Defining a direction: Electron transfer and catalysis in Escherichia coli complex II enzymes

Abstract

There are two homologous membrane-bound enzymes in Escherichia coli that catalyze reversible conversion between succinate/fumarate and quinone/quinol. Succinate;ubiquinone reductase (SQR) is a component of aerobic respiratory chains, whereas quinol:fumarate reductase (QFR) utilizes menaquinol to reduce fumarate in a final step of anaerobic respiration. Although, both protein complexes are capable of supporting bacterial growth on either minimal succinate or fumarate media, the enzymes are more proficient in their physiological directions. Here we evaluate factors that may underlie this catalytic bias.

Probing the Menasemiquinone Binding Mode to Nitrate Reductase A by Selective 2H and 15N Labeling, HYSCORE Spectroscopy, and DFT Modeling

“…[23] The latter value is the smallest ever reportedf or methyl protons of vitamin Km olecules bound to proteins or dissolved in protic solvents. [16,29,34,35,39,42,63,64] For the symmetric IP 4 -MSK model,t he calculated A iso ( 1 H Me ) % 7.5 MHz is very similar to the experimental value A iso ( 1 H Me ) % 7.8 MHz determined from X-band cw ENDOR spectra of MK 4 C À (4 refers to the number of isoprenoid units of the menasemiquinone radical anion)i n2 -isopropanol.…”

“…Therefore, the large decreaseb etween the value measured for MSK D compared with that for MSK 4 C À must originate from the stronga symmetry of the spin density distribution in MSK D .T his is illustrated in Figure6,w hich compares the Mulliken spin populations on the ring carbon and oxygen atoms in the Im-MSK model and in the IP 4 -MSK model.I tc an be seen that the stronger hydrogen bond to the carbonyl oxygen O1, as compared with oxygen O4, leads to an increase of spin density on carbon 3b ut a % 33 %d ecrease of the spin density on carbon 2, which carries the methyl group. This value is in close agreement with the % 29 %d ecrease that was estimated in our previous work by using the McConnell relationa nd the experimental A iso ( 1 H Me ) values for MSK D and for the corresponding radical measured in 2-propanol.…”

“…[4,5] Importantly, modulating thesei nteractions allows to tune the stability and the lifetimeo ft he highly reactive semiquinone intermediate, In vivo specific isotope labelinga tt he residue or substituent level is used to probe menasemiquinone (MSK) binding to the quinol oxidation site of respiratory nitrate reductase A( NarGHI) from E. coli.…”

Redox State of Flavin Adenine Dinucleotide Drives Substrate Binding and Product Release in Escherichia coli Succinate Dehydrogenase

“…It is widely believed that covalent attachment increases the E m value of the FAD cofactor by ~100 mV to facilitate electron transfer between its isoalloxazine ring and succinate/fumarate (16, 21, 54). This assertion is based on two key observations.…”

Oxidative Phosphorylation as a Target Space for Tuberculosis: Success, Caution, and Future Directions

“…They couple the oxidation of succinate to the reduction of quinone via both FAD and heme co-factors (44), thereby playing important roles in both carbon metabolism and pmf generation. M. tuberculosis encodes two SDH enzymes ( sdh1 , Rv0249c-Rv0247c ; sdh2 , Rv3316-Rv3319 ), as well as a separate fumarate reductase with possible bidirectional behaviour.…”