lecture 21 - glycolysis Flashcards
What is the first step of glycolysis?
formation of glucose-6-phosphate (G6-P) by phosphate transfer from ATP catalysed by hexokinase (glucokinase in liver) IRREVERSIBLE STEP
Phosphorylation important to:
activate glucose
accumulate G6-P in the cell when glucose levels outside are low (to maintain glucose concentration gradient for influx)
prevent efflux of glucose (G6-P cannot cross plasma membrane
Hexokinase traps glucose in the cell
Induced fit ensures specificity of hexokinase
The enzyme will only hydrolyze ATP when the presence of a substrate (e.g. glucose) induces its active conformation. This ensures that the generated Pi is always transferred to the substrate and thus no ATP is wasted.
What is the second step of glycolysis?
conversion of G6-P to fructose-6-phosphate (F6-P) by phosphohexose isomerase (also known as glucose-6-phosphate isomerase)
The conversion is through an open chain intermediate and involves the repositioning of the carbonyl oxygen from the aldehyde to the first ketone position.
What is the third step of glycolysis?
phosphate from ATP added to F6-P to form fructose 1.6 bisphosphate (F1,6-BP) + ADP catalysed by phosphofructokinase: IRREVERSIBLE STEP
The hexose becomes ‘symmetrical’ in respect to phosphate – ready to be converted to 3-carbon molecules.
Phosphofructokinase is allosterically regulated
PFK is the most intensely regulated glycolytic enzyme, so it controls the flux through this catabolic pathway.
In the presence of the allosteric activator ADP an arginine with a favourable positive charge is placed in the catalytic site. This attracts the negative charge of the substrate F-6-P. In the presence of allosteric inhibtion a glutamate’s negative charge repels the substrate.
What is the fourth step of glycolysis?
F1.6-BP broken down by aldolase into 2 x 3C compounds: glyceraldehyde 3-phosphate (GA3P) and dihydroxyacetonephosphate (DHAP)
Aldolase conveniently cuts the hexose into inter-convertible three carbon intermediates.
The rest of the pathway thus only has to oxidize three separate carbons to CO2 instead of 6 separate carbons, saving reaction steps.
What is the fifth step of glycolysis?
DHAP converted to GA3P via triose phoshate isomerase (reaction to GA3P favoured as GA3P removed by subsequent glycolysis reactions, shifting equilibrium)
At this point:
Used two ATPs
No oxidization
Generated two 3carbon intermediates: 2 X GAP
What is the sixth step of glycolysis?
GA3P converted to 1.3-bisphospholycerate by glyceraldehyde 3-phosphate dehydrogenase (reaction converts NAD+ to NADH)
The burning starts – oxidation of GAP
The reduction of NAD+ to NADH is tightly coupled to the production of 1,3-BPG. Therefore sufficient amounts of NAD+ are necessary for this reaction.
Under aerobic conditions NADH is used to generate ATP, which in turn regenerates NAD+. Under anaerobic conditions the oxidizing potential of NAD+ has to be regenerated by the reactions of fermentation.
Coupling of endergonic and exergonic reactions
The strongly exergonic oxidation helps to move the highly unfavourable phosphorylation forward. This allows creation of a high energy phosphate intermediate AND a molecule of NADH.
Thioester ensures that DG is kept high - The thioester intermediate’s free energy is between that of the reactants’ and the products’. This allows the reaction to proceed.
What is the seventh step of glycolysis?
ATP generated by donation of phosphate group from 1,3-biphosphoglycerate to ADP, catalysed by phosphoglycerate kinase, forming 3-phosphoglycerate
What is the eighth step of glycolysis?
conversion of 3-phosphoglycerate (3-PG) to 2-phosphoglycerate (2-PG) (near equilibrium) by phosphoglycerate mutase
What is the ninth step of glycolysis?
dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP) + H2O by enolase
What is the tenth step of glycolysis?
substrate level phosphorylation to generate ATP from PEP + ADP, forming pyruvate, catalysed by pyruvate kinase: IRREVERSIBLE.
Pyruvate kinase is tightly regulated allosterically, covalently, and at gene expression level by hormones
What does the overall pathway generate?
2 x ATP
2 x NADH
2 x pyruvate
per glucose
What are the three possible fates of pyruvate?
depends on degree of oxygenation and species
can be converted to:
acetyl coA, which enters the TCA cycle under aerobic conditions; 2CO2 evolved
ethanol + 2CO2: anaerobic e.g some gut bacteria, brewers’ yeast
lactate: anaerobic, e.g. muscle (carried in blood and converted back to pyruvate in liver by hepatic lactate dehydrogenase)
essential that pyruvate is metabolised further in order to regenerate the NAD+ required for further glycolysis to take place
Pyruvate -> acetyl coA step:
irreversible in animals and the carbon skeleton cannot be used to remake glucose
in plants and bacteria, this step can be reversed - the glyoxylate cycle
NADH generated in glycolysis can enter the electron transport chain
