Characterisation of the active/de-active transition of mitochondrial complex I

Abstract

Oxidation of NADH in the mitochondrial matrix of aerobic cells is catalysed by mitochondrial complex I. The regulation of this mitochondrial enzyme is not completely understood. An interesting characteristic of complex I from some organisms is the ability to adopt two distinct states: the so-called catalytically active (A) and the de-active, dormant state (D). The A-form in situ can undergo de-activation when the activity of the respiratory chain is limited (i.e. in the absence of oxygen). The mechanisms and driving force behind the A/D transition of the enzyme are currently unknown, but several subunits are most likely involved in the conformational rearrangements: the accessory subunit 39kDa (NDUFA9) and the mitochondrially encoded subunits, ND3 and ND1. These three subunits are located in the region of the quinone binding site. The A/D transition could represent an intrinsic mechanism which provides a fast response of the mitochondrial respiratory chain to oxygen deprivation. The physiological role of the accumulation of the D-form in anoxia is most probably to protect mitochondria from ROS generation due to the rapid burst of respiration following reoxygenation. The de-activation rate varies in different tissues and can be modulated by the temperature, the presence of free fatty acids and divalent cations, the NAD(+)/NADH ratio in the matrix, the presence of nitric oxide and oxygen availability. Cysteine-39 of the ND3 subunit, exposed in the D-form, is susceptible to covalent modification by nitrosothiols, ROS and RNS. The D-form in situ could react with natural effectors in mitochondria or with pharmacological agents. Therefore the modulation of the re-activation rate of complex I could be a way to ameliorate the ischaemia/reperfusion damage. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference. Guest Editors: Manuela Pereira and Miguel Teixeira.

Keywords: A/D transition; Conformational change; Ischaemia/reperfusion; Mitochondrial complex I; Thiol modification.

Graphical abstract

Fig. 1

Scheme illustrating the functional and structural aspects of the A/D transition of mitochondrial complex I. A. Time-course of the NADH:ubiquinone (Q) oxidoreductase reaction catalysed by the active (A), deactive (D^SH) and irreversibly modified (D^S⁎) forms of the enzyme curves. The A-form of the enzyme catalyses the fast physiological NADH:Q oxidoreductase reaction with a linear rate insensitive to cysteine-modifying reagents such as nitrosothiols, peroxynitrite or ROS. NADH:Q oxidoreductase reaction catalysed by the D-form of the enzyme (D^SH) proceeds with a lag phase when the D→A conversion takes place. The lag phase is significantly prolonged in the presence of divalent cations or at alkaline pH. Covalent modification of the Cys-39 residue of subunit ND3 of the D-form prevents complex I from undergoing turnover-dependent reactivation and irreversibly inhibits the enzyme (D^S⁎). B. Possible sequence of events in conditions of ischaemia or lack of oxygen. If oxygen is absent the A-form (A) spontaneously converts to the D-form (D^SH), which can be re-activated back in the case of reoxygenation (given substrate ubiquinone availability). Depending on the particular conditions in situ the ND3 thiol group residue can be reversibly S-nitrosated by nitrosothiols (D^SNO) or irreversibly oxidised by peroxynitrite or ROS (D^S⁎). In the latter case the enzyme is irreversibly inhibited making this the initial step of mitochondrial damage in I/R. S-nitrosated enzyme (D^SNO) can be reduced by mitochondrial glutathione and thioredoxin therefore further delaying the re-activation of the complex I at the early stages of reperfusion.