Cyanide resistant respiration pathway.pptx

Cyanide resistant pathway

1. CYANIDE-RESISTANT RESPIRATION 1

2. CYANIDE –RESISTANT RESPIRATION(CRR) INTRODUCTION Aerobic respiration of most organisms, including some plants is strongly inhibited by certain negative ions. It combines with iron in cytochrome oxidase. Two such ions are cyanide ( CN-) and azide (N3-) are particularly effective. Carbon monoxide (CO) also forms a strong complex with iron, preventing electron transport and poisoning respiration. Animal mitochondria thus indicates that respiration is cyanide- sensitive. 2

3. Cyanide resistant pathway cont … Because cytochrome oxidase (a3) which catalyses the electron transfer between cyt a3 and oxygen is rapidly inactivated by reaction with cyanide. It leads to inhibited oxygen uptake and ATP generation. However, respiration of a number of higher plants( potato, Arum, suaromatium ), several algae, fungi and a few bacteria are cyanide- resistant. So, these cyanide- resistant plants and microorganisms have an alternate insensitive non-cytochrome terminal pathway. 3

4. Cyanide resistant pathway cont … This is called alternative respiratory pathway or cyanide resistant pathway . It allows transfer of electrons. Here, electrons are transported from CoQ to oxygen through non-cytochrome oxidase enzyme. Instead of ATP formation, only transfer of heat is evolved . ATP is formed only in Complex 1 prior to CoQ . ADP+Pi =ATP Flavoprotein NADH CoQ Non- cytochrome oxidase O2 H2O 4

5. Cyanide resistant pathway cont … Thus, mitochondria of certain plants, possess two different routes by which electrons are transported from substrates like NADH and succinate to oxygen. The first conventional route is blocked by cyanide which inhibits cytochrome oxidase. The second route is not blocked by cyanide and is called cyanide-resistant electron transport chain and the process is called cyanide-resistant respiration. This pathway is also sensitive to antimycin A(an inhibitor of cyt b oxidation). Both cyanide and antimycin A direct electrons to alternative path. 5

6. Cyanide resistant pathway cont … Cyanide- resistant respiration through non-cytochrome oxidase is an adaptation for the survival of plants at high temperature. The capacity for CN- resistant electron transfer varies greatly in different plants. It is poorly developed in young tissues eg : in seedlings, resting seeds. Older tissues frequently possess a strongly CN- resistant component of O2 uptake. CN- poisoning in some tissues may even lead to stimulation of O2 uptake. 6

7. Cyanide resistant pathway cont … The cyanide resistant pathway in most plants is unclear. Because it doesn’t operate unless cytochrome oxidase of normal pathway is poisoned by cyanide, azide or CO. An exception is that it does operate when glycolysis and Krebs cycle occur unusually rapidly. Because then normal electron transport pathway cannot handle all electrons provided to it. This explains greatly in arum lillies , where rates of glycolysis and Krebs cycle are somehow decontrolled . And heat production occurs partly because normal respiration is so fast. 7

8. Cyanide-sensitive respiration: Cyanide-resistant respiration: The flow of electrons in the usual mitochondrial electron transport chain (in both plants and animals) during aerobic respiration is blocked by presence of cyanides which inhibit the activity of cytochrome oxidase. Plant mitochondria, however, differ from animal mitochondria in having an alternate oxidase system pathway through which terminal oxidation of reduced coenzyme continues even in presence of cyanides. 8

9. ALTERNATIVE OXIDASE AOX was isolated from skunk cabbage. It is an integral membrane protein complex which acts as an enzyme in cyanide-resistant pathway. Probably faces matrix side of membrane. Functions as a Ubiquinone:O2 oxidoreductase ie , accepts electrons from Ubiquinone pool and transfers them directly to oxygen to form water in addition to optional pathways for oxidation of NADH. The proton pumping complexes III and IV do not participate. Hence ATP is not formed. The calories are lost as heat. At the most only, first ATP is formed. 9

10. Alternative oxidase cont … It is commonly found in algae, fungi and some protozoa. This nuclear-encoded alternate oxidase is also found in plants. The enzyme is tightly bound to inner mitochondrial membrane. It diverts electrons from normal cytochrome pathway at level of ubiquinone pool. As there is no proton gradient is generated during this electron transfer, all free energy released is lost as heat without ATP formation. 10

11. ALTERNATIVE OXIDASE CONT… AOX is associated with a single polypeptide of approximately 32kDa. It is encoded by a single nuclear gene AOX1. The enzyme exists in inner membrane as a homodimer , in which two reduced SH- monomers are apparently held together by noncovalent interactions. The active site of enzyme have one or more transition metal centers as it has O2- reducing activity . It’s active form requires iron which form di-iron center in c-terminal domain. 11

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13. ALTERNATIVE OXIDASE CONT… The common substrate of cytochrome pathway and AOX is reduced ubiquinone. Partitioning of electrons to cytochrome pathway is linearly proportional to degree of ubiquinone reduction. AOX is not active until level of reduced ubiquinone reaches a high threshold value. This non-linear response of alternative pathway suggests that AOX would function as an ‘energy overflow’. Only become active in oxygen consumption when cytochrome pathway is saturated with electrons. Such conditions arise when respiratory substrate is abundant or when cellular ATP is high thus restricting oxidative phosphorylation. 13

