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4 – ATP is generated by photosynthesis

ATP synthesis catalyzed by ATP synthase is powered bythe transmembrane electrochemical proton potential difference, composed of twocomponents: the chemical and theelectrical one. The more protons are on one side of a membrane relativetothe other, the higher is the driving force for a proton to cross themembrane. As proton is a charged particle, its movement is alsoinfluenced by electrical field: transmembrane electrical potentialdifference will drive protons from positively charged side tothe negatively charged one.

Is the production of ATP in photosynthesis similar to the ..

In the absence of transmembrane pH difference equals the transmembraneelectrical potential difference and can be directly measured by severalexperimental techniques (i.e. permeate ion distribution,potential-sensitive dyes, electrochromic carotenoid bandshift, etc.).Each pH unit of the transmembrane pH gradient corresponds to 59 mVof .
For most biological membranes engaged in ATP synthesis the value lies between 120 and 200mV ( between 11.6 and19.3 kJ mol-1).

The mechanism of ATP synthesis ..

Traditionally the thermodynamics of ATP synthesis/hydrolysis isdescribed for the hydrolysis reaction:

The catalytic mechanism of ATP synthasemost probably involves rotation of Gamma subunit together with subunitEpsilon and -subunitoligomer relative to the rest of the enzyme. Such rotation wasexperimentally shown for ATP hydrolysis uncoupled to protontranslocation. Moreover, recent experiments revealed, that if Gammasubunit is mechanically forced into rotation, ATP synthesis takes placeeven without proton-translocating FO-portion.
It seems most probable that such rotation takes place . However, there is nodirect experimental evidence for such rotary mechanism in the intactenzyme under physiological conditions.
The proposed mechanism is the following:

ATP synthesis catalyzed by ATP synthase is powered bythe transmembrane electrochemical proton potential difference, composed of twocomponents: the chemical and theelectrical one. The more protons are on one side of a membrane relativetothe other, the higher is the driving force for a proton to cross themembrane. As proton is a charged particle, its movement is alsoinfluenced by electrical field: transmembrane electrical potentialdifference will drive protons from positively charged side tothe negatively charged one. A water mill is a good analogy: the difference between the water levelsbefore and after the dam provides potential energy; downhill water flowrotates thewheel; the rotation is used to perform some work (ATP synthesis in ourcase). Quantitatively is measured in Joules per mole (J mol-1) and isdefined as:
where the "" and "" indices denote the ositively and the egatively charged sides of thecoupling membrane; is Faraday constant(96 485 C mol-1); is the molar gas constant(8.314 J mol-1K-1), is the temperature in Kelvins, and is thetransmembrane electrical potential difference involts. The value of tells, how much energy is required (or is released, depending on thedirection of the transmembrane proton flow) to move 1 mol of protonsacross the membrane.
It is often more convenient to use not , but protonmotive force ():

At room temperature (25oC) the protonmotive force (inmillivolts, as well as )is:
In the absence of transmembrane pH difference equals the transmembraneelectrical potential difference and can be directly measured by severalexperimental techniques (i.e. permeate ion distribution,potential-sensitive dyes, electrochromic carotenoid bandshift, etc.).Each pH unit of the transmembrane pH gradient corresponds to 59 mVof .
For most biological membranes engaged in ATP synthesis the value lies between 120 and 200mV ( between 11.6 and19.3 kJ mol-1).
The catalytic mechanism of ATP synthasemost probably involves rotation of Gamma subunit together with subunitEpsilon and -subunitoligomer relative to the rest of the enzyme. Such rotation wasexperimentally shown for ATP hydrolysis uncoupled to protontranslocation. Moreover, recent experiments revealed, that if Gammasubunit is mechanically forced into rotation, ATP synthesis takes placeeven without proton-translocating FO-portion.
It seems most probable that such rotation takes place . However, there is nodirect experimental evidence for such rotary mechanism in the intactenzyme under physiological conditions.
The proposed mechanism is the following:
ATP synthase activity is specifically inhibited by several compounds(both organic and inorganic). Most of these inhibitors are very toxic, so great careand appropriate safety precautions are essential when working with them (it is not very surprising thatwe get unhappy when OUR ATP synthase is blocked!).Most inhibitors are specific for either proton-translocating FO-portion, or hydrophilicF1-portion, so the section below is divided accordingly. Oligomycin is the inhibitor that gave the name "FO" to the membrane-embedded portion of ATP synthase. The subscript letter "O" in FO(not zero!) comes from Oligomycin sensitivity of this hydrophobicphosphorylation Factor in mitochondria.
Oligomycin binds on theinterface of subunit and -ring oligomer and blocks the rotary proton translocation in FO. If the enzyme is well-coupled, the activity of F1is also blocked. Because of the latter phenomenon, a subunit of mitochondrial F1-portionthat connects F1 with FO was named Oligomycin-Sensitivity Conferring Protein (OSCP).This subunit is essential for good coupling between F1 and FO and makes the ATPase activity of F1 sensitive to FO inhibitor oligomycin, hence the name.
Oligomycin is specific for mitochondrial ATP synthase and in micromolar concentrationseffectively blocks proton transport through FO. This inhibitor also works in some bacterial enzymes that show highsimilarity to mitochondrial ATP synthase, e.g. enzyme from purple bacterium . But ATP synthase from chloroplasts and from most bacteria (including )has low sensitivity to oligomycin.
It should also be noted that oligomycin in high concentrations also affects the activity of mitochondrial F1. DCCD (abbreviation for Dicyclohexylcarbodiimide; also known as DCC, as N,N'-dicyclohexylcarbodiimide, as Bis(cyclohexyl)carbodiimide, and as 1,3-dicyclohexylcarbodiimide) is a small organic molecule thatcan covalently modify protonated carboxyl groups. When added to ATP synthase at pH above 8, DCCD almost exclusively reacts with the carboxyl group of the conserved acidic amino acid residue of subunit (that is why subunit is sometimes called "DCCD-binding protein"). that has elevated pK and can therefore be protonated at such a high pH. Modification of the carboxyl group in a single -subunit is enough to renderthe whole -ring oligomer inactive. Because DCCD covalently binds to -subunit,this inhibition is irreversible.
The carboxyl group of the conserved amino acid residue in subunit -subunit is present inall ATP synthases known so far. So DCCD is a universal inhibitor that can FO function in bacterial, mitochondrial and chloroplast enzymes. Moreover, V- and A-type proton-transporting ATPasesare also sensitive to DCCD for the same reason. Sodium-transporting ATP synthases are also effectively inhibited by DCCD.
At lower pH (1 and inactivates it. So this compound canbe considered as an inhibitor of both FO and F1. However, inhibition of FOis highly specific, well-defined, and requires much lower DCCD concentration so usually thisinhibitor is used as FO-specific.

