Electron Transport System or Respiratory Chain:
The hydrogen and electron transport system comprise many hydrogen and electron acceptors (Fig. 17-6). In fact oxidation reduction reactions in a biological system involve hydrogen and electron acceptors. Both hydrogen and electrons are passed from one acceptor to another. Thus, there are intermediate and final hydrogen acceptors. The final hydrogen acceptor is molecular oxygen.
Hydrogen (2H) is removed at various stages of glycolysis, oxidation of pyruvic acid and citric acid cycle. NAD is the initial compound for accepting hydrogen except in one instance, where the initial acceptor is FAD of flavoprotein.
The initial acceptors of hydrogen are coenzymes (NAD = nicotinamide adenine dinucleotide; FMN = flavin mononucleotide; CoQ = Co-enzyme Q; UQ = ubiquinone and cytochromes, b, Ci, c, and a, a3. Some workers have postulated the occurrence of two flavoproteins between NAD and CoQ and these are designated as FPN1 and FPN2.
It may be noted that the former flavoprotein has the redox potential similar to that of NAD/NADH system while the latter has a much higher one. A reference may be made to the structure of the mitochondrion and it may be recalled that the respiratory chain is situated in the Inner membrane of the mitochondrion (Fig. 17-7A, 7B). When treated with chemicals, the chain breaks down into four complexes and also two mobile carriers.
It consists of NADH dehydrogenase flavoprotein (FPN) with FMN as the prosthetic group. Flavoprotein is combined with non-heme iron of NADH dehydrogenase (Fe NHN).
This consists of succinate dehydrogenase, flavoprotein with FAD prosthetic group. The flavoprotein is combined with non-heme iron of succinate dehydrogenase (Fe NHS).
Mobile carriers exist between complexes I and III (CoQ) and II and III (UQ).
It comprises cytochrome b and cytochrome CI. Non-heme iron of complex III (FeNHR) is associated with cytochrome b.
Cytochrome a and a3 and bound copper constitute this complex. It is pertinent to mention that electrons follow either the pathway of complexes I, III and IV or II or III and IV.
In the following a brief account of the various initial hydrogen acceptors is given:
Nicotinamide Adenine Dinucleotide (NAD):
Hydrogen released from substrates other than succinate is accepted by NAD+ or NADP. The latter is phosphorylated form of NAD. The former was once referred to as Coenzyme I and the latter as Coenzyme II. Two nucleotides are there in NAD: nicotinamide ribose phosphate and adenine ribose phosphate (Fig. 17-7B).
It is worth remembering that N of nicotinamide is positively charged. Thus, in many instances NAD is also written as NAD+. Hydrogen atoms from the substrate NAD+ or NADP+ are reduced to NADH+ +H+. NAD is also referred to as acceptor molecule.
Further NADH is referred to as a reduced form of NAD. Similarly NADP is reduced to NADPH2. The FAD of flavoprotein also accepts hydrogen atoms from succinic acid and is converted to FADH2.
Flavoproteins are present in the complexes I and II. The flavoprotein of complex I is referred to as NADH dehydrogenase while the flavoprotein of complex II is called succinate dehydrogenase. The two complexes contain prosthetic groups FMN and FAD, respectively. FAD accepts 2H from succinate and is thus reduced to FADH2. Likewise FMN also accepts 2H from NADH2 and is reduced to FMNH2.
Coenzyme Q or Ubiquinone:
This is the carrier between flavoproteins and cytochrome. It is found in the mitochondria in the form of oxidized quinone under aerobic conditions. However, under anaerobic conditions it is found as reduced quinone. The reduced form is also called hydroquinone. Coenzyme Q has a polyisoprenoid side chain and the number of isoprenoid units vary from 6-10 in coenzyme Q.
As many as 30 cytochromes have been identified and they are named after their resemblance to the original three types e.g., at, a3, c1, c2, c3, etc. It is generally viewed that cytochromes a and a3 may be separate pigments or may be a single protein with two prosthetic groups.
Cytochrome oxidase represents a3. The cytochromes are large complex molecules of porphyrins containing heme, a tetrapyrrole compound with Fe. Ionic iron exists as ferrous (Fe++) or ferric (Fe+++) and these are interconvertible by a loss or gain of an electron. Fe of cytochrome oxidase acts as an electron donor or acceptor and thus the atom is reduced or oxidized.
