Function and Catalyzed Reaction
The primary function of creatine kinase in the body is to provide reversible phosphorylation of creatine. This process yields creatine phosphate, a vital high-energy compound that serves as a cellular energy reserve.
The chemical reaction involves the transfer of a phosphate group ($PO_3H_2$) from an ATP molecule directly to the amino group of a creatine molecule.
Reaction components:
- Substrates: creatine and ATP.
- Products: creatine phosphate and ADP.
- Enzyme: creatine kinase.
Structure and Isoforms
Structurally, the creatine kinase molecule is a dimer, meaning a protein composed of exactly two subunits. Two basic types of subunits exist, named after the English terms reflecting their primary tissue localization:
- M (Muscle) — muscle subunit.
- B (Brain) — brain subunit.
Various paired combinations of these subunits form three creatine kinase isoenzymes (isoforms). They differ in electrophoretic mobility—the rate of directed movement from the origin (cathode, "minus") to the anode ("plus") in an electric field:
- CK-1 (BB) — exhibits the highest electrophoretic mobility and migrates closest to the anode (+).
- CK-2 (MB) — a hybrid form with intermediate mobility.
- CK-3 (MM) — has the lowest mobility and remains closest to the origin, i.e., the cathode (-).
Tissue Localization
Each isoform has a strict tissue distribution, determining its role in the cellular metabolism of specific organs:
- The BB (Brain-Brain) isoform is localized exclusively in brain tissue.
- The MM (Muscle-Muscle) isoform is found predominantly in skeletal muscle.
- The MB (Muscle-Brain) isoform is specific to cardiac muscle (myocardium).
Clinical and Diagnostic Significance
Measuring the levels of various creatine kinase isoforms in blood plasma serves as a valuable diagnostic criterion in several acute conditions:
- Myocardial Infarction. When cardiomyocytes die, cell destruction occurs, and the MB isoform is actively released into blood plasma. Its elevation is considered one of the most important diagnostic markers for myocardial infarction.
- Skeletal Muscle Injury. Following physical trauma, crush syndrome, or other muscle damage, the amount of the muscle isoform MM increases in the blood accordingly.
- Central Nervous System Pathologies. Unlike the myocardium and skeletal muscles, the brain BB isoform is virtually undetectable in the blood during strokes and other CNS lesions. This is because the enzyme is physically unable to cross the blood-brain barrier. Consequently, measuring blood creatine kinase has no clinical diagnostic value for assessing brain pathologies.