ATP synthase in chloroplasts

The mechanism of ATP in chloroplasts closely resembles the process that takes place in mitochondria. The structure of the ATP synthase in chloroplasts is similar to that in mitochondria. 

Reflect and Apply What are the evolutionary implications of the similarity in structure and function of ATP synthase in chloroplasts and mitochondria ... 

The flow of electrons occurs in a similar manner from the excited pigment to cytochromes, quinones, pheophytins, ferridoxins, etc. The ATP synthase in the mitochondria of a bacterial system resembles that of the chloroplast—chloroplast proton translocating ATP synthase [37]. 

The mechanism of chloroplast ATP synthase is also believed to be essentially identical to that of its mitochondrial analog ADP and P, readily condense to form ATP on the enzyme surface, and the release of this enzyme-bound ATP requires a proton-motive force. Rotational catalysis sequentially engages each of the three JS subunits of the ATP synthase in ATP synthesis, ATP release, and ADP + Pj binding (Figs 19-24, 19-25). 

Although the focus of this chapter is on the CFq F ]-ATP synthase from chloroplasts, it should be noted that ATP synthase from bacteria such as Escherrichia cpli, from mitochondria, and as well as from chloroplasts (EcFn Fi. MEo Fi, and CFq F, respectively) are all remarkably conserved at the tertiary and quartemary level and have a substantial homology at the primary level also. To make this point, the ATP synthases from different species are often described as cousins. What is also quite extraordinary, although perhaps not surprising, is that component parts from different sources may even be combined to construct a so-called chimeric Fo F] complex. In fact, subunits from chloroplasts or cyanobacteria have been combined with those from E. coli to form an active ATP synthase, as have subunits from the... 

Of the species studied, Escherichia coli appears to have the simplest ATP synthase it is therefore considered a prototype for ATP synthase in other species. The Fq sector of E. coli ATP synthase consists ofthree different polypeptide subunits, a, b andc, with an approximate composition ofab2C9/,2 (see Fig. 3). The hydrophobic transmembrane protein complex Fq in chloroplasts also consists of similar kinds of subunits, commonly designated as I, n, HI and FV. In Fig. 3, these subunits are aligned with the corresponding a, b and c subunits ofE coli for comparison. It should be noted thatthemitochondrial Fq, on the other hand, may contain as many as ten subunits. 

Fig. 15. Conceptual development of a membane vesicle subjected to voltage pulses to create a potential difference across the membrane. (A) A1 pm-dlameter sphere of water is Imagined placed between two platinum electrodes 1 mm apart (B) The water sphere is replaced by a sphere of lipid (C) The Interior of the lipid sphere is replaced by a sphere of water, resulting in a lipid shell surrounding an aqueous medium to form the equivalent of a membrane vesicle. See text for details. (D) A schematic representation of a chloroplast thylakoid membrane containing ATP synthase to be subjected to voltage pulses and then the amount of ATP formed determined. Plots of actually measured ATP formation by voltage pulses (E) or light pulses (F) as a function of the number of pulses. (A), (B), (C), (E) and (F) from Witt (1987) Examples for the cooperation of photons, excitons, electrons, electric fields and protons in the photosynthesis membrane. Nouveau Journal deChimie 11 97 (D) adapted from Bauermeister, Schlodderand Graber(1988) Electric field-driven ATP synthesis catalyzed by the membrane-bound ATP-synthase from chloroplasts. Ber Bunsenges Phys Chem 92 1037.

In Fig. 15 (D), the imaginary membrane vesicle is replaced either by an actual chloroplast thylakoid-membrane vesicle containing ATP synthase, as shown in the example of Bauermeister, Schloddder and Graber" or by a liposome reconstituted with isolated CFo Fi-ATP synthase. In the actual experiment, two platinum electrodes each 5 cm in area were spaced 2 mm apart and filled with a chloroplast suspension. The cuvette containing the chloroplast suspension and the electrodes was then thermostated at 4 °C and kept from light. 

RE McCarty (1996) An Overview of the function, composition and structure of chloroplast ATP synthase, In DR Ort and CF Yocum (eds) Oxygenic Photosynthesis The Light Reactions, pp. 439-451. Kluwer. 

The structures of F class and V class ion pumps are sIm liar to one another but unrelated to and more complicated than P-class pumps. F- and V-class pumps contain several different transmembrane and cytosolic subunits. All known V and F pumps transport only protons. In a process that does not Involve a phosphoprotein Intermediate. V-class pumps generally function to maintain the low pH of plant vacuoles and of lysosomes and other acidic vesicles In animal cells by pumping protons from the cytosolic to the exoplasmic face of the membrane against a proton electrochemical gradient. F-class pumps are found In bacterial plasma membranes and In mitochondria and chloroplasts. In contrast to V pumps, they generally function to power the synthesis of ATP from ADP and Pj by movement of protons from the exoplasmic to the cytosolic face of the membrane down the proton electrochemical gradient. Because of their Importance In ATP synthesis in chloroplasts and mitochondria, F-class proton pumps, commonly called ATP synthases, are treated separately In Chapter 8. 

