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Phosphoglucose Isomerase. The second step in glycolysis is an isomerization: a reaction that changes the shape of a single molecule, but doesn't permanently add or remove any atoms. The enzyme phosphoglucose isomerase, shown here from PDB entry 1hox, takes glucose-6-phosphate and shuffles a few atoms, forming fructose-6-phosphate, shown here in yellow. The enzyme can do this reaction in either direction. So when glucose-6-phosphate is plentiful in the cell, it converts it to fructose-6-phosphate, and when fructose-6-phosphate is more common, the enzyme converts it back. Recently, researchers have discovered that this protein also plays other important roles outside of cells, acting not as an enzyme but rather as a molecular messenger. It is secreted by white blood cells and helps control the growth and motion of many types of cells. As researchers delve deeper and deeper into the human genome, they are discovering numerous examples of other "moonlighting" proteins that have one function in one place in the body and an entirely different function somewhere else.
Phosphofructokinase. At the third step of glycolysis, we reach the major point of regulation. Glucose-6-phosphate and fructose-6-phosphate, formed in the first two steps of glycolysis, are used by other cellular processes. But when phosphofructokinase adds a second phosphate to the sugar, it is committed for complete breakdown. Phosphofructokinase is like a miniature molecular computer that senses the levels of different molecules and decides if the time is right for breakdown of sugar. For instance, when ADP and AMP are common, the cell needs to make ATP, so the enzyme turns on. Phosphfructokinase is a mechanical computer, with moving parts. The bacterial enzyme shown at the right (from PDB entry 4pfk) is composed of four identical subunits. The forms in our own cells are even larger and more complex. The active site binds to the sugar, shown in orange, and an ATP, shown in red (this structure actually has ADP bound, along with a magnesium ion shown in green). Notice how each binding site for the sugar is composed of two different subunits, closing around either side of the molecule. Allosteric motions in phosphofructokinase: active state (left) and inactive state (right).The enzyme also has regulatory binding sites at the top and bottom--you can just see the other ADP molecules bound there, marked with asterisks. As shown below (looking from the side of the enzyme) the whole enzyme to shifts when ADP and other molecules bind to the regulatory sites. The active state is shown at left from PDB entry 4pfk, and an inactive structure is shown on the right from PDB entry 6pfk. When the enzyme shifts, the shape of the active site is changed and the enzyme switches on and off. [2]
Fructose 1,6-bisphosphate Aldolase. At this stage in glycolysis, the sugar molecule is primed and the cell is ready to start breaking it up. The fourth enzyme, fructose 1,6-bisphosphate aldolase, cuts the molecule in the middle, producing two similar pieces, each with a single phosphate attached. The enzyme also readily performs the reverse reaction, connecting these two smaller molecules to reform the phosphorylated fructose. In fact, the enzyme is named for this reverse reaction, which is an aldol condensation. The enzyme shown here, from PDB entry 4ald, is found in our muscle cells. It contains four identical subunits, each with its own active site. The active site uses a special lysine, number 229 in this particular form, to attack the sugar chain. As shown on the right in PDB entry 1j4e, this lysine forms a covalent bond with molecule during the cleavage reaction. This structure is frozen at a stage when the sugar molecule (with red oxygen, white carbon, and yellow phosphorus) has been cleaved and only half is left in the active site.
