Tuesday, December 18, 2007
How Cells Make ATP: Substrate-Level Phosphorylation
Monday's Molecule #56 was adenosine 5′-diphosphate, or ADP. ADP is the precursor for synthesis of adenosine 5′triphosphate, or ATP. ATP is one of the most important cofactors in biochemistry. It carries energy that drives other reactions to completion.
The synthesis reaction looks like this, where Pi is inorganic phosphate.
This reaction is the reverse of the energy-producing reaction where ATP is hydrolyzed to ADP and H2O. Since the energy-producing reaction produces a lot of energy (~45 kJ mol-1 under normal cellular conditions), it follows that the synthesis reaction requires the input of a great deal of energy.
There are several different ways that cells can make ATP from ADP and inorganic phosphate (Pi). one of the more common ways is when the phosphate group is transferred to ADP from a molecule that is more energetic than ATP. This form of biosynthesis is called substrate-level phosphorylation. The formation of ATP is coupled to the removal of a phosphate group from another molecule.
Here's an example from the gluconeogenesis/glycolysis pathway. The reaction catalyzed by phosphoglycerate kinase is readily reversible inside the cell. (In biochemistry courses, we say that it is a near-equilibrium reaction.)
When glucose is being synthesized (gluconeogenesis) the reaction goes from right to left and a molecule of ATP is used up to create 1,3-bisphosphoglycerate. During glycolysis, when glucose is being broken down, the reaction goes from left to right and a molecule of ATP is produced when the phosphate group on 1,3-bis phosphoglycerate is transferred to ADP.
This is an example of ATP synthesis by substrate-level phosphorylation. It's one of two such reactions in glycolysis and it's the main reason why the degradation of glucose can be used to produce useful energy. For example, when glucose is taken up from the blood stream by muscle cells and degraded to produce ATP that can be used in muscle contraction.