4.11 chu trình KrebsBây giờ mà chúng tôi có một nắm bắt cách ATP được thực hiện trong hô hấp, chúng tôicần phải xem xét các phản ứng quan trọng trong chuyển hóa cacbongắn liền với sự hình thành của ATP. Chúng tôi tập trung ở đây là trên các citricchu trình axit, còn được gọi là chu trình Krebs, một con đường quan trọng tronghầu như tất cả các tế bào.Sự hô hấp của GlucoseSinh hóa bước đầu trong việc hô hấp của glucose cácgiống như những người glycolysis; Tất cả các bước từ glucose để pyruvat(Hình 4,14) đều giống nhau. Tuy nhiên, trong khi trong quá trình lên menPyruvat là giảm và chuyển đổi thành sản phẩmbài tiết, hô hấp pyruvat là mầu để CO2. Con đườngbởi pyruvat mà hoàn toàn bị ôxi hóa để CO2 được gọi là cácchu trình Krebs (CAC), được tóm tắt trong hình 4.21.Pyruvat là decarboxylated đầu tiên, dẫn đến việc sản xuấtCO2, NADH, và năng lượng phong phú chất acetyl-CoA (hình4.12). Nhóm axetyl acetyl-CoA sau đó kết hợp với các4-Bon hợp chất oxalacetate, tạo thành axít citric hợp chất của sáu-cacbon. Một loạt các phản ứng sau, và hai bổ sungCác phân tử khí CO2, ba thêm NADH và một FADH được thành lập.Cuối cùng, oxalacetate tái sinh để trở lại như một axetylTìm, do đó hoàn thành chu kỳ (hình 4.21).CO2 phát hành và nhiên liệu cho giao thông vận tải điện tửQuá trình oxy hóa của pyruvat để CO2 yêu cầu hoạt động phối hợpchu trình axit citric và chuỗi vận tải điện tử. Đối với mỗiPyruvat phân tử bị ôxi hóa thông qua chu trình axit citric, baCO2 molecules are released (Figure 4.21). Electrons released during the oxidation of intermediates in the citric acid cycle aretransferred to NAD1 to form NADH, or to FAD to form FADH2.This is where respiration and fermentation differ in a major way.Instead of being used in the reduction of pyruvate as in fermentation (Figure 4.14), in respiration, electrons from NADH andFADH2 are fuel for the electron transport chain, ultimately resulting in the reduction of an electron acceptor (O2) to H2O. Thisallows for the complete oxidation of glucose to CO2 along with amuch greater yield of energy. Whereas only 2 ATP are producedper glucose fermented in alcoholic or lactic acid fermentations(Figure 4.14), a total of 38 ATP can be made by aerobically respiring the same glucose molecule to CO2 1 H2O (Figure 4.21b)Biosynthesis and the Citric Acid CycleBesides playing a key role in catabolism, the citric acid cycle playsanother important role in the cell. The cycle generates several keycompounds, small amounts of which can be drawn off for biosynthetic purposes when needed. Particularly important in thisregard are -ketoglutarate and oxalacetate, which are precursorsof several amino acids (Section 4.14), and succinyl-CoA, needed toform cytochromes, chlorophyll, and several other tetrapyrrole compounds (Figure 4.16). Oxalacetate is also important because it canbe converted to phosphoenolpyruvate, a precursor of glucose. Inaddition, acetate provides the starting material for fatty acid biosynthesis (Section 4.15, and see Figure 4.27). The citric acid cycle thusplays two major roles in the cell: bioenergetic and biosynthetic.Much the same can be said about the glycolytic pathway, as certainintermediates from this pathway are drawn off for various biosynthetic needs as well (Section 4.134.12 Catabolic DiversityThus far in this chapter we have dealt only with catabolism bychemoorganotrophs. We now briefly consider catabolic diversity, some of the alternatives to the use of organic compounds aselectron donors, with emphases on both electron and carbonflow. Figure 4.22 summarizes the mechanisms by which cellsgenerate energy other than by fermentation and aerobic respiration. These include anaerobic respiration, chemolithotrophy, andphototrophy.Anaerobic RespirationUnder anoxic conditions, electron acceptors other than oxygencan be used to support respiration in certain prokaryotes. Theseprocesses are called anaerobic respiration. Some of the electronacceptors used in anaerobic respiration include nitrate (NO32,reduced to nitrite, NO22, by Escherichia coli or to N2 byPseudomonas species), ferric iron (Fe31, reduced to Fe21 byGeobacter species), sulfate (SO422, reduced to hydrogen sulfide,H2S, by Desulfovibrio species), carbonate (CO322, reduced tomethane, CH4, by methanogens or to acetate by acetogens), andeven certain organic compounds. Some of these acceptors, forexample Fe31, are often only available in the form of insolubleminerals, such as metal oxides. These common minerals, widelydistributed in nature, allow for anaerobic respiration in a widevariety of microbial habitats.Because of the positions of these alternative electron acceptorson the redox tower (none has an as positive as the O2/H2Ocouple; Figure 4.9), less energy is released when they are reducedinstead of oxygen (recall that DG09 is proportional to ;Section 4.6). Nevertheless, because O2 is often limiting or absentin many microbial habitats, anaerobic respirations can be veryimportant means of energy generation. As in aerobic respiration,anaerobic respirations involve electron transport, generation of aproton motive force, and the activity of ATPase.ChemolithotrophyOrganisms able to use inorganic chemicals as electron donors arecalled chemolithotrophs. Examples of relevant inorganic electron donors include H2S, hydrogen gas (H2), Fe21, and NH3.Chemolithotrophic metabolism is typically aerobic andbegins with the oxidation of the inorganic electron donor(Figure 4.22). Electrons from the inorganic donor enter an electron transport chain and a proton motive force is formed in exactly the same way as for chemoorganotrophs (Figure 4.19).However, one important distinction between chemolithotrophsand chemoorganotrophs, besides their electron donors, is theirsource of carbon for biosynthesis. Chemoorganotrophs useorganic compounds (glucose, acetate, and the like) as carbonsources. By contrast, chemolithotrophs use carbon dioxide(CO2) as a carbon source and are therefore autotrophs (organisms capable of biosynthesizing all cell material from CO2 asthe sole carbon source). We consider many examples ofchemolithotrophy in Chapter 13.PhototrophyMany microorganisms are phototrophs, using light as an energysource in the process of photosynthesis. The mechanisms bywhich light is used as an energy source are complex, but the endresult is the same as in respiration: generation of a proton motiveforce that is used to drive ATP synthesis. Light-mediated ATPsynthesis is called photophosphorylation. Most phototrophsuse energy conserved in ATP for the assimilation of CO2 as thecarbon source for biosynthesis; they are called photoautotrophs.However, some phototrophs use organic compounds as carbon sources with light as the energy source; these are the photoheterotrophs (Figure 4.22).As we discussed in Chapter 2, there are two types of photosynthesis: oxygenic and anoxygenic. Oxygenic photosynthesis, carried out by cyanobacteria and their relatives and also by greenplants, results in O2 evolution. Anoxygenic photosynthesis is asimpler process used by purple and green bacteria that does notevolve O2. The reactions leading to proton motive force formation in both forms of photosynthesis have strong parallels, as wesee in Chapter 13.
The Proton Motive Force
and Catabolic Diversity
Microorganisms show an amazing diversity of bioenergetic
strategies. Thousands of organic compounds, many inorganic
compounds, and light can be used by one or another microorganism as an energy source. With the exception of fermentations,
in which substrate-level phosphorylation occurs (Section 4.8),
energy conservation in respiration and photosynthesis is driven
by the proton motive force.
Whether electrons come from the oxidation of organic or
inorganic chemicals or from phototrophic processes, in all
forms of respiration and photosynthesis, energy conservation is
linked to the pmf through ATPase (Figure 4.20). Considered in
this way, respiration and anaerobic respiration are simply metabolic variations employing different electron acceptors. Likewise,
chemoorganotrophy, chemolithotrophy, and photosynthesis are
simply metabolic variations upon a theme of different electron
donors. Electron transport and the pmf link all of these
processes, bringing these seemingly quite different forms of
metabolism into a common focus. We pick up on this theme in
Chapters 13 and 14.
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