Methionine sulfoxide reductases and cholesterol transporter STARD3 constitute an efficient system for detoxification of cholesterol hydroperoxides

Catalytic Mechanism of Mycobacterium tuberculosis Methionine Sulfoxide Reductase A

The oxidation of Met to methionine sulfoxide (MetSO) by oxidants such as hydrogen peroxide, hypochlorite, or peroxynitrite has profound effects on protein function. This modification can be reversed by methionine sulfoxide reductases (msr). In the context of pathogen infection, the reduction of oxidized proteins gains significance due to microbial oxidative damage generated by the immune system. For example, Mycobacterium tuberculosis (Mt) utilizes msrs (MtmsrA and MtmsrB) as part of the repair response to the host-induced oxidative stress. The absence of these enzymes makes Mycobacteria prone to increased susceptibility to cell death, pointing them out as potential therapeutic targets. This study provides a detailed characterization of the catalytic mechanism of MtmsrA using a comprehensive approach, including experimental techniques and theoretical methodologies. Confirming a ping-pong type enzymatic mechanism, we elucidate the catalytic parameters for sulfoxide and thioredoxin substrates (kcat/KM = 2656 ± 525 M-1 s-1 and 1.7 ± 0.8 × 106 M-1 s-1, respectively). Notably, the entropic nature of the activation process thermodynamics, representing ∼85% of the activation free energy at room temperature, is underscored. Furthermore, the current study questions the plausibility of a sulfurane intermediate, which may be a transition-state-like structure, suggesting the involvement of a conserved histidine residue as an acid-base catalyst in the MetSO reduction mechanism. This mechanistic insight not only advances our understanding of Mt antioxidant enzymes but also holds implications for future drug discovery and biotechnological applications.

Cholesterol is more readily oxidized than phospholipid linoleates in cell membranes to produce cholesterol hydroperoxides

Cholesterol is an essential component of cell membranes and serves as an important precursor of steroidal hormones and bile acids, but elevated levels of cholesterol and its oxidation products have been accepted as a risk factor for maintenance of health. The free and ester forms of cholesterol and fatty acids are the two major biological lipids. The aim of this hypothesis paper is to address the long-standing dogma that cholesterol is less susceptible to free radical peroxidation than polyunsaturated fatty acids (PUFAs). It has been observed that cholesterol is peroxidized much slower than PUFAs in plasma but that, contrary to expectations from chemical reactivity toward peroxyl radicals, cholesterol appears to be more readily autoxidized than linoleates in cell membranes. The levels of oxidation products of cholesterol and linoleates observed in humans support this notion. It is speculated that this discrepancy is ascribed to the fact that cholesterol and phospholipids bearing PUFAs are localized apart in raft and non-raft domains of cell membranes respectively and that the antioxidant vitamin E distributed predominantly in the non-raft domains cannot suppress the oxidation of cholesterol lying in raft domains which are relatively deficient in antioxidant.