Terminal Oxidation:
It is the name of oxidation found in aerobic respiration that occurs towards the end of catabolic process and involves the passage of both electrons and protons of reduced coenzymes to oxygen. It produces water.
Terminal oxidation consists of two processes— electron transport and oxidative phosphorylation….
i. Electron Transport Chain (Fig. 14.4):
Inner mitochondrial membrane contains groups of electron and proton transporting enzymes. In each group the enzymes are arranged in a specific series called electron transport chain (ETC) or mitochontrial respiratory chain or electron transport system (ETS). An electron transport chain or system is a series of coenzymes and cytochromes that take part in the passage of electrons from a chemical to its ultimate acceptor.
The passage of electrons from one enzyme or cytochrome to the next is a downhill journey with a loss of energy at each step. At each step the electron carriers include flavins, iron sulphur complexes, quinones and cytochromes.
Most of them are prosthetic groups of proteins. Quinones are highly mobile electron carriers. Inner mitochondrial membrane possesses five complexes. Complex V is connected with ATP synthesis (F0-F1particle).
Complexes I to IV are involved in electron transport:
(i) NADH-Q reductase or NADH- dehydrogenase complex,
(ii) Succinate Q-reductase complex,
(iii) QH2-cytochrome с reductase complex, and
(iv) Cytochrome с oxidase complex.
NADH-Q reductase (or NADH-dehydrogenase) has two prosthetic groups, flavin mononucleotide (FMN) and iron sulphur (Fe-S) complexes. Both electrons and protons pass from NADH to FMN. The latter is reduced. However, FMNH2 breaks to release protons (H+) and electrons.
NADH + H+ + FMN → FMNH2 + NAD+
FMNH2→FMN + 2H+ + 2 e– transport chain.
Electron now moves to the FeS complex and from there to a quinone. The common quinone is co-enzyme Q, also called ubiquinone (UQ).
2e–+2Fe3+ S → 2Fe2+S
2Fe2+ S + Q → 2 Fe3+ S + Q2-
Charged ubiquinone picks up protons and passes it into the outer chamber with the help of Cyt b.
FADH2 produced during reduction of succinate also hands over its electrons and protons to ubiquinone or co-enzyme Q through FeS complex. The enzyme is succinate-Q reductase complex.
FADH2 + 2Fe3+ S → 2 Fe2 + S + 2H+ + FAD
2Fe2+ S + Q +2H+ → 2 Fe3+ S + QH2
QH2-cytochrome с reductase complex has three components— cytochrome b, FeS complex and cytochrome с1. Coenzyme Q may also be involved between FeS complex and cytochrome c1. Reduced ubiquinone or ubiquinol (QH2) is oxidised with the passage of protons to the outside and handing over the electrons to cytochrome с via cytochrome b- C1 complex.
Cytochrome c1 hands over its electron to a small protein called cytochrome c. Like coenzyme Q, cytochrome с is also mobile carrier of electrons that transfers electrons between complex III and IV.
Cytochrome с oxidase complex contains cytochrome a and cytochrome a3. Cytochrome a3 also possesses two copper centres. The latter help in transfer of electron to oxygen.
Oxygen is the ultimate acceptor of electrons. It becomes reactive and combines with protons to form metabolic water.
2H++ О2-→ H2O
Energy released during passage of electrons from one carrier to the next is made available to specific trans membrane complexes, which pump protons (H+) from the matrix side of the inner mitochondrial membrane to the outer chamber.
There are three such sites corresponding to three enzymes present in the electron transport chain (NADH-Q reductase, QH2– cytochrome с reductase and cytochrome c-oxidase).
This increases proton concentration in the outer chamber or outer surface of the inner mitochondrial membrane. It creates a proton gradient
The difference in the proton concentration on the outer and inner sides of the inner mitochondrial membrane creates an electric potential across the membrane with inner surface becoming negative as compared to outer surface. The electrochemical potential gradient created across the membrane due to high H+ concentration on one side is called proton motive force (PMF, ∆p).
ii. Oxidative Phosphorylation (Fig. 14.5):
Oxidative phosphorylation is the synthesis of energy rich ATP molecules with the help of energy liberated during oxidation of reduced co-enzymes (NADH FADH2) produced in respiration. The enzyme required for this synthesis is called ATP synthase. It is considered to be fifth complex of electron transport chain.
ATP synthase is located in F1 or head piece of F0-F1 or elementary particles. The particles are present in the inner mitochondrial membrane. ATP-synthase becomes active in ATP formation only where there is a proton gradient having higher concentration of H+ or protons on the F0 side as compared to F, side.
Increased proton concentration is produced in the outer chamber or outer surface of inner mitochondrial membrane by the pushing of protons with the help of energy liberated by passage of electrons from one carrier to another.
Transport of the electrons from NADH over ETC helps in pushing three pairs of protons (5 pairs as per latest estimate) to the outer chamber while two (latest estimate-three) pairs of protons are sent outwardly during electron flow from FADH2 (as the latter donates its electrons further down to the ETC).
Higher proton concentration in the outer chamber causes the protons to pass inwardly into matrix or inner chamber through the inner membrane. The latter possesses special rotating proton channels in the region of F0 (base) of the F0 — F1, particles.
The flow of protons through the F0 channel induces F1, particle to function as АТР-synthase. The energy of the proton gradient is used in attaching a phosphate radicle to ADP by high energy bond. This produces ATP. Oxidation of one molecule of NADH2 produces 3 ATP molecules while a similar oxidation of FADH2 forms 2 ATP molecules.
