kinase

Meaning: A kinase is a transferase that moves a phosphate group from ATP onto a substrate. Protein kinases attach it to the hydroxyl group of serine, threonine, or tyrosine. The added negative charge changes the protein's shape and activity. Metabolic kinases act on small molecules, such as hexokinase converting glucose to glucose-6-phosphate, which traps glucose inside the cell.

Equation: substrate + ATP → substrate–phosphate + ADP, where the substrate is the molecule being phosphorylated, ATP is adenosine triphosphate, and ADP is adenosine diphosphate (amounts in mol). Mg²⁺ is typically a cofactor.

On the MCAT: Signal transduction passages describe receptor tyrosine kinases, kinase cascades, or protein kinase A activated by cAMP. You predict what happens when a kinase is inhibited or a mutation leaves it constitutively active (signaling stays on). Metabolism questions use hexokinase and phosphofructokinase-1 in glycolysis. Reversible phosphorylation is the standard example of covalent regulation of enzymes.

Common confusion: A kinase is not a phosphatase. A phosphatase removes a phosphate by hydrolysis and reverses the kinase's effect. A kinase is also not a phosphorylase, which cleaves bonds using inorganic phosphate, as glycogen phosphorylase does, and does not use ATP. Finally, phosphorylation does not always activate: it activates glycogen phosphorylase but inhibits glycogen synthase.