that absorb light and generate ATP during photosynthesis.

Explain how exergonic oxidation of glucose is coupled to endergonic synthesis of ATP; ..

The "" and "" indices denote the ositively and the egatively charged sides of thecoupling membrane.
The pH value is important: the pK value for 2-+i- is 7.2, while thecorresponding pK values for phosphate in ADP and ATP are close to 6.9.
This means that in the pH interval of 6.9-7.2 the prevailingreaction will not include trapping of protons:

Chloroplast ATP synthase and the enzyme from some photosyntheticbacteriahave 2 different, although similar, -typesubunits in the protontranslocating FO portion, namely and, one copy ofeach.
High homology is found for most of the ATP synthase subunits fromdifferentbacteria and chloroplasts.

Mitochondrial enzyme is much more complex; are described at the moment. Some of these subunits have high homology to bacterial andchloroplast counterparts, especially subunits Alpha, Beta and Gamma inthe F1 portion and subunits and in the FOportion. Many subunits are unique for the mitochondrial enzyme (see for details).However, the catalytic and proton translocating "core" of the enzyme isstill highly homological to that of bacterial and chloroplast ATPsynthase. The overall topology of the enzyme is also quite similar.

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ATP and Energy Storage - Biology in Motion

To make a long story short, the primary function of ATP synthase in most organisms is ATP synthesis. Hence the name. However, in some cases the reverse reaction, i.e. transmembrane proton pumpingpowered by ATP hydrolysis is more important. A typical example: anaerobic bacteria produce ATP byfermentation, and ATP synthase uses ATP to generate protonmotive force necessary for ion transportand flagella motility.
Many bacteria can live both from fermentation and respiration or photosynthesis. In such case ATP synthasefunctions in both ways.
An important issue is to control ATP-driven proton pumping activity of ATP synthase in order to avoid wasteful ATP hydrolysis under conditions when no protonmotive force can be generated (e.g. leakydamaged membrane, uncoupler present, etc.). In such case ATP hydrolysis becomes a problem,because it can quickly exchaust the intecellular ATP pool. To avoid this situation,all ATP synthases are equipped with regulatory mechanisms that suppress the ATPaseactivity if no protonmotive force is present. The degree of ATP hydrolysis inhibitiondepend on the organism. In plants (in chloroplasts), where it is necessary to preserveATP pool through the whole night, the inhibition is very strong: the enzyme hardly has anyATPase activity. In contrast, in anaerobic bacteria where ATP synhase is the maingenerator of protonmotive force, such inhibition is very weak. Mitochondrial ATP synthase is somewhereinbetween.

ATP Synthase (FoF1-complex): Home

The Gibbs energy change under such conditions (temperature 310oK,or 37oC) will be

oi+-1

This figure, calculated from the actual concentrations of thereaction components, reflects the energy available as a driving forcefor any other process coupled to ATP hydrolysis under given conditions.
It follows that the same 49.6 kJ mol-1 must be provided bythe proton transport across the membrane down the electrochemicalgradient to maintain such a high ATP/ADP ratio. If we assume that 3protons are transported per each ATP molecule synthesized, atransmembrane H+ electrochemical gradient of 49.6 / 3= 16.5kJ mol-1(i.e., of 171 mV) is necessary.

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