Final answer:
LDH-A4, or M4, has a low Km value for pyruvate, resulting in a higher affinity and an efficient conversion of pyruvate to lactate in skeletal muscles. This is advantageous for high-intensity, anaerobic activities. In contrast, cardiac muscle LDH1 has a higher Km, suitable for its normal aerobic function.
Step-by-step explanation:
Lactate dehydrogenase (LDH) plays a critical role in the conversion of pyruvate to lactate, an essential process during anaerobic respiration. Skeletal muscles frequently operate under anaerobic conditions, especially during intense exercise when oxygen supply is limited. The LDH5 isoenzyme, also known as LDH-A4 or M4, is more abundant in skeletal muscles and has a low Km value for pyruvate, indicating a high affinity for pyruvate. This allows for a more efficient conversion of pyruvate to lactate, thus enabling the muscle to continue to generate energy under anaerobic conditions, which can be critical during short bursts of high-intensity activity.
By contrast, the cardiac muscle predominantly contains the LDH1 or H1 form of the enzyme, which has a higher Km value, meaning it has a lower affinity for pyruvate and converts it to lactate at a slower rate. This is suitable for the heart, which usually operates aerobically.
In conditions such as myocardial infarction, the presence of LDH1 in plasma can be indicative of cardiac damage due to this isoenzyme's tissue-specific distribution. However, the higher conversion of pyruvate to lactate by LDH-A4 in skeletal muscles represents an evolutionary advantage for activities that require energy in the absence of sufficient oxygen.