Abstract text (incl. figure legends and references)

Scattering of electrons by atoms in electron microscopy produces coulomb potential maps and hence EM maps reflect the charge of atoms in protein complexes. At resolution ranges between 5 Å and 10 Å, atomic scattering amplitudes are usually weaker the more negatively charged atoms are. Here, we compare the cryo EM structure of Rhodobacter capsulatus formate dehydrogenase (FDH) in the air oxidized, sodium azide inhibited state with an enzyme that is reduced by treatment with NADH. FDH catalyzes the reversible oxidation of formate to carbon dioxide. The electrons gained in this reaction at the active site molybdenum atom are transferred via an electron-transfer pathway formed by numerous FeS clusters and flavin mononucleotide (FMN) onto nicotinamide adenine dinucleotide (NAD+). At a resolution of about 3.25 Å, the difference map between the oxidized and NADH reduced states qualitatively visualizes negative charges on electron accepting atoms in the electron transfer chain of the reduced map.