The synthesis of 15N- and deuterium-substituted, spin-labeled analogues of NAD+ and their use in EPR studies of dehydrogenases

Nitroxide spin-spin interactions: applications to protein structure and dynamics

Measurement of the distance between two spin label probes in proteins permits the spatial orientation of elements of defined secondary structure. By using site-directed spin labeling, it is possible to determine multiple distance constraints and thereby build tertiary and quaternary structural models as well as measure the kinetics of structural changes. New analytical methods for determining interprobe distances and relative orientations for uniquely oriented spin labels have been developed using global analysis of multifrequency electron paramagnetic resonance data. New methods have also been developed for determining interprobe distances for randomly oriented spin labels. These methods are being applied to a wide range of structural problems, including peptides, soluble proteins, and membrane proteins, that are not readily characterized by other structural techniques.

Interaction of glyceraldehyde-3-phosphate dehydrogenase with AMP as studied by means of a spin-labeled analog

The binding of a spin-labeled AMP analog to tetrameric glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle is described. The spin label, perdeuterated and 15N-substituted 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, was attached to C-8 of AMP (C8-SL-AMP). Up to 8 equivalents of C8-SL-AMP bind per enzyme tetramer, i.e., 2 per monomer. Combining sites are the adenine subsite of the coenzyme-binding domain and the phosphate site. Glyceraldehyde 3-phosphate causes a conformational change in the enzyme that brings C8-SL-AMP molecules bound to adjacent R-axis-related subunits closer to one another by 0.2-0.3 nm and allows for spin-spin interaction between the nitroxide radicals. Similar, but less pronounced structural changes take place upon lowering the pH from 8 to 7. Addition of a single equivalent of NAD+ to a complex of the enzyme with 7.6 equivalents of C8-SL-AMP leads to the release of almost 4 C8-SL-AMP molecules. This supports our previous findings that binding of just one NAD+ molecule induces conformational changes in all four subunits.

Interaction of spin-labeled nicotinamide adenine dinucleotide phosphate with chicken liver fatty acid synthase

The spatial relationships between the four reduced nicotinamide adenine dinucleotide phosphate (NADPH) binding sites on chicken liver fatty acid synthase were explored with electron paramagnetic resonance (EPR) and spin-labeled analogues of NADP+. The analogues were prepared by reaction of NADP+ with 2,2,5,5-tetramethyl-1-oxy-3-pyrroline-3-carboxylic acid, with 1,1'-carbonyldiimidazole as the coupling reagent. Several esterification products were characterized, and the interaction of the N3' ester of NADP+ with the enzyme was examined in detail. Both 1H13, 14N and 2H13, 15N spin-labels were used: the EPR spectrum was simpler, and the sensitivity greater, for the latter. The spin-labeled NADP+ is a competitive inhibitor of NADPH in fatty acid synthesis, and an EPR titration of the enzyme with the modified NADP+ indicates four identical binding sites per enzyme molecule with a dissociation constant of 124 microM in 0.1 M potassium phosphate and 1 mM ethylenediaminetetraacetic acid (pH 7.0) at 25 degrees C. The EPR spectra indicate the bound spin-label is immobilized relative to the unbound probe. No evidence for electron-electron interactions between bound spin-labels was found with the native enzyme, the enzyme dissociated into monomers, or the enzyme with the enoyl reductase sites blocked by labeling the enzyme with pyridoxal 5'-phosphate. Furthermore, the EPR spectrum of bound ligand was the same in all cases. This indicates that the bound spin-labels are at least 15 A apart, that the environment of the spin-label at all sites is similar, and that the environment is not altered by major structural changes in the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)