Final answer:
LDH tetramers are expressed differently across vertebrate tissues, with variations like LDH1 in the heart and LDH5 in the liver and skeletal muscle. These isoenzymes have different substrate affinities which contribute to their specific roles in metabolism.
Step-by-step explanation:
Expression of LDH Tetramers in Vertebrates
Lactate dehydrogenase (LDH) is an enzyme involved in the glycolytic pathway, facilitating the conversion of lactate to pyruvate. In vertebrates, LDH exists in multiple isoenzyme forms, known as tetramers, due to their four polypeptide structure. These isoenzymes include combinations of 'H chain' (heart-specific) and 'M chain' (muscle-specific) and are expressed differently across tissues. For instance, LDH₁ or H₄ is mainly found in the cardiac muscle, while LDH₅ or M₄ is abundant in skeletal muscle and liver. The evolutionary sequences of these isoenzymes, such as LDH-A, LDH-B, and LDH-C, also differ among species like the African frog and humans.
The affinity for substrates differs among these isoenzymes. For example, the skeletal muscle isoenzyme LDH5 has a lower Km value for pyruvate, indicating a higher affinity, which leads to a greater rate of pyruvate to lactate conversion. Conversely, the cardiac isoenzyme LDH1 has a higher Km value for pyruvate and thus a lower affinity, resulting in a reduced rate of conversion. The distribution and function of these isoenzymes reflect the metabolic requirements of different tissues.