Creatine phosphate is an organic compound that provides a quick source of energy for muscle fibers to contract when they need an initial burst of energy. It is also found in the brain and provides a similar burst of energy for neurons. Another term for this compound is phosphocreatine, which is abbreviated as PCr or Pcr.
The most common description for the action of creatine phosphate is with muscle fibers. The initial energy for muscle contraction comes from the high-energy compound adenosine triphosphate (ATP). The power of ATP comes from its three high-energy phosphate bonds. Muscle fibers, however, only contain a small reservoir of ATP. Most of their energy store is kept in a pool of creatine phosphate.
The cells also have the precursor to ATP, which is adenosine diphosphate (ADP). This compound has two high-energy bonds. The addition of another high-energy phosphate bond to ADP creates ATP.
Creatine that has a phosphate attached to it is referred to as being phosphorylated, and this phosphate bond is a high-energy one like that of ATP. The phosphorylated creatine transfers its phosphate to ADP to form ATP, leaving unphosphorylated creatine. When the muscle cells have the energy of ATP, they can act in the time it takes for alternate energy sources to be activated. If all of the creatine phosphate is used up, the cells can still produce ATP by an alternate method of energy production that is much less efficient.
This process is catalyzed by the enzyme creatine phosphokinase, also known as creatine kinase. The reaction is reversible. The enzyme can either add a phosphate to creatine to make creatine phosphate, or remove one to make creatine, depending on the needs of the cell.
When the cells are resting, they build up their stores of creatine phosphate. This is done by removing a phosphate from ATP and adding it to creatine, creating ADP as a by-product in the process. Muscle and brain cells are the classic examples of tissues that utilize this type of system, but other tissues that rapidly use ATP also use creatine phosphate as an energy store. These include spermatozoa and the photoreceptor cells of the retina.
The reversible phosphorylation of creatine phosphate is carried out by one of several different types of creatine kinases. There is one type specialized for muscles (M) and another for the brain (B). Each molecule of creatine kinase is made up of two sub-units, which can be composed of varying combinations of the different types. Creatine kinase of the MB type is assayed clinically in blood tests for emergency patients, and specifically for patients suspected of having had a heart attack or kidney failure.