Ferredoxin:NADP+ oxidoreductase in junction with CdSe/ZnS quantum dots: characteristics of an enzymatically active nanohybrid

Ferredoxin:NADP(+) oxidoreductase (FNR) is a plant and cyanobacterial photosynthetic enzyme, also found in non-photosynthetic tissues, where it is involved in redox reactions of biosynthetic pathways. In vivo it transfers electrons to nicotinamide adenine dinucleotide phosphate (NADP(+)), forming its reduced version, NADPH, while in vitro it can also use NADPH to reduce several substrates, such as ferricyanide, various quinones and nitriles. As an oxidoreductase catalyzing reaction of a broad range of substrates, FNR may be used in biotechnological processes. Quantum dots are semiconductor nanocrystals of a few to several nanometers diameter, having very useful luminescent properties. We present the spectroscopic and functional characteristics of a covalent conjugation of FNR and CdSe/ZnS quantum dots. Two types of quantum dots, of different diameter and emission maximum (550 and 650 nm), were used for comparison. Steady-state fluorescence and gel electrophoresis confirmed efficient conjugation, while fluorescence correlation spectroscopy (FCS) allowed for determination of the conjugates' radii. The nanohybrids sustained enzymatic activity; however, changes in maximal reaction rates and Michaelis constant were found. Detailed analysis of the kinetic parameters showed 6that the changes in the enzyme activity depend on the substrate used for activity measurement but also on the size of the quantum dots. The presented nanohybrids, as the first example using plant and photosynthetic enzyme as a protein partner, may became a tool to study photosynthesis as well as other biosynthetic and biotechnological processes, involving enzymatically catalyzed electron transfer.