Energy generation: First Stage of Glycolysis Flashcards
What is the overall process of glycolysis?
Glucose (C6H12O6) —> Pyruvate (2x C3H3O3)
Net generation of ATP= Energy
What happens during glycolysis to NAD and what is themes important catabolic process?
Glucose becomes oxidised to form pyruvate= Reducing agent as it reduces NAD+ –> NADH
Other carbohydrates can ‘feed in’ to the pathway at the level of glucose
Most important: Starch degradation
What is the first step of glycolysis?
What is used in the process?
Glucose + ATP —> Glucose 6-phosphate + ADP + H+
Catalysed by hexokinase enzyme which phosphorylate the 6th position of sugar= Induced fit
One molecule of ATP is used in the reaction
Large -ve ΔG = Effectively irreversible in vivo
Importance of step: TRAPS glucose inside the cell as G-6-P cannot be transported
What is the second STEP of glycolysis?
Glucose 6-phosphate —> Fructose 6-biphosphate
Isomerisation by phosphoglucose isomerase
Becomes open chained form, then COH is converted to COCH2OH and then it goes back to closed form again
What is the third STEP of glycolysis?
Fructose 6-phosphate + ATP –> Fructose 1,6-biphosphate + ADP + H+
MOST IMPORTANT STEP= Catalysed by phosphofructokinease= Very large -veΔG= Irreversible= Can directly control the amount of glucose in the glycolytic pathway + Small changes in concentration will easily change the equilibrium
What are the other two most important dietary sugars which can enter the glycolytic pathway via phosphorylation?
Fructose= Present in fruits
Galactose= Milk and polysaccharides in plant tissues
Need to covert these into forms which can be fed into energy generation
How can galactose enter (‘feed in’) the glycolytic pathway?
When does it enter?
Hydrolysis of disaccharide lactose (in milk) —> Galactose and Glucose
Are epimers (stereoisomers) therefore to convert galactose into glucose 6-phosphate= Galactose-glucose interconversion pathway
1) Galactose is phosphorylated by galactokinase= Galactose 1-phosphate= Use of ATP
2) Transfer of a uridyl group
3) Converted back to UDP-galactose 4-epimerase
4) Converted to glucose 6-phosphate
Enters: Second step go glycolysis as glucose 6-P
What happens if there is defect in galactose metabolism in humans?
Lactose intolerance
Low levels of lactase= High build up of lactose= Metabolised by bacteria in the gut
Leads to flatulence, bloating and diarrhoea
What else can be caused if there is a defect in galactose metabolism in humans?
Galactoseamia
Absence of one of the enzymes converting galactose to glucose= Toxic substances accumulate galactitol
Leads to damage to liver, eyes, kidneys and brain
How does fructose ‘feed in’ to the glycolytic pathway?
Hydrolysis of sucrose —> Fructose + glucose
Muscle and adipose tissue= Fructose can be phosphorylated by hexokinase= Fructose 6-phosphate=
Enters: third step of glycolysis
How does stored intracellular carbohydrate in the form of starch enter the glycolytic pathway?
Intracellular starch= Glycogen
Used to mobilise carbohydrate stores in cells such as liver and muscle
Glycogen degradation —> Phosphorylated hexose sugar directly by glycogen phosphorylase
Glycogen –> glucose 1-phosphate + Glycogen with one less residue
THEN: Glucose 1-phosphate is then converted to glucose 6-phosphate by phosphoglucomutase
Phosphoglucomutase:
1) Dephosphorylates serine = enzyme becomes inactivated
2) Enzyme donates the phosphate from the serine to glucose 1-phosphate= Glucose 1,6-biphosphate
3) The phosphate group on 1st carbon is removed
Enters: Second step of glycolysis
What is the overall stage 1 of glycolysis?
Glucose —> Fructose 1,6-biphosphate
2 molecules of ATP are used up during this stage
Why is ATP consumed during this stage?
1) Addition of polar, hydrophilic phosphate group prevents metabolites from diffusing out of the cell through the membrane
2) Addition of charged phosphate groups allows compounds to bind to enzymes with higher affinity
