Enzyme Activity Measurement for Homoserine Dehydrogenase Using Spectrophotometric Assays

Creative Enzymes is one of the few companies in the world that provide spectrophotometric assays for homoserine dehydrogenase. Its convoluted catalysis mechanism stops most companies from accurately measuring and characterizing homoserine dehydrogenases. With a strong team of enzymologists, Creative Enzymes is able to perform precise and fast measurement of the enzymatic activity of homoserine dehydrogenase. The quality of our results is assured with the most advanced spectrophotometric instruments.

A homoserine dehydrogenase (EC 1.1.1.3, HSD, or HSDH) is an enzyme that catalyzes the oxidation of L-homoserine, using NAD or NADP as the redox cofactor, to produce L-aspartic 4-semialdehyde. The enzyme belongs to the family of oxidoreductases. The enzyme also catalyzes the reversed reaction to give homoserine as the product, and the equilibrium is controlled by thermodynamics. The enzyme is involved in many biological pathways:

• Glycine, serine and threonine metabolism
• Cysteine and methionine metabolism
• Lysine biosynthesis
• Biosynthesis of secondary metabolites
• Microbial metabolism in diverse environments
• Biosynthesis of antibiotics

The aspartate metabolic pathway is involved in both storage of asparagine and in the synthesis of aspartate-family amino acids, therefore it is used for nitrogen and carbon storage and utilization. In this pathway, homoserine dehydrogenase catalyzes the third step, where aspartate beta-semialdehyde is reduced into homoserine, which is an essential intermediate in the biosynthesis of threonine, isoleucine, and methionine. In photosynthetic aspartate accumulates during the day and are used to synthesize other amino acids. At night, aspartate is converted to asparagine for storage. In mammals the enzyme is absent. Therefore mammals have to intake threonine, methionine, and isoleucine from external sources, because these essential amino acids are synthesized through homoserine dehydrogenase.

Although some research has been done to explore the molecular basis of its catalysis activity, the precise mechanism of complete homoserine dehydrogenase catalysis remains unknown. The reaction has been postulated to proceed through a bi-bi kinetic mechanism, where the NAD or NADP cofactor binds the enzyme first, followed by substrate binding and product dissociation. The cofactor then dissociates from the enzyme by the end of the reaction. Specially, homoserine dehydrogenase exhibits multi-order kinetics at various levels of the substrate. Although both NAD and NADP can be used as the redox cofactor, a specific homoserine dehydrogenase may exhibit preference on one over the other. These complications make activity measurement difficult for homoserine dehydrogenases. After years of tests and exploration, Creative Enzymes developed proper activity assays for the enzyme, based on deep understanding of its structure and properties. Therefore, our service provides the most reliable assay results.

Due to the important biological role of homoserine dehydrogenases and the fact that it is absent in mammals, the enzyme has been found valuable in nutritional, veterinary, and agricultural applications. However, the activity test is so challenging that even the activity assay kit has not become available in the market. To help customers solve the problem, Creative Enzymes offers spectrophotometric assays to accurately measure the enzymatic activity, which is critical to developing products with homoserine dehydrogenases.

Figure: The active site of homoserine dehydrogenase from Saccharomyces cerevisiae, showing the substrate homoserine (magenta, sphere) and the cofactor NAD (light blue). PDB: 1EBU
Reference: Stierand, K., & Rarey, M. (2010). ACS Medicinal Chemistry Letters, 1(9), 540–545. http://doi.org/10.1021/ml100164p