METAL AS A NOVEL TYPE OF THE ENZYME-SUBSTRATE - METALLIC CADMIUM PHOTOGENERATED IN THE SYSTEM CDS-FORMATE AS A SUBSTRATE OF THE NAD-DEPENDENT HYDROGENASE

The process of NAD+ photoreduction under the coupled action of CdS sem iconductor and NAD-dependent hydrogenase from hydrogen-oxidizing bacte rium Alcaligenes eutrophus may be divided into light and dark stages. At the first stage, illumination of the system leads to the photooxida tion of the sacrificial electron donor and results in the reduction of the semiconductor surface. At the second dark stage NAD+ is reduced t o NADH in the presence of hydrogenase. Atoms of metallic Cd(0) are sho wn to be the true substrate of the enzymatic reaction. The prerequisit e for the electron transfer from Cd(0) to hydrogenase is enzyme adsorp tion on the semiconductor surface. the redox center of the hydrogenase reacting with Cd(0) atoms resides on the flavin-containing heterodime r of the protein. The activity of the hydrogenase immobilized on CdS i n the reaction of NAD+ reduction by metallic Cd is close to the enzyme activity with the physiological substrates in solution. Thus, the fir st example of a metal being the substrate of the enzymatic process is presented.