Balance Sheet of ATP:
There is net gain of 2 ATP molecules during glycolysis and 2 ATP (GTP) molecules during double Krebs cycle. Glycolysis also forms 2 NADH2. Its reducing power is transferred to mitochondria for ATP synthesis.
For this a shuttle system operates at the inner mitochondrion membrane. (i) NADH2 → NAD NADH2. (ii) NADH, FAD FADH2. The former operates in liver, heart and kidney cells. No energy is spent. The second method occurs in muscle and nerve cells.
It lowers the energy level of 2NADH2 by 2ATP molecules total of 10 NADH2 and 2 FADH2 molecules are formed in aerobic respiration. They help in formation of 34 ATP molecules.
The net gain from complete oxidation of a molecule of glucose in muscle and nerve cells is 36 ATP molecules (10 NADH2 = 30 ATP, 2 FADH = ATP, four formed by substrate level phosphorylation in glycolysis and Krebs cycle and two consumed in transport of the NADH2 molecules into mitochondria). In aerobic prokaryotes, heart, liver, and kidneys, 38 ATP molecules are produced per glucose molecule oxidized.
Passage of ATP, molecules from inside of mitochondria to cytoplasm is through facilitated diffusion. Since, one ATP molecule stores 34 kJ or 8.15 kcal/mole, the total energy trapped per gm mole of glucose is 1292 kJ or 309.7 kcal with an efficiency of 45%. The rest of the energy is lost as heat.
ATP (Fig. 14.8)
It is adenosine triphosphate. Adenosine triphosphate is formed of an adenine (a purine), a ribose (a 5-carbon sugar) and a row of three phosphate radicals attached to ribose.
The complex formed from’ adenine and ribose is called adenosine. ATP was discovered by Karl Lohmann in 1929. Its functioning through build up and hydrolysis of high energy phosphate bond was discovered by Fritz Lipmann (1941). Lipmann is called father of ATP cycle.
In ATP the last two phosphate radicals are attached by bonds of high transfer potential. They are also called energy rich bonds. The bond between second and third phosphate radicals possesses an energy equivalent of 8.15 Kcals/mole (7.3 Kcal/mole according to early estimates) while the bond linking the second phosphate radical with the first one has an energy equivalent of 6.5 Kcal/mole.
They are represented by the squiggle sign (~) proposed by Lipmann (1941). The last phosphate can be very easily broken up and synthesised.
The easily available from of energy present in high energy bounds of ATP (and other energy carriers like GTP, UTP or CTP) is know as biologically useful energy. Hence ATP can function as energy currency of the living cells.
Synthesis of ATP:
ATP is synthesised from ADP (adenosine diphosphate) and inorganic phosphate (Pi). The reaction is called phosphorylation. It is endergonic or energy requiring. Phosphorylation is of three types— substrate level phosphorylation, oxidative phosphorylation and photophosphorylation. Substrate level phosphorylation or ATP synthesis is directly linked to the liberation of energy in chemical reactions of respiration.
1: 3-diphosphoglycerate + ADP ⇋ 3-phosphoglycerate + ATP
Phosphoenol pyruvate + ADP → Pyruvate + ATP
Succinyl CoA + ADP + Pi → Succinate + CoA + ATP
Oxidative phosphorylation is linked to terminal oxidation of reduced coenzymes (NADH and FADH2) in respiration. The coenzymes release H+ ions and electrons.
The electrons pass over a series of carriers, called electron transport chain, before combining with oxygen to make it reactive. The energy released during electron transport is used in creating a proton gradient. The proton gradient activates ATP synthase of F0-F1particles resulting in synthesis of ATP.
Photophosphorylation occurs on the thylakoids of chloroplasts. In the primary photochemical reaction an electron is extruded by chlorophyll a on the receipt of radiation energy. The electron passes over a transport chain of carriers.
Sufficient energy is released when the electron passes between cytochrome b and cytochrome f (cyclic photophosphorylation) or plastoquinone to cytochrome f (noncyclic photophosphorylation). It creates a proton gradient. The latter activates ATP synthase of F0-F1, complex to produce ATP.
Functions:
(i) It can store small packets of energy as soon as the energy becomes available so that wastage of energy is minimised,
(ii) ATP makes energy available at a spot away from the area of release of energy,
(iii) By its accumulation at a spot, it makes available large and continuous supply of energy for carrying out heavy work,
(iv) ATP releases small amount of energy required for building new chemical bonds during anabolism,
(v) It helps in driving energetically un-favourable processes like absorption of inorganic solutes,
(vi) ATP acts as a phosphorylating agent for activating certain metabolites like sugars,
(vii) It maintains bio-electric potential of cellular membranes,
(viii) ATP energises the membrane carriers for influx and efflux of biochemical,
(ix) It energises the enzyme luciferase in bioluminescent organisms for liberation of light.
Utility of Step-wise Oxidation:
(i) There is a step-wise release of chemical bond energy which is very easily trapped in forming ATP molecules,
(ii) Cellular temperature is not allowed to rise,
(iii) Wastage of energy is reduced,
(iv) There are several intermediates which can be used in production of a number of bio-chemicals,
(v) Through their metabolic intermediates different substances can undergo respiratory catabolism,
(vi) Each step of respiration is controlled by its own enzyme. The activity of different enzymes can be enhanced or inhibited by specific compounds. This helps in controlling the rate of respiration and the amount of energy liberated by it.
No comments:
Post a Comment