²⁻ shields the negative charges on the phosphate groups of ATP, which would otherwise repel the nucleophilic attack by the glucose –OH group. Secondly, the Mg²⁻ complex increases the electrophilicity of the terminal (gamma) phosphorus atom of ATP, making it more accessible to nucleophilic attack, thereby facilitating the transfer of the phosphate group to glucose to form glucose-6-phosphate, the first step of glycolysis.