Function and essence of the catalyzed reaction
The main task of creatine kinase in the body is to ensure the reversible phosphorylation of creatine. As a result of this process, creatine phosphate is formed, which is a crucial high-energy compound serving as an energy reserve for the cell.
The chemistry of the 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 of the enzyme
In its chemical structure, the creatine kinase molecule is a dimer, meaning a protein consisting of exactly two subunits. Nature has provided two basic types of such subunits, which derived their names from English terms reflecting their primary localization:
- M (Muscle) — muscle subunit.
- B (Brain) — brain subunit.
Various paired combinations of these subunits form three isoenzymes (isoforms) of creatine kinase. They differ from each other in electrophoretic mobility — the speed of directed movement from the start (cathode, "minus") to the anode ("plus") in an electric field:
- CK1 (BB) — has maximum electrophoretic mobility and is located closest to the anode (+) during analysis.
- CK2 (MB) — a heterodimeric form characterized by intermediate mobility.
- CK3 (MM) — has minimal mobility and remains closest to the start, i.e., the cathode (-).
Tissue localization
Each isoform has its own strict tissue assignment, which determines its role in the cellular metabolism of specific organs:
- The BB isoform (Brain-Brain) is localized exclusively in brain tissues.
- The MM isoform (Muscle-Muscle) is predominantly found in skeletal muscle.
- The MB isoform (Muscle-Brain) is specific to the heart muscle (myocardium).
Clinical and diagnostic significance
Determining the level of various creatine kinase isoforms in blood plasma is a valuable diagnostic criterion in a number of acute conditions:
- Myocardial infarction. Upon the death of cardiomyocytes, cell destruction occurs, and the MB isoform is actively released into the blood plasma. Its elevation is considered one of the most important diagnostic markers of infarction.
- Skeletal muscle damage. In physical trauma, crush syndrome, or other muscle damage, the amount of the muscle isoform MM predictably increases in the blood.
- Central nervous system pathologies. Unlike the myocardium and muscles, in strokes and other CNS lesions, the brain isoform BB is practically undetectable in the blood. The reason lies in the fact that the enzyme is physically unable to penetrate the blood-brain barrier. Consequently, determining creatine kinase in the blood has no clinical and diagnostic significance for assessing brain pathologies.
Briefly: answers to topical questions
How long after a myocardial infarction does the CK-MB level reach its maximum in the blood?
The creatine kinase level reaches its maximum by 24 hours after the development of a myocardial infarction.
Dynamics of enzyme activity in the blood:
- Onset of elevation — expected 3–6 hours after the attack (acts as an early marker of damage).
- Peak activity — occurs by 24 hours (rapidly rises and reaches the peak first).
- Decline — after reaching the peak, activity decreases relatively quickly.
In myocardial infarction, destruction of cardiomyocytes occurs along with an elevation in plasma of the specific MB isoform of creatine kinase.
To easily remember the localization of the isoforms, focus on the English letters: M (Muscle) — muscles, B (Brain) — brain. The MM isoform works in skeletal muscles, BB in the brain, and their hybrid MB took a bit from both and works in the body's "motor" — the myocardium.
Class and exam questions
What is the essence of the reaction catalyzed by creatine kinase?
The enzyme catalyzes the reversible phosphorylation of creatine using ATP. The phosphate group is transferred to the amino group of creatine, resulting in the formation of the high-energy compound creatine phosphate and an ADP molecule.
Why does the creatine kinase level not rise in stroke?
The brain contains the BB isoform. When brain tissue is damaged, this enzyme cannot overcome the blood-brain barrier and enter the systemic circulation, which is why it is not detected in blood plasma.
Which creatine kinase isoform has the highest electrophoretic mobility?
The brain isoform CK1 (BB) has maximum mobility. During electrophoresis, it moves faster than the others from the cathode to the anode.
What else to study on this topic
- Reaction chemistry: the mechanism of phosphate group transfer from ATP
- Principles of separating enzyme isoforms by electrophoresis
- Specificity of the MB fraction as a marker of myocardial infarction
- Blood-brain barrier: reasons for impermeability to the BB fraction
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