Stage 3 Synthesis of ATP Protons move down their concentration gradient from the thylakold lumen to the stroma through the FqFi complex (ATP synthase), which couples proton movement to the synthesis of ATP from ADP and Pj. The mechanism whereby chloroplast FqFi harnesses the proton-motive force to synthesize ATP is identical with that used by ATP synthase in the inner mitochondrial membrane and bacterial plasma membrane (see Figures 8-24 and 8-26). 

Mitochondrial ATP synthase is an F-type ATPase (see Fig. 11-39 Table 11-3) similar in structure and mechanism to the ATP synthases of chloroplasts and eubac-teria. This large enzyme complex of the inner mitochondrial membrane catalyzes the formation of ATP from ADP and Pj, accompanied by the flow of protons from the P to the N side of the mem-... 

In this work we describe single site, single turnover ATP hydrolysis catalyzed by the reconstituted, reduced, and active ATP synthase from chloroplasts. 

The ATP-synthase from chloroplasts carries out single site ATP hydrolysis, when the enzyme is in the active, reduced state. The reaction mechanism can be described by the scheme shown in Fig. 3, which is similar to that obtained for mitochondrial (2,5). However, the rate constants differ significantly for CFqF ATP binding is slower and the product release (Pj and ADP) is faster. This implies that the concentration of the enzyme-bound species is always much lower than for MF. The rate of ATP hydrolysis depends supralinearly on the ATP concentration. This indicates that even at low ATP concentrations a second ATP-binding site plays a role in ATP hydrolysis. The function of this site is now under investigation. 

ISOLATION OF THE CATALYTIC (p) SUBUNIT OF THE CHLOROPLAST ATP SYNTHASE IN A HYDROLYTICALLY ACTIVE FORM... 

Functional Studies on the Reaction Pattern of the H -ATP-Synthase in Spinach Chloroplasts... 

Isolation of the Catalytic (p) Subunit of the Chloroplast ATP Synthase in a Hydrolytically... 

FIGURE 10 Electron transport and ATP synthesis in chloroplasts. The jagged arrows represent light striking the two photosystems (PS I and PS II) in the thylakoid membrane. Other members of the electron transport chain shown are a quinone (Q), the cytochrome complex (heO plastocyanin (PC), and an iron-sulfur protein (FeS). The chloroplast ATP synthase is shown making ATP at the expense of the electrochemical proton gradient generated by electron transport. 

The thylakoid membrane enzyme that couples ATP synthesis to the flow of protons down their electrochemical gradient is called the chloroplast ATP synthase (see Fig. 10). This enzyme has remarkable similarities to ATP synthases in mitochondria and certain bacteria. For example, the subunits of the chloroplast ATP synthase have 76% amino acid sequence identity with the subunits of the ATP synthase of the bacterium E. coli. 

If chloroplasts were incubated for five minutes in the dark, CF1 contained 2 moles of eosin/mol CF1 The additional eosin was also bound to the beta-subunit and CF1 retained its full Ca -ATPase acivity as well as its ability to reconstitute the ATP-synthase in depleted membranes. But under light incubation two additional eosin-SCN(a total of four) were und to the beta- and to the gamma-subunit of CF1. In this case the Ca -ATPase of isolated CF1 was inhibited by 70% of the control and the ability of CF1 to... 

F-ATPases (including the H+- or Na+-translocating subfamilies F-type, V-type and A-type ATPase) are found in eukaryotic mitochondria and chloroplasts, in bacteria and in Archaea. As multi-subunit complexes with three to 13 dissimilar subunits, they are embedded in the membrane and involved in primary energy conversion. Although extensively studied at the molecular level, the F-ATPases will not be discussed here in detail, since their main function is not the uptake of nutrients but the synthesis of ATP ( ATP synthase ) [127-130]. For example, synthesis of ATP is mediated by bacterial F-type ATPases when protons flow through the complex down the proton electrochemical gradient. Operating in the opposite direction, the ATPases pump 3 4 H+ and/or 3Na+ out of the cell per ATP hydrolysed. 

Transporting ATP synthase [EC 3.6.1.34] in plants, also referred to as chloroplast ATPase and CFiCFo-ATPase, catalyzes the hydrolysis of ATP to produce ADP and orthophosphate. When coupled with proton transport the reverse reaction results in the synthesis of ATP by this multisubunit complex. CFi, isolated from the rest of the membrane-bound complex, retains the ATPase activity but not the proton-translocating activity. 

The synthesis of ATP is catalyzed by the enzyme ATP synthase (or FiFq-ATP synthase) the Fj portion of this enzyme was first isolated by Racker and coworkers in 1960 [4]. ATP synthase is present in abundance in the membranes of animal mitochondria, plant chloroplasts, bacteria and other organisms. ATP synthesized by our ATP synthase is transported out of mitochondria and used for the function of muscle, brain, nerve, liver and other tissues, for active transport, and for synthesizing myriad compounds needed by the cell. Since the pool of adenosine phosphates in the body is limited, the use of ATP must be continually compensated by its synthesis, and an active person synthesizes his own body weight of ATP every day. The synthesis of ATP is the most prevalent chemical reaction in the body [5]. This is indeed a very important reaction. How exactly does it occur ... 

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