glycolysis Flashcards
describe glycolysis
its the central pathway for carbohydrate metabolism, because all other sugars can be converted to glucose
its ancient process
its sued by vast array of organisms such as mammals, bacteria and plants
its anaerobic process, as evolved before lots oxygen present in earths atmosphere
give a quick overview of glycolysis what it uses and produces
it involves breakdown of glucose (c6) into 2 pyruvate molecules (c3), resulting in 2 molecules of ATP being produced
where does glycolysis occur
in cytosol of mammalian cells
how many reactions are there in glycolysis
10 which are essentially the same in all cells and are divided into 2 phases
summarise the 2 phases of glycolysis
phase 1: converts glucose into two glyceraldehyde 3-phosphate molecules and is energy requiring (catabolic)
phase 2: generates two pyruvate molecules and is energy producing (anabolic)
name the products of glycolysis
2 pyruvate
4 ATP
2 NADH
2 water
what are the 3 irreversible reactions in glycolysis pathway and why need them
needed as allow process to move forward and progress
- Glucose —>Glucose-6-phosphate
Hexokinase
- Fructose-6-phosphate—-> Fructose-1,6-bisphosphate
Phosphofructokinase
- Phosphoenolpyruvate —-> Pyruvate
Pyruvate kinase
describe the hexokinase reaction 1
glucose is irreversibly phosphorylated into glucose-6-phosphate, this is powered by ATP hydrolysis (the transfer of a phosphate group from ATP onto other molecule is called phosphate transfer)
its important to trap glucose inside cell
describe the phosphoglucose isomerase reaction 2
this is the second step in the isomerisation of glucose-6-phosphate to fructose-6-phosphate
the reaction is the conversion of aldose sugar into ketose sugar
- aldose = aldehyde group
- ketose = ketone group
describe the phosphofructokinase reaction 3
fructose-6-phosphate is phosphorylated again to generate fructose-1,6-bisphophate
thus is the 2nd phosphoryl transfer reaction of glycolysis
what does allosteric mean
enzymes that have an additional binding site for effector molecules other than the active site.
- ATP is an allosteric effector
- high levels inhibit the enzyme
- low levels activate it
how is phosphofructokinase an isoenzyme and allosteric
it exists as a tetramer:
- the tetramer is composed of L and M subunits
- M4, M3L, M2L2, and L4 all exist
- combinations of these subunits are called isoenzymes
- muscles are rich in M4
liver is rich in L4
describe aldolase reaction 4
fructose -1,6-bisphosphate is cleaved into two 3-carbon fragments:
- dihydroxyacetone phosphate - a ketose
- glyceraldehyde 3-phiosphate - an aldose
describe the triose phosphate isomerase reaction 5
the subsequent reaction uses only glyceraldehyde 3-phosphate.
- dihydroxyacetone phosphate (DHAP) is converted to glyceraldehyde-3-phosphate. when this is used, the equilibrium is restored by conversion of DHAP to glyceraldehyde 3-phosphate
describe a summary of phase 1
its a catabolic (energy investing) phase as 2 molecules of ATP have been used
glucose is converted to two molecules of glyceraldehyde 3-phosphate. the key intermediate in this reaction is fructose-1,6-bisphosphate. the enzyme that catalyses the reaction, phosphofructokinase, is subject to allosteric control
describe glyceraldehyde 3-phosphate dehydrogenase reaction 6
its the first reaction that begins conversion to pyruvate, involving oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. this reaction involves adding an inorganic phosphate group
NAD+ is reduced to NADH
describe phosphoglycerate kinase reaction 7
1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase
this step involves another reaction where ATP is produced by ADP phosphorylation. As 1,3-bisphosphoglycerate transfers a phosphate group to ADP known as substrate level phosphorylation.
the reaction is catalysed by phosphoglycerate kinase and pays off the ATP dept created by priming reactions in the first place
describe phosphoglyceromutase reaction 8
3-phosphoglycerate is isomerized to 2-phosphoglycerate. this is by a mutase catalysing the migration of a functional group within the substrate
describe enolase reaction 9
enolase = phosphopyruvate hydrolase
2-phosphoglycerate lose one water molecule, forming phosphenolpyruvate
enolase catalyses the reaction and requires a mg2+ cofactor. this requires 3 isoenzymes:
- alpha alpha or NNE (non-neural enolase), found in tissues such as liver, brain, kidney, spleen, adipose. and is present at some level in normal human cells
- beta beta or MSE (muscle-specific enolase), enzyme largely restricted to muscle where present at very high levels
- gamma gamma or NSE (neuron-specific enolase), in very high levels in neurons and neural tissue, accounting for as much as 3% of total soluble protein. its expressed at much lower levels in mammalian cells
describe pyruvate kinase reaction 10
phosphoenolpyruvate contains a high energy bond. as transfers its phosphate group to ADP producing ATP and pyruvate (substrate level phosphorylation)
this reaction is catalysed by pyruvate kinase
give a summary of phase 2
its anabolic (energy producing)
2 pyruvate molecules produced for each molecule of glucose entering pathway. these reactions involve production of two NADH molecules and net production of two ATP for each glucose molecule
what are the metabolic fates of NADH and pyruvate produced in glycolysis in mammals
NADH can be recycled via aerobic or anaerobic pathways
- aerobic (oxygen present), NAD is reoxidised in electron transport pathway, making ATP in oxidative phosphorylation. three ATP molecules made from one NADH molecule
- anaerobic (no oxygen), NADH is reoxidized by lactate dehydrogenase (LDH) which provides NAD+ for more glycolysis
describe the anaerobic metabolism of pyruvate
under anaerobic conditions, such as actively contracting muscle, the most important pathway for regeneration of NAD+ is reduction of pyruvate to lactate by lactase dehydrogenase
describe the aerobic metabolism of pyruvate
aims to get pyruvate into citric acid cycle where the products of glucose can be completely oxidised, leading to production of many ATP
the 2 steps required for this are:
- convert pyruvate into acetyl-CoA
- put the acetyl CoA into the citric acid cycle
