when the 6 carbon molecules are broken into three carbon molecules then where will be the second half that is 3 carbon molecules? explain
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Answer:We now start our consideration of the glycolytic pathway. This pathway is common to virtually all cells, both prokaryotic and eukaryotic. In eukaryotic cells, glycolysis takes place in the cytosol. This pathway can be thought of as comprising three stages (Figure 16.3). Stage 1, which is the conversion of glucose into fructose 1,6-bisphosphate, consists of three steps: a phosphorylation, an isomerization, and a second phosphorylation reaction. The strategy of these initial steps in glycolysis is to trap the glucose in the cell and form a compound that can be readily cleaved into phosphorylated three-carbon units. Stage 2 is the cleavage of the fructose 1,6-bisphosphate into two three-carbon fragments. These resulting three-carbon units are readily interconvertible. In stage 3, ATP is harvested when the three-carbon fragments are oxidized to pyruvate.Phosphoryl transfer is a fundamental reaction in biochemistry and is one that was discussed in mechanistic and structural detail earlier (Section 9.4). Kinases are enzymes that catalyze the transfer of a phosphoryl group from ATP to an acceptor. Hexokinase, then, catalyzes the transfer of a phosphoryl group from ATP to a variety of six-carbon sugars (hexoses), such as glucose and mannose. Hexokinase, like adenylate kinase (Section 9.4.2) and all other kinases, requires Mg2+ (or another divalent metal ion such as Mn2+) for activity. The divalent metal ion forms a complex with ATP.
The results of x-ray crystallographic studies of yeast hexokinase revealed that the binding of glucose induces a large conformational change in the enzyme, analogous to the conformational changes undergone by NMP kinases on substrate binding (Section 9.4.3). Hexokinase consists of two lobes, which move toward each other when glucose is bound (Figure 16.4). On glucose binding, one lobe rotates 12 degrees with respect to the other, resulting in movements of the polypeptide backbone of as much as 8 Å. The cleft between the lobes closes, and the bound glucose becomes surrounded by protein, except for the hydroxyl group of carbon 6, which will accept the phosphoryl group from ATP. The closing of the cleft in hexokinase is a striking example of the role of induced fit in enzyme actionInduced Fit in Hexokinase. Image mouse.jpg As shown in blue, the two lobes of hexokinase are separated in the absence of glucose. The conformation of hexokinase changes markedly on binding glucose, as shown in red. The two lobes of the enzyme come together and surround (more...)
The glucose-induced structural changes are significant in two respects. First, the environment around the glucose becomes much more nonpolar, which favors the donation of the terminal phosphoryl group of ATP. Second, as noted in Section 9.4.3, the substrate-induced conformational changes within the kinase enables it to discriminate against H2O as a substrate. If hexokinase were rigid, a molecule of H2O occupying the binding site for the-CH2OH of glucose would attack the γ phosphoryl group of ATP, forming ADP and Pi. In other words, a rigid kinase would necessarily also be an ATPase. It is interesting to note that other kinases taking part in glycolysis—pyruvate kinase, phosphoglycerate kinase, and phosphofructokinase—also contain clefts between lobes that close when substrate is bound, although the structures of these enzymes are different in other regards. Substrate-induced cleft closing is a general feature of kinases.
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16.1.2. The Formation of Fructose 1,6-bisphosphate from Glucose 6-phosphate
The next step in glycolysis is the isomerization of glucose 6-phosphate to fructose 6-phosphate. Recall that the open-chain form of glucose has an aldehyde group at carbon 1, whereas the open-chain form of fructose has a keto group at carbon 2. Thus, the isomerization of glucose 6-phosphate to fructose 6-phosphate is a conversion of an aldose into a ketose. The reaction catalyzed by phosphoglucose isomerase includes additional steps because both glucose 6-phosphate and fructose 6-phosphate are present primarily in the cyclic forms. The enzyme must first open the six-membered ring of glucose 6-phosphate, catalyze the isomerization, and then promote the formation of the five-membered ring of fructose 6-phosphate.
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A second phosphorylation reaction follows the isomerization step. Fructose 6-phosphate is phosphorylated by ATP to fructose 1,6-bisphosphate (F-1,6-BP). The prefix bis- in bisphosphate means that two separate monophosphate groups are present, whereas the prefix di- in diphosphate (as in adenosine diphosphate) means that two phosphate groups are present and are connected by an anhydride bond.The second stage of glycolysis begins with the splitting of fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). The products of the remaining steps in glycolysis consist of three-carbon units rather than six-carbon units
Explanation:
Answer:
The strategy of these initial steps in glycolysis is to trap the glucose in the cell and form a compound that can be readily cleaved into phosphorylated three-carbon units. Stage 2 is the cleavage of the fructose 1,6-bisphosphate into two three-carbon fragments.