Creatine Kinase Definition
Creatine phosphokinase, or simply creatine kinase, is an enzyme found in many tissues which helps regulate the concentration of ATP (adenosine triphosphate) within a cell. To do so, creatine kinase moves a phosphate group from ATP to creatine, forming phosphocreatine. This molecules stores the phosphate group in a stable form.
Creatine Kinase Overview
In cells which need a lot of ATP, it is more economical to store the ATP as a less reactive molecule until it is needed. Thus, the phosphate group is transferred from ATP by creatine kinase (CK) to a creatine molecule. The end products are ADP (adenosine diphosphate), and PCr (phosphocreatine). This reaction is reversible, and when ATP is needed, it can easily be regenerated by the enzyme from the stored pool of PCr.
PCr is much more stable than ATP, and cannot be directly used by most enzymes. Instead, the enzyme must reverse the reaction to move a phosphate group back to a molecule of ADP to form more ATP. In effect, this gives muscle cells and other cells with a high demand for energy the ability to store the energy without disrupting the equilibrium of ATP. This allows the cell to continue producing ATP and storing the extra energy for when it is needed.
Creatine Kinase Levels
The level of creatine kinase within the blood is measured in Units of enzyme activity (U) per Liter of blood (L), or U/L. A typical range is between 20 and 200 U/L. However, certain conditions can elevate these levels severely. For instance, if you have suffered muscle damage in an accident, your levels may be much higher, in the tens of thousands. Other neuromuscular disorders also cause elevated levels of CK. Your doctor should always be consulted to discuss your specific level of creatine kinase.
Creatine Kinase Function
Creatine Kinase is found primarily in tissues which require a lot of ATP. Muscle cells, nerve cells, and even sperm cells are examples of highly active cells which contain large amounts of the enzyme. This is because these cells must use a large amount of ATP to complete their work. Because of the nature of ATP, it must remain at certain concentrations to preserve the function of certain biochemical pathways. Therefore, the energy of ATP must be held in another place until it is needed. This place is PCr, which is simply a creatine molecule attached to a phosphate group.
The energy held in this bond can be efficiently and quickly converted by CK either to or from ATP. When there is too much ATP, the enzyme functions to lower the concentration by converting ATP to ADP. It stores the extra phosphate on a creatine molecule, creating PCr. The pool of PCr in the cell is much larger than the amount of ATP. For this reason, it is considered a “fuel tank” or energy storage and utility system. As the mitochondria produce ATP through oxidative phosphorylation, the energy is transferred to PCr molecules, which are distributed to the cell.
Using CK to maintain the “energy reservoir” is an efficient way to save up the ATP produced, without creating disruptive conditions for the cell. This is known as the PCr circuit. The chemical equation of the effects of CK can be seen below.
Structure of Creatine Kinase
Creatine kinase, like all proteins, is a specific chain of amino acids. When folded properly, this chain takes on a three-dimensional form, which gives it the ability to interact with certain molecules. The amino acids in CK are specific in that when folded, they increase the interaction ability of the enzyme with both creatine and phosphocreatine (PCr). Another site on CK is dedicated for interaction with ATP and ADP. As both molecules attach to the enzyme, it will either take a phosphate group from ATP and add it to creatine or take a phosphate group from PCr and transfer it to ADP. The end result is either the creation or usage of ATP.
There are multiple types of CK, coded by different genes. While these forms of creatine kinase differ in their amino acid structure, their function remains similar. However, slight subtleties in function allow the creatine kinase to operate in different environments.
Different cells even have different versions of creatine kinase, likely based on their function. The different forms of creatine kinase all perform the same function but under different conditions. These different forms are necessary to manage the energy reservoir in many different types of cell. In most cells, this reservoir of PCr is maintained at a concentration much higher than that of ATP. This makes it possible to do a lot of work.
Creatine Kinase Test
The different forms of creatine kinase make it a useful diagnostic took. Like other enzymes, CK is leaked into the bloodstream when a cell becomes damaged. If many cells are damaged at the same time, a detectable level of creatine kinase and other enzymes can be detected in the blood. Doctors can determine which form of CK is in the blood, which can give them clues as to which organs are being damaged.
A serum CK test can detect many conditions, such as a heart attack, muscle breakdown, and even autoimmune diseases which are attacking certain organs and tissues. After a heart attack, for instance, the level of enzyme rapidly spikes in the blood. Further, a doctor can determine that it is a combination of muscular and brain creatine kinase. This is evidence that the heart has been damaged. Because the enzyme rapidly disappears from the blood, it can be used as an indicator to determine when a damaging event happened in the system. This can help find the cause of major events.