Just like a car engine, your cells need fuel to power the reactions that keep them going. The most important fuel your cells burn is a simple sugar called glucose. A 10-step series of reactions called glycolysis breaks molecules of glucose down into molecules of pyruvate, extracting a little energy along the way. The first step in glycolysis involves an enzyme called hexokinase.
Hexokinase
The name hexokinase may sound a little complicated, but it becomes simpler if you break it down. Enzymes are catalysts that act on molecules called substrates. An enzyme that attaches a phosphate group to its substrate or phosphorylates it is called a kinase. Glucose is a hexose, meaning it's a simple sugar with a total of six carbon atoms. Consequently, hexokinase is quite literally an enzyme that phosphorylates the most well-known hexose of them all -- glucose.
Energy
Attaching a phosphate group to glucose requires energy in the form of a molecule called ATP. By breaking a high-energy bond in ATP and reattaching a phosphate group from that molecule to glucose, hexokinase takes energy from ATP and stores it in the glucose molecule. Although enzymes can catalyze both the forward and reverse reactions, the amount of energy released in this first reaction is so great that the reaction only runs forwards, practically, and not in reverse.
Role
Breaking up ATP to phosphorylate glucose represents an energy investment for the cell. This investment will ensure a payoff later, however, because the cell will harvest more ATP from glycolysis than it spends. The payoff from later reactions in a pathway called the citric acid cycle is even greater still. Moreover, the phosphorylated glucose generated by hexokinase can also find use in other important pathways, so even after phosphorylation the glucose molecule is not yet irretrievably committed to glycolysis.
Trapping
There's another important reason for phosphorylating glucose. Cells don't actively take up glucose; rather, they allow it to diffuse across with the aid of a carrier protein embedded in the membrane. If the glucose concentration inside the cell is greater than the concentration outside, the carrier protein will actually enable glucose to recross the membrane and leave the cell -- an undesirable outcome from the cell's point of view. Phosphorylated glucose cannot fit into the carrier protein and make its escape, so once glucose has been phosphorylated it remains trapped inside the cell, awaiting its metabolic fate.
References
- "Lehninger Principles of Biochemistry"; David L. Nelson and Michael M. Cox; 2008
- "Organic Chemistry, Structure and Function"; Peter Vollhardt et al.; 2011
