Answer:
Phase 1: Preparation, regulation and energy expenditure
In the initial preparatory phase of glycolysis (investment phase), glucose is phosphorylated twice by ATP and cleaved into two phosphate trioses. [2] At this stage, the cell spends two molecules of ATP, the Mg2 + cation is indispensable for the reactions, and five biochemical reactions are processed. No energy is stored, on the contrary, two ATP molecules are invested in phosphorylation reactions.
Reaction 1: hexokinase
In the first reaction, the glucose entering the tissues is phosphorylated to the hydroxyl group at C6, with the energy expenditure of an ATP molecule, giving rise to glucose-6-phosphate and ADP. [1] This reaction, catalyzed by the hexokinase enzyme, is irreversible under physiological conditions due to its highly negative ΔG °. It is one of the three steps that regulate glycolysis. The phosphorylation of glucose in the first reaction prevents it from leaving the cell again (glycolysis takes place in the cell's cytosol). By adding a phosphate group to glucose, it becomes a negatively charged molecule and it is impossible to passively pass through the cell membrane, keeping it trapped within the cell.
Glucose-6-phosphate is a branching point in carbohydrate metabolism. It is a precursor to almost all routes that use glucose, including glycolysis, via pentose phosphate and glycogen synthesis. From an opposite point of view, it can also be generated from other carbohydrate metabolism routes, such as glycogenolysis (glycogen breakdown), via pentose phosphate and gluconeogenesis (synthesis of glucose from non-carbohydrates).
Hexokinases, enzymes that catalyze glucose phosphorylation, are a family of tissue-specific isoenzymes that differ in their kinetic properties. The isoenzyme found in the liver and cells of the pancreas has a much higher Km than other hexokinases and is called glycokinase. Kinases are enzymes that catalyze the transfer of a terminal phosphoryl group from ATP to a nucleophile acceptor. In the case of hexokinase, the acceptor is a hexose, usually D-glucose, although hexokinase can catalyze the phosphorylation of other common hexoses, such as D-fructose and D-mannose. Hexokinase, like many other kinases, requires Mg2 + for its activity, since the true substrate of the enzyme is not ATP-4, but MgATP-2. In many cells, part of the hexokinase is bound to porins in the outer mitochondrial membrane, which give these enzymes early access to the newly synthesized ATP as it leaves the mitochondria.
Reaction 2: phosphohexose isomerase
In the second reaction, catalyzed by the enzyme glycosphosphate isomerase (also called phosphoxose isomerase), glucose-6-phosphate, an aldose, is converted into a reversible isomerization process in fructose-6-phosphate, a ketosis, thus allowing a site input for dietary fructose in glycolysis. This isomerization plays a critical role in the general chemistry of the glycolytic pathway, since the rearrangement of the carbonyl and hydroxyl groups at C-1 and C-2 is a necessary preparation for the next two steps. The phosphorylation that occurs in the next reaction (reaction 3) requires that the group at C-1 be first converted from a carbonyl to an alcohol and, in the subsequent reaction (reaction 4), the cleavage of the bridge between C-3 and C-4 by aldolase requires a carbonyl group at C-2.
Reaction 3: phosphofructokinase-1 (PFK1)
In reaction number 3, the cell invests another ATP molecule to phosphorylate fructose-6-phosphate and convert it to fructose-1,6-bisphosphate. This is also an irreversible and control reaction of this metabolic pathway, catalyzed by the enzyme phosphofrutokinase, which is the enzyme pacemaker of glycolysis. This step occurs to make the molecule symmetrical for the cleavage reaction in the next step.
Reaction 4: aldolase
In reaction 4, fructose-1,6-bisphosphate is cleaved into two trioses: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. This reaction is catalyzed by the enzyme aldolase.
Reaction 5: triosphosphate isomerase
Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are isomers that are easily interconnected by the enzyme triosphosphate isomerase. Then, the conversion of dihydroxy ketone P into glyceraldehyde 3P occurs, the only triosis that can continue to be oxidized.
Phase 2: ATP production and oxidation
In the ATP generation phase (yield), glyceraldehyde-3-phosphate (a phosphate triosis) is oxidized by NAD and phosphorylated using inorganic phosphate. The high-energy phosphate bridge generated in this step is transferred to the ADP to form ATP. The remaining phosphate is also rearranged to form another high-energy phosphate bridge that is transferred to the ADP. As there are two moles of phosphate triosis formed, the result
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