GLYCOLYSIS: Definition, Reactions, and 10 Steps for Class 10th and 11th
Definition:
Glykys is a Greek word that means “sweet” or “sugar,” and lysis means “breakdown.”
This pathway consists of a series of non-oxygenated glucose breakdown processes that occur during respiration. Both aerobic and anaerobic respiration are affected by this. G.Embden, O.Meyerhof, and J K Parnas were the first to discover the EMP pathway. There are ten steps in all, all of which are catalyzed by separate enzymes. Steps 1, 3, and 10 are irreversible, but the remaining reactions are reversible. The above-mentioned three stages are thermodynamically infeasible. Because the cytosol contains enzymes that catalyze glycolysis, the cytosol is the glycolysis site. Glycolysis is the breakdown of glucose.
2 Pi + Glucose + 2 NAD+ + 2 ADP —————— 2 Pyruvate + 2 NADH + 2 ATP + 2H2O + 4H +
This is accomplished in the following manner:-
The phase of preparation (ATP consuming phase)
1. In the first phase of the reaction, glucose is phosphorylated in the presence of Mg ++ by ATP catalyzed by the enzyme Hexokinase.
Glucose +ATP—-hexokinase, Mg ++ ———Glucose 6- phosphate +ADP
2 . Phosphohoglucoisomerase (PGI) is an enzyme that transforms glucose 6 phosphates to fructose 6 phosphate.
Glucose 6-p —–PGI————-Fructose 6-p
3 . In the presence of Mg ++, fructose 6-p is phosphorylated by another ATP-producing Fr. 1, 6-bisphosphate, which is catalyzed by the enz. Phosphofructokinase (PFK).
Fr.6-P + ATP ——-PFK– Mg ++ —- Fr.1, 6-BP + ADP
4. The enz. aldolase splits FR.1,6- BP into one molecule of glyceraldehyde 3-phosphate (Gly3P) and one molecule of dihydroxyacetone phosphate (DHAP).
FR.1, 6- BP ====-aldolase====Gly 3-P + DHAP
5. The enz.Triosephosphate isomerase catalyzes the interconversion of Gly 3-P and DHAP (TPI). Only Gly 3P is used in subsequent conversion because the reaction is reversible. DHAP easily transforms to Gly-3P, allowing additional reactions to proceed.
Gly 3-P——-TPI ———DHAP
B.ATP generating phase
6. Gly 3-P dehydrogenase reacts with each Gly 3-P and produces 1, 3-biphosphoglycerate with the addition of inorganic phosphate (Pi). This reaction combines two reactions:
(a) NAD+ oxidation of glyceraldehydes to glycerate, and
(b) Orthophosphate combining with the carboxylic group to generate acyl-phosphate. It’s worth noting that one of the two molecules of Gly3-P derives from the DHAP reversal.
2 Gly3-P+2NAD+ + pi ——-Gly3P dehydrogenase————2 1,3-Biphosphoglycerate+NADH+H+
7. In the presence of Mg ++, the enzyme 3P-glyceric kinase (PGK) converts two molecules of 1,3-BPG into two molecules of 3-Phoshoglycerate (3-PG).
2 1,3-BPG +2ADP ——PGK—Mg++ ——-2, 3-PG + 2ATP
8. The enzyme enz. Phosphoglycerate Mutase converts two molecules of 3-PGlycerate to two molecules of 2-PGlycerate(2-PG) (PGM)
2,3-PG———-PGM————–2,2-PG.
9. The enz. Enolase dehydrates two molecules of 2-PG to two molecules of Phosphoenolpyruvate (PEP).
2, 2-PGly ——-enolase- Mg ++ ———2 Phsphoenolpyruvate + 2H2O
10. In the presence of Mg++, enz. Pyruvate kinase (PK) acts on two molecules of PEP to create two molecules each of ATP and Pyruvate.
2 PEP + 2 ADP ———-PK-, Mg++, K + —–2 Pyruvate + 2 ATP
It’s worth noting that each oxidized glucose molecule produces two Pyruvate molecules. Splitting Fructose 1, 6-biP yields one molecule of Gly 3-P and one molecule of DHAP (STEP4). In step 5, DHAP is also transformed to Gly-3P.
In Glycolysis, the entire energy balance sheet indicates a net gain of 2 ATP.
Consumption of ATP per molecule of glucose used ———2
ATP produced ———-2X2=4 molecules (step 7 & 10)
ATP gain per mol of glucose consumed —— 2
Pyruvate’s Post-Glycolysis Fate
Pyruvate enters mitochondria during eukaryotic aerobic respiration and is oxidatively decarboxylated by an enzyme called pyruvate dehydrogenase (PD). It is a multienzyme complex that results in decarboxylation and dehydrogenation of Pyruvate by combining three separate enzymes: (PD; E1), dihydrolipoyl transacetylase (E2; DLT), and dihydrolipoyl dehydrogenase (E3; DLD). The mitochondrial matrix is home to this enzyme. Mitochondria are missing in prokaryotes, like bacteria, and PD is found in the cytosol.
Thiamine pyrophosphate (TPP), Lipoamide (LA), acetyl-CoA, FAD, and NAD+ is required for oxidative decarboxylation of pyruvate.
The steps of response are as follows:
Pyruvate +TPP—E1——–PD—Hydroxyethylthiamine pyrophosphate(HETPP) + CO2
HETPP + LA (ox)—E2 — DLT—Acetyl- dihydrolipoamide + TPP
Acetyl-dihydrolipoamide + CoA—–E2–Acetyl-CoA + LA (red.) LA (red.)
+ FAD——–E3——- LA (ox.) +FADH2
FADH2 + NAD+ ——–E3——FAD + NADH+H+
Because one molecule of glucose yields two molecules of pyruvate, the whole oxidative decarboxylation of pyruvate reaction can be summarised as:
2 Pyruvate+2CoA+2NAD + ——-(PD)—–2acetyl- CoA +2CO2+2NADH+2H+
(CoA—Coenzyme A)
Pyruvate is metabolized to ethyl alcohol and CO2 in anaerobic respiration, which is present in yeasts. Yeasts, on the other hand, are facultative anaerobes that can breathe aerobically but anaerobically in anaerobic conditions.
C 3H 4O3 (Pyruvic acid) ——Pyruvic decarboxylase—CH 3CHO (Acetaldehyde)+CO2
CH3CHO + NADH+H + —-Alcohol dehydrogenase— C 2H 5OH (ethyl alcohol)+ NAD +
This reaction is used in the synthesis of ethyl alcohol by yeasts. The process is called Alcoholic Fermentation and has industrial value. This is utilized in the fermentation of sugar using yeasts in the alcohol industry. Bread is leavened by the CO2 produced in the process, which is vital for bakeries. Lactic acid, produced by Lactobacilli, is another byproduct of anaerobic respiration.
Lactic acid bacteria (Lactobacilli) use the following reaction (Lactic acid fermentation) to convert Pyruvate to Lactate:
C3H4O3 + NADH+ H+ ———Lactate dehydrogenase——C3H6O3 + NAD+
Pyruvic acid —————— Lactic acid
Mammals, unlike yeasts, lack Pyruvate decarboxylase and so are unable to create ethanol from pyruvate. Lactate dehydrogenase catalyzes the conversion of pyruvate to lactate, which is a reversible process (reaction as above). Lactate is also created in the skeletal muscles when they are worked hard. Lactate dehydrogenase converts it to pyruvate after transporting it through the bloodstream to the liver. Oxygen is required for further pyruvate metabolism. Lactic acid accumulates when there is an insufficient supply of oxygen. During and after exercise, lactic acid produces muscle pain.