ABSTRACT
The first point to be stressed in this chapter is that the entire chain of the oxidation-reduction reactions always requires hydrogen atom transfer. Although some steps in this reaction chain can occur as a result of only electron transfer, generally, along the entire chain, it always happens that reaction steps can be passed only by hydrogen atom transfer or proton-coupled electron transfer. A proton is required as an inalienable partner for an electron passing along the electron transfer chain to fulfill the electroneutrality principle and to facilitate the conformation transformations in enzymes, for which proton transfer could also appear as a useful tool. Enzymes are proteins that catalyze (i.e., increase the rates of chemical reactions). In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. Since enzymes are selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell. However, the outstanding role of hydrogen atom very often is not properly underlined. Since protons are always present in an aqueous media of the cell, it is usually assumed that the required number of the protons could be consumed from the aqueous media. Because of this, often only electron transfer is considered in the oxidation process, which sometimes hides the role of the protons and incorrectly presents the process. The oxidation reaction proceeds inside the cell in several stages. Under the action of special enzymes two electrons are transferred from the food product to any primary acceptor. Some other enzymes transfer the electrons further along the chain of the electron transfer to the secondary acceptor and so on. The process finishes with the formation of a water molecule for which each oxygen atom requires two electrons and two protons. As in photosynthesis, the oxidized forms, NAD+ and NADP+, of the molecules NAD and NADP play a very important roles here as primary electron acceptors. Similarly, FAD and FMN molecules play an important role in the oxidation-reduction process. A primary acceptor of electrons flavin adenine dinucleotide (FAD) is a redox cofactor involved in several important reactions in
metabolism. FAD can exist in two different redox states, to which it converts by accepting or donating electrons. The molecule consists of a riboflavin moiety (vitamin B2) bound to the phosphate group of an ADP molecule. The flavin group is bound to ribitol, a sugar alcohol, by a carbon-nitrogen bond, not a glycosidic bond. Thus, riboflavin is not technically a nucleotide; the name flavin adenine dinucleotide is a misnomer [90-92]. FAD can be reduced to FADH2 by accepting two hydrogen atoms (a net gain of two electrons). Via participation of the enzymes, FAD performs dehydrogenation of substances that contain –C-C-groups. Flavin mononucleotide (FMN) is a biomolecule that, during catalytic cycle, undergoes the reversible interconversion of oxidized (FMN), semiquinone (FMNH) and reduced (FMNH2) forms in the various oxidoreductases. FMN is a stronger oxidizing agent than NAD and is particularly useful because it can take part in both one-and two-electron transfer [93]. In all those reactions the electrons and the protons act together providing the redox process. Their partnership looks natural since they have small sizes, are mobile and with the same value of electric charge of opposite sign. Studying the oxidation process in a mitochondria helps in deducing the functions of hydrogen in an oxidation process. 5.2 Mitochondrion and Its Functions
5.2.1 Mitochondrion Design The main function of a mitochondria is to provide the cells with energy via the reaction called oxidative phosphorylation of food substrates; a mitochondrion functions as specific energy power station in a cell. Each mitochondria is surrounded by two membranes (Fig. 5.1). The membrane architecture is an inalienable feature in living organisms; it makes it possible to separate electric charges and to create a proton gradient across the membrane or, in other words, to charge the biological battery whose energy is employed for the synthesis of ATP molecules. And is it a function of the mitochondria to generate ATP molecules? How do they perform this function?