Redox potential is the tendency for the release or acceptance of electrons. The world is derived from reduction-oxidation and represents the ratio of the reduced form to the oxidized form. It is expressed as volts.
Different compounds of the hydrogen transport system possess different redox potentials. As a general rule electrons flow from the high electronegative components to the high electropositive components. Thus, a compound which is a reducing agent in one reaction becomes an oxidizing agent in another.
Redox System of the Respiratory Chain:
Based on the redox potential of coenzymes and prosthetic groups, the enzymes of the respiratory chain may be arranged. This sequence (Fig. 17-8) is well documented in animals while in plants it still remains to be confirmed.
As will be made out the first redox system is NADH+ H+ /NAD+ which has a strong negative potential (—0.32 volt), shows the highest electron pressure and thus the largest reduction potential. Hydrogen is therefore, easily passed on to flavoproteins which occupy lower position on the redox scale and they accept hydrogen (E0 = 0.00 volt). This decrease of the electron pressure equals a change in the free energy of ΔG0 = 12000 cal/mole.
By following the energy gradient hydrogen has moved to lower or more positive carrier. It is believed that NADH+H+ emanating from mitochondria can donate its hydrogen directly to the respiratory chain.
The hydrogen of NADH+H+ obtained during glycolysis in the cytoplasm in all probability enters the mitochondria having been attached to some metabolites which deliver it to the respiratory chain. NAD system seems to be present to a relatively large extent in mitochondria. The proportion of NAD to mitochondria is 40: 1 in plant mitochondria.
NADP system appears to be missing. The flavoprotein contains FMN or FAD. They enable the enzymes to transfer hydrogen. Hydrogen may be removed directly from the substrate or accepted from NADH+H+. There is also NADH-cytochrome C reductase which is a complex enzyme system.
It is firmly attached to mitochondria membranes and is difficult to isolate and characterize. Possibly there are several NADH reductases which differ in their prosthetic groups, in the number of iron atoms, active SH group, etc. The reduced flavoproteins are oxidized by a quinone and converted to hydroquinone. A coenzyme Q seems also to be active in the respiratory chain.
Cytochromes of several types cause oxidation of reduced quinones or reduced flavoproteins. They are c, b, a and are apparently groups of closely related components. As will be seen from Fig. 17-9 the transfer of hydrogen atoms changes to pure electron transport because hydrogen is ionized.
In general two cytochrome equivalents are needed for one transfer step. No specific function is ascribed to cytochrome b and it is believed to have specific role in plants. Cytochrome a finally catalyzes the transfer of electrons to oxygen. This enzyme can directly react with oxygen and acts like anendoxidase. The oxygen ion (Cr) reacts with two hydrogen ions (H+) and water is formed.
In plants the knowledge of the plant mitochondria respiratory chain is limited. The possible mechanism is shown in Fig. 17-9. There are, however, differences with regard to redox enzymes involved especially of the cytochrome components.
Hydrogen and Electrons Transfer:
From the substrate the hydrogen atoms are transferred to NAD and from the latter they are transferred to FMN of flavoprotein 1. Then the hydrogen atom undergoes ionization and thus it is spilt into electron and proton.
In further stages only electrons are transferred to coenzyme Q. From coenzymes Q it goes to cytochromes b, c1, c, a, and a3. In this process the proton is released free. Following is the sequence of compounds in the hydrogen transport system:
NAD → FMN → CoQ → Cyt.b → Cyt. c1 → Cyt.c → Cyt. a → Cyt. a3
As the hydrogen atom or electron passes down the chain, at each step, there is simultaneous oxidation of one coenzyme and reduction of another. The compounds of the hydrogen transfer system act as oxidizing and then reducing agents. Thus, substance in the respiratory chain is alternately oxidized and reduced. NADH2 and FMNH2 lose hydrogen.
But coenzyme Q and different cytochromes lose electrons to become oxidized. Two electrons are released at one time. The different cytochromes accept one electron at one time. Consequently two molecules each of the enzymes and the cytochrome system accept the two molecules.
After transfer from the cytochrome a3, both an electron and a proton are combined to produce hydrogen. This hydrogen is ultimately accepted by oxygen molecule and thus water is formed. The need and the essentiality of oxygen for respiration would be clear soon after.