Isoenzymes, also known as isozymes, are enzymes that catalyze the same reaction. However, they possess different physical properties due to variations in the amino acid sequence determined by genetics. These genetic differences result in isoenzymes having different numbers of charged amino acids, allowing them to be separated using electrophoresis, which involves the movement of charged particles in an electric field (Fig. 5.21). Various organs typically have characteristic proportions of different isoenzymes. Hence, analyzing the pattern of isoenzymes present in the plasma can help identify the specific site of tissue damage. For instance, the levels of creatine kinase (CK) in the plasma are commonly measured for diagnosing myocardial infarction (MI). This diagnostic approach is particularly valuable when interpreting the electrocardiogram (ECG) becomes challenging, such as in cases with a history of heart disease.

1.
The quaternary structure of isoenzymes: Many isoenzymes consist of different subunits combined in different ways. For example, CK exists in three isoenzymes. Each isoenzyme is a dimer composed of two polypeptides, namely B and M subunits, arranged in one of three combinations: CK1 = BB, CK2 = MB, and CK3 = MM. Each CK isoenzyme exhibits a distinctive electrophoretic mobility (refer to Fig. 5.21). [Note: The BB isoform is predominantly found in the brain, while skeletal muscle contains mostly MM isoform. In cardiac muscle, about one third is MB, with the rest being MM.] 

2.
Diagnosing myocardial infarction: Biomarkers, which are proteins with cardiac specificity, are measured in the blood to aid in the diagnosis of MI. The CK2 (MB) isoenzyme accounts for more than 5% of the total CK activity only in myocardial muscle. The presence of this hybrid isoenzyme in plasma is almost exclusive to myocardial infarction. Following an acute MI, CK2 starts appearing in the plasma within 4–8 hours after the onset of chest pain, reaches its peak activity at around 24 hours, and returns to baseline levels within 48–72 hours (Fig. 5.22). Troponins T (TnT) and I (TnI) are regulatory proteins involved in muscle contractility. Cardiac-specific isoforms (cTn) of these proteins are released into the plasma as a response to cardiac damage. They are highly sensitive and specific indicators of cardiac tissue damage. cTn levels increase in plasma within 4–6 hours after an MI, peak between 24–36 hours, and remain elevated for 3–10 days. Elevated cTn levels, along with the clinical presentation and characteristic ECG changes, are currently considered the "gold standard" for diagnosing an MI.

X. CHAPTER SUMMARY
Isoenzymes (also called isozymes) are enzymes that catalyze the same reaction. However, they do not necessarily have the same physical properties because of genetically determined differences in amino acid sequence. For this reason, isoenzymes may contain different numbers of charged amino acids, which allows electrophoresis (the movement of charged particles in an electric field) to separate them (Fig. 5.21). Different organs commonly contain characteristic proportions of different isoenzymes. The pattern of isoenzymes found in the plasma may, therefore, serve as a means of identifying the site of tissue damage. For example, the plasma levels of creatine kinase (CK) are commonly determined in the diagnosis of myocardial infarction (MI). They are particularly useful when the electrocardiogram (ECG) is difficult to interpret such as when there have been previous episodes of heart disease.
1.
Isoenzyme quaternary structure: Many isoenzymes contain different subunits in various combinations. For example, CK occurs as three isoenzymes. Each isoenzyme is a dimer composed of two polypeptides (called B and M subunits) associated in one of three combinations: CK1 = BB, CK2 = MB, and CK3 = MM. Each CK isoenzyme shows a characteristic electrophoretic mobility (see Fig. 5.21). [Note: Virtually all CK in the brain is the BB isoform, whereas it is MM in skeletal muscle. In cardiac muscle, about one third is MB with the rest as MM.] 2.
Diagnosis of myocardial infarction: Measurement of blood levels of proteins with cardiac specificity (biomarkers) is used in the diagnosis of MI. Myocardial muscle is the only tissue that contains >5% of the total CK activity as the CK2 (MB) isoenzyme. Appearance of this hybrid isoenzyme in plasma is virtually specific for infarction of the myocardium. Following an acute MI, CK2 appears in plasma within 4–8 hours following onset of chest pain, reaches a peak of activity at ~24 hours, and returns to baseline after 48–72 hours (Fig. 5.22). Troponins T (TnT) and I (TnI) are regulatory proteins involved in muscle contractility. Cardiac-specific isoforms (cTn) are released into the plasma in response to cardiac damage. They are highly sensitive and specific for damage to cardiac tissue. cTn appear in plasma within 4–6 hours after an MI, peak in 24–36 hours, and remain elevated for 3–10 days. Elevated cTn, in combination with the clinical presentation and characteristic changes in the ECG, are currently considered the “gold standard” in the diagnosis of an MI.
X. CHAPTER SUMMARY