Expression, Purification, and in vitro Enzyme Activity Assay of a Recombinant Aldehyde Dehydrogenase from Thermus thermophilus, Using an Escherichia coli Host

Based on previous in-depth characterisation, aldehyde dehydrogenases (ALDH) are a diverse superfamily of enzymes, in terms of both structure and function, present in all kingdoms of life. They catalyse the oxidation of an aldehyde to carboxylic acid using the cofactor nicotinamide adenine dinucleotide (phosphate) (NAD(P)⁺), and are often not substrate-specific, but rather have a broad range of associated biological functions, including detoxification and biosynthesis. We studied the structure of ALDH_Tt from Thermus thermophilus, as well as performed its biochemical characterisation. This allowed for insight into its potential substrates and biological roles. In this protocol, we describe ALDH_Tt heterologous expression in E. coli, purification, and activity assay (based on Shortall et al., 2021). ALDH_Tt was first copurified as a contaminant during caa3-type cytochrome oxidase isolation from T. thermophilus. This recombinant production system was employed for structural and biochemical analysis of wild-type and mutants, and proved efficient, yielding approximately 15–20 mg/L ALDH_Tt. For purification of the thermophilic his-tagged ALDH_Tt, heat treatment, immobilized metal affinity chromatography (IMAC), and gel filtration chromatography were used. The enzyme activity assay was performed via UV-Vis spectrophotometry, monitoring the production of reduced nicotinamide adenine dinucleotide (NADH).