Biology subjects

Samhan-Arias, A. K.

Publications and source records attributed to Samhan-Arias, A. K..

2 recordsLinked to original sources

bioRxiv2026

Ubiquinone is a hysteretic modulator of the NADH:cytochrome b5 reductase activity of human Cb5R

BackgroundCytochrome b5 reductase is a flavoprotein that transfers electrons from NADH to multiple electron acceptors, such as cytochrome b5 or ubiquinone. Hysteresis is a phenomenon characterized by a slow transition between active and inactive catalytic states, leading to a lag phase in enzymatic activity. In this study, the effect of the soluble analogue of ubiquinone named 2,3 dimethoxy-5-methyl-1,4 benzoquinone (CoQ0) on the NADH:Cb5 reductase activity of recombinant human soluble Cb5R, using recombinant human soluble Cb5 as a substrate was evaluated. The aim of this study was to determine whether ubiquinone exerts a hysteretic modulation of this activity based on previous studies supporting that microsomal reduction of cytochrome b5 is controlled by redox hysteresis. ResultsThe NADH:cytochrome b5 reductase activity of Cb5R was characterized at different concentrations of Cb5R, cytochrome b5, and CoQ0 by monitoring the reduction of cytochrome b5. The addition of CoQ0 induced the appearance of a lag phase, whose duration increased with the concentration of CoQ0 and decreased with higher concentrations of cytochrome b5 or Cb5R. Additionally, a concentration-dependent decrease in the maximum rate of reduction and the appearance of positive cooperativity was observed in the presence of CoQ0 which resulted in leading to lower KM values for cytochrome b5. This suggests the formation of a CoQ0:Cb5R complex altering the interaction between the reductase and cytochrome b5 which increases the affinity for cytochrome b5. Cyclic voltammetry data support the formation of CoQ0/protein complex that could be responsible for the hysteretic behavior. ConclusionsThese results support the hypothesis that CoQ0 is a hysteretic modulator and inhibitor of the NADH: cytochrome b5 reductase activity of human Cb5R.

biochemistry↗

bioRxiv2026

Redox hysteresis controls the NADH-dependent reduction of cytochrome b5 in rat microsomes

In enzymology, hysteresis is manifested as a time-dependent shift in the kinetic behavior of an enzyme. Through hysteresis, the activation or inhibition of a biological pathway can be regulated by a molecule or metabolite that acts as a hysteretic modulator of the enzyme within that metabolic route. This mechanism of regulation contrasts with those that act on gene expression leading to modulation of enzyme protein levels. Through hysteresis, the amplitude of natural oscillations in metabolic pathways can be adjusted according to the levels of a metabolite that might be beneficial for cells. At physiological level, the slow response of hysteretic enzymes to changes, in the cellular levels of substrates, allows a time-dependent buffering effect on certain metabolites. Understanding the mechanisms and properties of hysteretic enzymes has been important for developing new therapies and improving our understanding of these enzymes in biological systems. However, due to their complex kinetics, the study of hysteretic enzymes has remained a challenge over time. In this study, we characterized the reduction of cytochrome b5 by NADH-dependent microsomal enzymes from rat liver using recombinant purified cytochrome b5, coenzyme Q10 and coenzyme Q0, as substrates, to mimic the conditions found in biological membranes, where competition between cytochrome b5 and other substrates might influence their reduction. We found a lag-time-dependent behavior in the cytochrome b5 reduction compatible with the existence of hysteretic modulation induced by endogenous molecules present in these membranes. Our data suggest that at least for the case of coenzyme Q10, fluctuations in its levels may impact metabolic pathways in which reduced cytochrome b5 levels play a key for the function of the cytochrome b5-dependent route.

biochemistry↗