Glycobsis is the initial step of catabolic chemical reactions. It occurs almost in all the cells. Six-carbon glucose molecule is broken do” ii into too molecules pyruvate durnig glycolysis. Pyruvate is a three carbon compound. There is net production ol bi‘o molecuces or ATP during glycolysis. Each molecule of glucose product, lour molecules or P during glycolysis. But to A FP molecules arc used to rearrange theglucose molecule. They form 11’n,k six-carbon compounds. Therefore, the net energy yield from glycolys1; is only two ATP molecules. Glycolvsis does not efficiently harvest eno.gy from glucose. But it was the only to generate KIT molecules for midions of years during the anaerobic stages of early life on earth. Folio.% ing reactions take place during glycolysis:
(a) Energy Investment Phase
I. Step Glucose enters the cell. It is phosphorylated,by the enzyme
hexoldnase. This enzyme transfers a phosphate group front ATP to the .sugar. The plasma membrane is impermeable to ions. So the electrical charge of the phosphate group traps the sugar in the cell. Phosphorylation of glucose makes the molecule more chemically react is e.
- Step 2: Glucose 6-phosphate is rearranged by enzyme isomerase. It is converted it to its isomer. fructose 6-phosphate
- Step 3: In this step, another molecule of ATP is used in glycolysis. An enzyme phosphofructokinase transfers a phosphate group from ATP to the sugar.
- Step 4: This is reaction from which glycolysis gets its name. An enzyme aldolasethreaks the sugar molecule into two different three-carbon sugars: glyceraidehydes phosphate and dilly droxyacetone phosphate. These two sugars are isomers of each other.
5‘. Step 5: Another enzyme catalyzes the reversible conversion between the two three—carbon sigars.. The reaction reaches equilibrium in test tube. .[he next enzyme in glycolysis uses only glyceraldehyde phosphate as its substrate.
(b) Energy-Yielding Phase
- Step 6: An enzyme now catalyzes two reactions. First, glyceraldehyde phosphate is oxidized tiw the transfer of electrons and FI to NAD. It forms NADI I . This reaction is very exergonic kcal/mol). The enzyme uses this energy to attach a phosphate group to the oxidized substrate. It forms 1,3 bisphosphogly eerie acid The source of the phosphate is inorganic phosphate. It is always present in the cytosol.
- Step 7: Finally… glycolysis produces sonic A IT. The phosphate group added in the previous step is transferred to ADP. It break 1,3 bisphosphoglyeeric acid to form 3 phosithogi) curate. This compound is not a sugar. The carbonyl group that characterizes a sugar has been oxidized to a carboxyl group. It is an org:mic acid.
–8. Step 8: An enzyme mutase relocates the remaining phosphate
group. It forms 2-phosphoglycerate. It prepares the substrate for the next reaction.
- Step 9: An enzyme enolase removes a water molecule and form a double bond in the substrate. It forms phosphoenol pyruvate. or PEP. This makes the substrate very unstable.
- Step 10: An enzyme pyruvate kinase transfers the phosphate group from PEP to ADP. So it produces more ATP. Since this step occurs twice for each glucose molecule, so there is a net gain of two ATP molecules. Thus glucose has been broken down and oxidized to two molecules of pyruvate. It is the end-product of the glycolytic pathway.