![]() Pressure is the force divided by the area on which the force is exerted, and temperature is measured with a thermometer. We have examined pressure and temperature based on their macroscopic definitions. Solve problems involving the distance and time between a gas molecule’s collisions.Solve problems involving mixtures of gases.Explain the relations between microscopic and macroscopic quantities in a gas.This allows more protein chains, and thus more active sites, to be packed into the same space.By the end of this section, you will be able to: By packing many chains together into a tight complex, the protein reduces the surface that must be wetted by the surrounding water. So, why do plants build a large complex? The answer might lie in the crowded conditions under which rubisco performs its job. In fact, photosynthetic bacteria build a smaller rubisco (shown on the right, taken from PDB entry 9rub ) composed of only two chains, which performs its catalytic task just as well. Rubisco, however, seems to be rigid as a rock, with each of the active sites acting independently of one another. Often, the interactions between the different chains are used to regulate the activity of the enzyme in the process known as allostery. Many enzymes form similar symmetrical complexes. ![]() The protein shown here is taken from spinach leaves (coordinates may be found in the PDB entry 1rcx the tobacco enzyme may be found in 1rlc ). Plants and algae build a large, complex form of rubisco (shown on the left), composed of eight copies of a large protein chain (shown in orange and yellow) and eight copies of a smaller chain (shown in blue and purple). The plant cell must then perform a costly series of salvage reactions to correct the mistake. Rubisco then attaches the oxygen to the sugar chain, forming a faulty oxygenated product. But in rubisco, an oxygen molecule can bind comfortably in the site designed to bind to carbon dioxide. In proteins that bind oxygen, like myoglobin, carbon dioxide is easily excluded because carbon dioxide is slightly larger. Unfortunately, oxygen molecules and carbon dioxide molecules are similar in shape and chemical properties. Rubisco also shows an embarrassing lack of specificity. This makes rubisco the most plentiful single enzyme on the Earth. Chloroplasts are filled with rubisco, which comprises half of the protein. Plant cells compensate for this slow rate by building lots of the enzyme. Typical enzymes can process a thousand molecules per second, but rubisco fixes only about three carbon dioxide molecules per second. In spite of its central role, rubisco is remarkably inefficient. But one out of every six molecules is skimmed off and used to make sucrose (table sugar) to feed the rest of the plant, or stored away in the form of starch for later use. Most of the phosphoglycerate made by rubisco is recycled to build more ribulose bisphosphate, which is needed to feed the carbon-fixing cycle. Phosphoglycerates are familiar molecules in the cell, and many pathways are available to use it. Rubisco then clips the lengthened chain into two identical phosphoglycerate pieces, each with three carbon atoms. Rubisco takes carbon dioxide and attaches it to ribulose bisphosphate, a short sugar chain with five carbon atoms. Inside plant cells, the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco, shown here from PDB entry 1rcx ) forms the bridge between life and the lifeless, creating organic carbon from the inorganic carbon dioxide in the air. ![]() Powered by the energy of sunlight, plants perform this central task of carbon fixation. In order to be useful, this oxidized carbon must be "fixed" into more organic forms, rich in carbon-carbon bonds and decorated with hydrogen atoms. Unfortunately, carbon in the earth and atmosphere is locked in highly oxidized forms, such as carbonate minerals and carbon dioxide gas. All of our molecular machines are built around a central scaffolding of organic carbon.
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