Formation of Supplementary Metal-Binding Centers in Proteins under Stress Conditions

In many proteins, supplementary metal-binding centers appear under stress conditions. They are known as aberrant or atypical sites. Physico-chemical properties of proteins are significantly changed after such metal binding, and very stable protein aggregates are formed, in which metals act as "cross-linking" agents. Supplementary metal-binding centers in proteins often arise as a result of posttranslational modifications caused by reactive oxygen and nitrogen species and reactive carbonyl compounds. New chemical groups formed as a result of these modifications can act as ligands for binding metal ions. Special attention is paid to the role of cysteine SH-groups in the formation of supplementary metal-binding centers, since these groups are the main target for the action of reactive species. Supplementary metal binding centers may also appear due to unmasking of amino acid residues when protein conformation changing. Appearance of such centers is usually considered as a pathological process. Such unilateral approach does not allow to obtain an integral view of the phenomenon, ignoring cases when formation of metal complexes with altered proteins is a way to adjust protein properties, activity, and stability under the changed redox conditions. The role of metals in protein aggregation is being studied actively, since it leads to formation of non-membranous organelles, liquid condensates, and solid conglomerates. Some proteins found in such aggregates are typical for various diseases, such as Alzheimer's and Huntington's diseases, amyotrophic lateral sclerosis, and some types of cancer.

Role of Nitric Oxide-Derived Metabolites in Reactions of Methylglyoxal with Lysine and Lysine-Rich Protein Leghemoglobin

Carbonyl stress occurs when reactive carbonyl compounds (RCC), such as reducing sugars, dicarbonyls etc., accumulate in the organism. The interaction of RCC carbonyl groups with amino groups of molecules is called the Maillard reaction. One of the most active RCCs is α-dicarbonyl methylglyoxal (MG) that modifies biomolecules forming non-enzymatic glycation products. Organic free radicals are formed in the reaction between MG and lysine or Nα-acetyllysine. S-nitrosothiols and nitric oxide (^•NO) donor PAPA NONOate increased the yield of organic free radical intermediates, while other ^•NO-derived metabolites, namely, nitroxyl anion and dinitrosyl iron complexes (DNICs) decreased it. At the late stages of the Maillard reaction, S-nitrosoglutathione (GSNO) also inhibited the formation of glycation end products (AGEs). The formation of a new type of DNICs, bound with Maillard reaction products, was found. The results obtained were used to explain the glycation features of legume hemoglobin-leghemoglobin (Lb), which is a lysine-rich protein. In Lb, lysine residues can form fluorescent cross-linked AGEs, and ^•NO-derived metabolites slow down their formation. The knowledge of these processes can be used to increase the stability of Lb. It can help in better understanding the impact of stress factors on legume plants and contribute to the production of recombinant Lb for biotechnology.

Electrophilic Signaling: The Role of Reactive Carbonyl Compounds

Reactive carbonyl compounds (RCC) are a group of compounds with clearly pronounced electrophilic properties that facilitate their spontaneous reactions with numerous nucleophilic reaction sites in proteins, lipids, and nucleic acids. The biological functions of RCC are determined by their concentration and governed by the hormesis (biphasic reaction) principle. At low concentrations, RCC act as signaling molecules activating defense systems against xenobiotics and oxidizers, and at high concentrations, they exhibit the cytotoxic effect. RCC participate in the formation of cell adaptive response via intracellular signaling pathways involving regulation of gene expression and cytoplasmic mechanisms related to the structure-functional rearrangements of proteins. Special attention in this review is given to the functioning of electrophiles as mediators of cell general adaption syndrome manifested as the biphasic response. The hypothesis is proposed that electrophilic signaling can be a proto-signaling system.

Dinitrosyl iron complexes: Formation and antiradical action in heart mitochondria

Mitochondria are widely known as a major source of reactive oxygen and nitrogen species for the cardiovascular system. Numerous studies established that superoxide anion radical production by heart mitochondria is only slightly suppressed under conditions of deep hypoxia, but is completely blocked under anoxia. It was found also that dinitrosyl iron complexes (DNIC) compare favourably with other physiologically active derivatives of nitric oxide (NO). DNIC with glutathione effectively scavenge superoxide radicals generated by mitochondria at different partial pressures of oxygen. Under conditions of simulated hypoxia, the synthesis of thiol-containing DNIC takes place in mitochondria and is concomitant with a significant decrease in the concentration of NO metabolites at the reoxygenation step. Free NO required for DNIC synthesis is generated in the reaction of S-nitrosothiols with superoxide or during single-electron oxidation of the nitroxyl radical (HNO) by coenzyme Q. Plausible mechanisms of antiradical effects of DNIC and their protective role in oxidative stress induced by hypoxia/reoxygenation of myocardial tissues are considered. © 2018 BioFactors, 44(3):237-244, 2018.