14. REGULATION OF AOX It has two mechanisms: First mechanism: Certain alpha- keto acids (particularly pyruvate) stimulate AOX activity. The stimulation is independent of pyruvate metabolism. ie , pyruvate interacts with AOX protein. Succinate and malate are also able to stimulate AOX activity. But these organic acids are converted into pyruvate by malic enzyme. 14

15. REGULATION OF AOX CONT… Second mechanism: AOX exists in inner mitochondrial membrane as a covalently linked dimer. The dimer when covalently linked by disulphide bond is a less active form of enzyme. Reduction of disulphide bond produces more active form. Only more active reduced form of enzyme is suitable for pyruvate activation. The fully reduced and activated enzyme has a higher affinity for reduced ubiquinone. Fully reduced and activated enzyme has higher affinity for reduced ubiquinone. 15

16. REGULATION OF AOX CONT… The second hypothesis is : Thermogenesis Based largely on certain members of family Aracae . This heat generating alternative pathway is useful in attracting insects for pollinating flowers of voodoo lily. Also plays a significant role in plants when they fail to carry out mitochondrial respiration due to drought, chilling, phosphate deficiency, osmotic stresses etc . 16

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18. FUNCTIONS OF AOX To balance carbon metabolism and electron transport: Any metabolic condition that leads to accumulation of reduced ubiquinone, mitochondrial NADH/NADPH or pyruvate. Has potential to increase electron flow to alternative pathway. Such condition arise only there is an imbalance between upperstream respiratory carbon metabolism and downstream electron transport. Such imbalance cause change in supply of or demand for carbon, reducing power and ATP. 18

19. FUNCTIONS OF AOX CONT… Activation of AOX by adjusting mitochondrial electron transport prevent excessive reduction of respiratory chain components which results in harmful active oxygen species. AOX act to shunt electrons from reduced ubiquinone thus prevent generation of active oxygen species. AOX activation by pyruvate regulate pyruvate level. Pyruvate accumulation by AOX cause increased rate of electron transport thus bringing down pyruvate levels. In transgenic plants lacking AOX, inhibition of cytochrome pathway by antimycin A results in production of large quantities of ethanol. 19

20. SIGNIFICANCE OF AOX FRUIT RIPENING Cyanide resistant respiration is responsible for climacteric in fruits (remarkable increase in respiration during and just before ripening). Climacteric is induced by ethylene. Latter may act to implement cyanide resistant respiration in ripening fruit. Production of H2O2 and superoxide increases oxidation and breakdown of membrane which are necessary activities in ripening process. 20

21. SIGNIFICANCE OF AOX CONT… SEED GERMINATION In early stages of germination, respiration is CN-resistant. AOX plays a role in germination. Cyanide has been to stimulate lettuce and Amaranthus seed germination. AOX inhibitors such as hydroxamates (SHAM or CLAM) inhibit germination by about 50 percent. At later stages of germination, however, respiration becomes sensitive to cyanide. 21

22. SIGNIFICANCE OF AOX CONT… THERMOGENESIS In aroids, heat is produced in inflorescences to volatilize odiferous compounds formed in them, usually auxins and indoles to attract pollinating insects. Production of heat is due to CN- resistant respiration in mitochondria of thermogenic tissues of inflorescence. These tissue require ATP for normal metabolic processes which is likely to be produced by CN-resistant respiration rather than by normal respiration. Since P/O ratio of CN-resistant respiration is one( in case of NAD-linked substrates). 22

23. SIGNIFICANCE OF AOX CONT… It is zero(in case of succinate) in contrast to three for conventional respiration. Such cells will have to oxidize three times as much fuel to get required amount of ATP. Since energy conservation is less, the tissues produce much waste as energy that is not conserved as ATP. The amount of heat produced in thermogenic tissues may be high as 51 degree celsius with an atm.temp, of 15 degree celsius. 23

24. SIGNIFICANCE OF AOX CONT… DEFENCE AGAINST FUNGAL INFECTION IN POTATO Infection of potato by late blight fungus, Phytophthora infestans leads to biosynthesis of sesquiterpenoid stress metabolites viewed as Phytoalexins. Pretreatment of tubers with ethylene or O2 enhances tissue resistant to fungus, presumbly by phytoalexins production possibly through alternate path. According to these, alternative path plays a role in defense against pathogen infection. 24

25. REFERENCE 1. Mohr, H., Schopfer, P. (2006). Plant Physiology . Springer (India) Pvt. ltd. 2. Salisbury, F. B., Ross, C.W. (1986). Plant Physiology (3 rd ed.). CBS Publishers and Distributors Pvt. ltd. 3. Verma, V. (2007). Textbook of Plant Physiology . A.P. Offset Pvt. ltd., India. 4. Salisbury, F.B., Ross, C.W. (2004). Plant Physiology (4 th ed.). Thomson Asia Pvt.ltd., Singapore. 5. Hopkins, G., William. (1995). Introduction To Plant Physiology . John Wiley and Sons, Inc. 6. Ghosh, A.K., Mukherji, S. (1996). Plant Physiology. New Central Book Agency Pvt. ltd. 25