ALDH2

Related Reading

Acetaldehyde dehydrogenase, abbreviated ALDH, a type of aldehyde dehydrogenase, is responsible for catalyzing the oxidation of acetaldehyde to acetic acid. Ethanol dehydrogenase in the liver is responsible for oxidizing ethanol (a component of wine) to acetaldehyde, and the resulting acetaldehyde is further converted to harmless acetic acid (a component of vinegar) catalyzed by acetaldehyde dehydrogenase as a substrate.

Introduction

Acetaldehyde can be covalently bonded to some proteins, phospholipids, nucleic acids, etc. in the body. The oxidation of acetaldehyde in the liver and other organs is catalyzed by ALDH, a polypeptide tetramer, and although there are at least seven different genes encoding ALDH (ALDH1, ALDH2, ALDH7) based on the tetrameric structure and other biochemical characteristics, only ALDH1 and ALDH2 are considered "true" ALDH based on substrate specificity and subunit composition. "ALDH2 is located in mitochondria, while ALDH, ALDH3, and ALDH4 are located in cytosol. ADH and ALDH2 are pure tetramers, consisting of subunits with a molecular weight of 54,000. Human erythrocytes contain an ALDH that is identical to the ALDH within the hepatic cytosol.

Figure 1. Alcohol metabolism and enzymes that strongly impact alcohol consumption. (Wang Q., et al. 2021)

Structure

The active site of ALDH2 is divided into two halves by the nicotinamide ring of NAD+. The enzyme encoded by the human ALDH2 gene is a tetrameric enzyme containing three structural domains; two dinucleotide-binding structural domains and a three-stranded beta-sheet domain. Adjacent to the A side of the nicotinamide ring (Pro-R) are three cysteine clusters (Cys301, Cys302, and Cys303), and adjacent to the B side (Pro-S) are Thr244, Glu268, Glu476, and an ordered water molecule bound to Thr244 and Glu476.

Properties

ALDH2 is a zinc-containing enzyme. Its molecule consists of two subunits, one subunit located in the active center of the enzyme and the other acts as a stabilizer of the quaternary structure. In the presence of coenzyme, it catalyzes the dehydrogenation of certain primary or secondary alcohols, aldehydes and ketones, including ethanol, and catalyzes the dehydrogenation of n-butyraldehyde, cinnamaldehyde and benzaldehyde at a greater rate than acetaldehyde. The hydrogen shed is accepted by NAD, making it a reduced coenzyme I. Serum ethanol dehydrogenase activity is a diagnostic indicator of parenchymal cell damage in acute hepatitis. Serum enzyme activity is negative in normal subjects or in patients without secondary liver disease.

Functions

The ethanol dehydrogenase in the liver is responsible for the oxidation of ethanol (a component of alcohol) to acetaldehyde, and the resulting acetaldehyde is further converted to harmless acetic acid (i.e., a component of vinegar) catalyzed by acetaldehyde dehydrogenase as a substrate. Acetaldehyde is more toxic than ethanol and is one of the main causes of hangovers. Moreover, acetaldehyde is suspected to be carcinogenic, and it has been linked to the development of human tumors. The main enzyme responsible for the conversion of acetaldehyde in the body is acetaldehyde dehydrogenase (ALDH) in the liver. There is a clear difference in the catalytic rate between ALDH1 and ALDH2.