Studies on the Energy Metabolism of Herpetomonads

Redman, Christopher A (1991) Studies on the Energy Metabolism of Herpetomonads. PhD thesis, University of Glasgow.

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Herpetomonas ingenoplastis is an intriguing and unusual trypanosomatid with many anaerobic features, including the lack of a complete cytochrome chain, the presence of an acristate mitochondrion and the ability to grow equally well under aerobic and anaerobic conditions. In this study, I compared the glucose catabolism of H. ingenoplastis and the aerobic H. muscarum , with the aim of providing greater insight into the mechanisms whereby H. ingenoplastis manages to meet its energetic requirements under aerobic and anaerobic conditions. The results of this study showed that both H. ingenoplastis and H. muscarum can consume glucose. This correlates with the discovered presence of enzymes of both the Embden-Meyerhof pathway and the pentose-phosphate shunt. The activities of these enzymes are similar in the two organisms. In both species the specific activities of the Embden-Meyerhof pathway enzymes, hexokinase (HK), phosphoglucoseisomerase (PGI) and phosphofructokinase (PFK), were found to be much higher than the activities of two enzymes of the pentose-phosphate shunt, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Glucose consumption varied with gaseous conditions. Anaerobic conditions caused a reduction in glucose consumption, compared with aerobic conditions, in the case of H. muscarum. The addition of CO2 caused an increase in the rate of glucose consumption from that under argon (anaerobic) alone. Similarly for H. ingenoplastis, glucose consumption was reduced under anaerobic conditions compared with aerobic conditions and was stimulated by the presence of CO2. Herpetomonas muscarum was found to produce succinate and acetate as major excretory products of glucose catabolism, and ethanol and propionate as minor excretory products. Compared with aerobic conditions succinate production increased under aerobic conditions in the presence of CO2 by about 2-fold and up to 6-fold in the case of anaerobic conditions in the presence of CO2. Acetate production increased slightly under the two conditions containing high CO2. Herpetomonas ingenoplastis produced propionate as a major end product of glucose catabolism, along with succinate, acetate and ethanol. Ethanol production was found to be greatest under aerobic and aerobic plus CO2 conditions, while succinate and propionate were found to be produced in the greatest quantities under anaerobic plus CO2 conditions. The possibility that propionate production may help supply the energy needs of H. ingenoplastis is discussed. One factor that may be important in the energy metabolism of both herpetomonads are the CO2-fixation pathways. Two CO2-fixing enzymes, 'malic' enzyme (ME) and phosphoenolpyruvate carboxykinase (PEPCK), were found at high activities in both organisms. This correlated with succinate production in the presence of CO2. There were differences between the two species with respect to the tricarboxylic acid (TCA) cycle enzymes. Notable was the presence of fumarate reductase (FR) and the absence of succinate dehydrogenase (SDH), in the case of H. ingenoplastis. Evidence produced suggests that the TCA cycle of H. ingenoplastis operates primarily in the reverse direction. Herpetomomas muscarum was found to have both FR and SDH, suggesting flux through the TCA cycle in both directions. Succinate dehydrogenase/fumarate reductase ratios of 5. 02 and <0. 08 for H. muscarum and H. ingenoplastis, respectively, suggest that H. ingenoplastis is an organism that prefers anaerobic conditions, while H. muscarum is a facultative anaerobe. Differences between H. muscarum and H. ingenoplastis were also found at the subcellular level with respect to studies on the possible presence of glycosomes in the two herpetomonads. Some evidence was obtained for the presence of glycosomes in H. muscarum, in that HK, PGI and malate dehydrogenase (MDH) all were recovered, in part, in the particulate fraction. The distribution of the enzymes of H. ingenoplastis was found to be very different, suggesting that either this organism lacks glycosomes or that the glycosomes are much more labile that those of H. muscarum. Both FR and SDH were found to be particulate in H. muscarum which is consistent with a location in the mitochondrial membrane, perhaps as part of the electron transport chain. The FR activity in H. ingenoplastis seemed to be approximately equally divided between the soluble and particulate fractions. Pyruvate kinase is possibly the major site of glycolytic control in H. ingenoplastis. This enzyme requires MgCI2 to function and is activated by fructose-1,6-bisphosphate (F-1,6-P2) and fructose-2,6-bisphosphate (F-2,6-P2). The enzyme has an apparent Km for ADP of 1.3 mM, much higher than that of other trypanosomatids, and the activity shows sigmoid kinetics with respect to phosphoenolpyruvate (PEP) concentration. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Zoology, Physiology
Date of Award: 1991
Depositing User: Enlighten Team
Unique ID: glathesis:1991-78367
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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