Introduction to Glycolysis

What's the point of all the phosporylation?

So far we have ignored the fact that the majority of intermediates on the pathway are phosphorylated. In fact ATP has to be temporarily invested to add phosphate groups. Here are several important advantages to phosphorylation:

• Transport. Cells often have transport systems built into their membranes for uptake of fuel molecules such as glucose. Phosphorylated glucose is no longer recognised by the glucose transport system and is therefore trapped in the cell.
• Binding. Substrates must be recognised by the active site of the enzyme which operates on them. There are two factors which affect this - shape and charge distribution. The strength of the interation can depend much on the charges. Monosacharides have polar bonds, allowing distinctive hydrogen bonds to form. This allows an enzyme to be highly specific. However, to provide a strong interaction between substrate and enzyme a large, electrically charged functional group is needed. So a phosphate group is a useful handle on to the substrate. This is particularly true of three carbon sugars which are capable of forming only a small number of hydrogen bonds with the active site of an enzyme.
• Synthesis of ATP. At one crucial step in glycolysis inorganic phosphate is used to phosphorylate the substrate. In the subsequent step the phosphate is transfered to ADP to make ATP. This form of ATP synthesis is called substrate level phosphorylation because the phosphate group is tranfered to ADP directly from the substrate.
• Control & Efficiency. Phosphorylation and dephosphorylation steps may be arranged which are irreversible. These can be placed at the start and finish of a pathway to aid efficiency. It's advantageous to prevent an equilibrium situation occuring where glucose or intermediates are left unprocessed. This would occur if the total free energy change were evenly distributed across all the steps. The irreversible steps are also good control points. If the enzyme is switched on the product is commited to the pathway. If the enzyme is switched off any product left in the system will be 'pulled' through by the irreversible step at the end.