Lysine biosynthesis in the yeast Candida maltosa: properties of some enzymes and regulation of the biosynthetic pathway

Abstract

AbstractFour enzymes of the lysine biosynthetic pathway of Candida maltosa were investigated and the regulatory pattern was established. α‐Aminodipate reductase (Mr 160,000), saccharopine reductase (Mr 90,000) and saccharopine dehydrogenase (Mr 45,000) were separated from each other. Homocitrate synthase was sensitive to the separation methods used. The formation of saccharopine reductase was constitutive, whereas the synthesis of homocitrate synthase, α‐aminoadipate reductase and saccharopine dehydrogenase were regulated by the general control of amino acid biosynthesis. l‐Lysine (Ki = 5.0 mm), l‐thialysine (Ki = 15 mm) and dl‐α‐aminoadipate (Ki = 5.1 mm) inhibited homocitrate synthase, the first enzyme of the pathway. The inhibition was competitive with respect to 2‐oxoglutarate. The Km‐values of the enzyme were estimated to be 0.033 mm for acetyl‐CoA and 0.025 mm for 2‐oxoglutarate. The Michaelis constants of the α‐aminoadipate reductase were calculated to be 0.20 mm for α‐aminoadipate, 0.50 mm for ATP, 0.10 mm for NADPH and 3.8 mm for MgCl2. The Km‐values of the saccharopine reductase were 0.55 mm for saccharopine and 0.13 mm for NADP+, respectively. The values of the saccharopine dehydrogenase were estimated to be 1.6 mm for l‐lysine, 0.66 mm for 2‐oxoglutarate and 0.23 mm for NADH. Furthermore, a number of amino acids were found to be effectors of the enzyme reactions.