Which one of the following is the systematic name for Lactate dehydrogenase ?

Question

Accepted Answer

The correct option is 1: E.C.1.1.1.27

Enzymes are systematically named and classified by the Enzyme Commission (E.C.) system, which assigns a four-part number to each enzyme based on the chemical reaction it catalyzes. This system is crucial for standardizing biochemistry across the scientific community. Lactate dehydrogenase (LDH) catalyzes a reversible reaction, specifically the conversion of L-lactate to pyruvate and back, using the coenzyme NAD+/NADH. This reaction involves the transfer of a hydrogen atom (and electrons), making it an oxidation-reduction process. The E.C. number E.C.1.1.1.27 is precisely derived from this functional activity. The first digit, '1', immediately identifies LDH as an Oxidoreductase, the class of enzymes that catalyze the transfer of electrons from one molecule to another. The second digit, '1', signifies that the enzyme acts upon the primary functional group, the C-OH group (alcohol group), of the donor molecule, which is L-lactate. The third digit, also '1', specifies that NAD+ or NADP+ is used as the electron acceptor. Finally, the fourth number, '27', is the unique serial number given to Lactate dehydrogenase within this specific sub-subclass, confirming E.C.1.1.1.27 as its definitive systematic name. Knowing how to decipher the E.C. code is a fundamental skill for understanding enzyme function.

The incorrect option is 2: E.C.1.1.1.1

This E.C. number also belongs to the Oxidoreductase class (1), acts on a C-OH group (1), and uses NAD+ or NADP+ as an acceptor (1), placing it in the same sub-subclass as LDH. However, the serial number '1' designates this enzyme as Alcohol dehydrogenase, which catalyzes the conversion of primary and secondary alcohols to aldehydes or ketones. While the overall reaction type is similar (a redox reaction), the specific substrates differ, meaning E.C.1.1.1.1 and E.C.1.1.1.27 are distinct enzymes that perform different roles in metabolism. The specificity of the fourth digit is what differentiates Lactate dehydrogenase from Alcohol dehydrogenase.

The incorrect option is 3: E.C.2.3.1.6

The classification E.C.2.3.1.6 is incorrect because the first digit, '2', immediately places this enzyme into the Transferase class, not the Oxidoreductase class to which Lactate dehydrogenase belongs. Transferases are enzymes that catalyze the transfer of a functional group (like an amino, carboxyl, or acetyl group) from one molecule to another, which is not the primary function of LDH. The remaining digits for this option, 3.1.6, refer to an enzyme that transfers an acyl group (3) and specifically is Acetyl-CoA C-acetyltransferase, also known as thiolase. Thiolase is centrally involved in the catabolism of fatty acids through beta-oxidation, a biochemical pathway entirely separate from the reversible conversion of lactate and pyruvate.

The incorrect option is 4: E.C.1.11.1.6

Although this E.C. number begins with '1', indicating it is an Oxidoreductase, the second digit is '11', which places it in a different subclass of oxidation-reduction enzymes. The subclass '11' is reserved for enzymes that act on a peroxide as an acceptor, which are known as Peroxidases. More specifically, E.C.1.11.1.6 is the systematic name for Catalase. Catalase is an extremely important enzyme that breaks down the toxic reactive oxygen species hydrogen peroxide into water and oxygen. This function, which is a disposal mechanism for a harmful cellular byproduct, is fundamentally different from the glycolytic role of Lactate dehydrogenase.