Meaning of glycolysis:
Glycolysis means glucose catabolism (breakdown). In glycolysis glucose(6 carbon compound) degraded to pyruvate(3 carbon compound). Free energy releases in this catabolic process which is converted into ATP and NADH.
- Fermentation is the anaerobic degradation of glucose to give ATP.
- organisms first live in the atmosphere without or in a very less oxygen environment.
- In Evolution the chemistry of this reaction sequence has been completely conserved, the glycolytic enzymes of vertebrates are closely similar.
- Glycolysis differs only in regard to regulation and in the subsequent metabolic fate of pyruvate formed.
History and Important discoveries:
In 1897 Buchner’s discover fermentation in yeast. which leads to understand aerobic respiration also later on by many scientists.
- Development of a method of enzyme purification
- Importance of coenzyme NADH, ATP, and other Phosphorylated groups.
Glycolysis is the sole source of metabolic energy to various mammalian tissues and different cell types like Erythrocytes, Renal medulla, Brain and sperm cells, etc. Some plant tissues that store starch like potato tubers and aquatic plants derive most of their energy from glycolysis.
An Overview of Glycolysis:
it has 2 phases. 10 steps are involved in this process ; 5 steps in first phase i.e. Preparatory phase ( conversion of glucose to glyceraldehyde 3 phosphate) and 5 steps in second phase i.e. Payoff phase.
Three types of chemical transformations are note worthy
- Degradation of the carbon skeleton of glucose to yield pyruvate
- phosphorylation of ADP to ATP
- Transfer of hydride ion to NAD+ to NADH
Fates of pyruvate:
1: In Aerobic organisms
pyruvate is oxidized to CO2 and acetyl co A formed. which is then utilized in citric acid cycle.
2: In Anaerobic conditions:
- pyruvate is converted into lactate by lactic acid fermentation. IN Hypoxia (less oxygen condition) muscles functions under it. NADH does not re-oxidized to NAD+ but NAD+ is required as an electron acceptor for oxidation of pyruvate. pyruvate is reduced by NADH to lactate and releases NAD+ which is further used in glycolysis to continue the process.
- pyruvate is converted into ethanol + CO2 by alcoholic fermentation. Occur in microorganisms, plant tissues, and yeast, etc.
ATP formation Coupled to glycolysis:
Glycolysis is an irreversible process. energy remains in the pyruvate. during glycolysis the energy transfer to pyruvate which can be recovered in the citric acid cycle. glycolysis releases a small fraction of glucose but in actuality the 60% of energy is recovered as ATP.
Importance of phosphorylated intermediates:
- plasma membrane Lacks phosphorylated transporters so it remains in the cell. No further energy requires after the initial formation of the phosphoryl group to retain them in the cell
- the phosphorylated group is essential for enzymatic action, conservation of metabolic energy like energy releases during ATP breakage is conserved as glucose 6 phosphate and High energy phosphoryl groups donate energy to form ATP from ADP
- They provide binding energy, Mg+2 complex requires and use binding energy of phosphoryl compounds to form a complex with ATP, ADP, Glucose 6 phosphate, etc.
Preparatory phase of glycolysis:
1: Phosphorylation of glucose
- glucose is phosphorylated at carbon 6 to yield glucose 6 phosphate. with ATP as a phosphoryl donor.
- This is an irreversible reaction and catalyzed by hexokinase.
- kinases enzymes catalyze the transfer of terminal phosphoryl group from ATP to nucleophile
- hexokinase require Mg+2 for its activity. Mg+2 shields the negative charge of ATP making it easy for nucleophiles to attack on terminal phosphate group.
- Hexokinase undergoes changes in shape, induced-fit when it binds to glucose
- 2 proteins move closer, this movement brings bound ATP closer to a molecule of glucose also bound to the enzyme and blocks access to water
- Hexokinase is a soluble cytosolic protein. it is present in the cell of all organisms.
2: Conversion of glucose 6 phosphate to fructose 6 phosphate:
- reversible reaction, phosphohexose isomerase catalyzes the isomerization of glucose 6 phosphate to fructose 6 phosphate.
- this isomerization is necessary for the next steps.
3: Phosphorylation of fructose 6 phosphate to 1,6 Bisphosphate:
Phosphofructokinase-1 catalyzes the transfer of phosphoryl group from ATP to fructose 6 phosphate to yield Fructose 1,6 Bisphosphate.
- PFK1 is a regulatory enzyme. It is the major point of regulation in glycolysis.
- Its activity increases whenever there is less amount of ATP present in the cell or a large amount of ADP or AMP.
- Fructose2,6 bisphosphate is an allosteric activator of PFK1.
4: Cleavage of Fructose 1,6 Bisphosphate
- Aldolase Catalyzes the reversible reaction. Fructose cleaved into Glyceraldehyde 3 phosphate and dihydroxyacetone.
5: Interconversion of Triose phosphate
Dihydroxyacetone is isomerize to G3P by triose phosphate isomerase.
The prep phase is completed at this reaction and the hexose molecule has been phosphorylated at C-1 and C-2 and then cleaved to form 2 molecules of G3P.
The Pay off phase:
in this phase some of the energy of glucose molecule is conserved in the form of ATP.
6: oxidation of Glyceraldehyde 3 phosphate to 1,3 Bisphosphoglycerate
- This reaction is catalyzed by Glyceraldehyde 3 phosphate dehydrogenase.
- Much of the energy of G3P is conserved by the formation of the acyl phosphate group at C1 of 1,3bisphosphoglycerate.
- NAD+ is the hydrogen acceptor and its reduction is proceeded by the transfer of hydride ion from aldehyde group of G3P to its ring forming NADH.
- G3P is covalently bounded to dehydrogenase enzyme during the reaction
- Aldehyde group of G3P reacts with _SH group of an essential Cys residue in active site producing a thiohemiacetal.
- NADH leaves and NAD+ enters the reaction and Pi also. this phosphoryl attack forms the 1,3 bisphosphoglycerate.
7: Phosphoryl transfer of 1,3 Bisphosphoglycerate to ADP
The enzyme phosphorylate kinase transfers the high energy phosphoryl group from the oxyl group of 1,3 BPG to ADP forming ATP and 3,phosphoglycerate.
the overall energy in step 6 and 7 is exergonic .
8: Conversion of 3 phosphoglycerate to 2 phosphoglycerate
phosphoglycerate mutase catalyzes the reaction. This is reversible reaction phosphate group shifts from C1 to C2 in glycerate. Mg+2 is essential for this reaction.
9: dehydration of 2 Phosphoglycerate to phosphoenolpyruvate
- PEP is the compound that has a high potential to transfer phosphoryl group.
- Enolase enzyme is used in the removal of water molecule from 2 PG
- A high energy phosphate bond is produced. The reaction is reversible.
- Enolase requires Mg++.
10: transfer of phosphoryl group from PEP to ADP
- This reaction is catalyzed by the pyruvate kinase. which require K+ and MG++or Mn++
- Phosphoenol pyruvate (PEP) is dephosphorylated to pyruvate, by pyruvate kinase.
- First PEP is made into a transient intermediary of enol pyruvate; which is spontaneously isomerized into keto pyruvate, the stable form of pyruvate.
- One mole of ATP is generated during this reaction. This is substrate level phosphorylation.
- The pyruvate kinase is a key glycolytic enzyme. This step is irreversible.
Reference: Lehninger principles of biochemistry
Animation video is taken as Reference from Youtube for better understanding.