metabolism 3 & 4

Question 1

What are the common metabolic reactions (6)

Answer 1

1. oxidation-reduction → oxidation = removal of hydrogen and electrons, reduction = acceptance of hydrogen and electrons
2. ligation → to join molecules covalently
3. isomerisation
4. group transfer → transfer of functional groups within the same molecule
5. hydrolysis → split bond through the addition of water
6. addition or removal of functional groups → removal to create a double bond or addition of a functional group across a double bond.

Question 2

What are the features of glycolysis (7)

Answer 2

1. catabolic process that breaks down glucose and other monosaccharides
2. occurs in the cytoplasm
3. glucose is broken down to produce ATP
4. unique characteristic: aerobic (presence of oxygen) or anaerobic (absence of oxygen)
5. It is divided into two distinct stages.
   
   1. conversion of glucose into two molecules of glyceraldehyde 3-phosphate (steps 1 → 5) and requires energy
   2. glyceraldehyde 3-phosphate is oxidised to pyruvate, and energy is produced (steps 6 → 10) and produces energy

Question 3

What occurs during stage 1 of glycolysis (5)

Answer 3

1. Phosphorylation—phosphorylation of glucose at carbon 6 to produce glucose 6-phosphate, catalysed by hexokinase. This prevents glucose from diffusing out of the cell. It requires energy as the kinase enzyme catalyses the hydrolysis of the terminal phosphate group of ATP, converting it to ADP. (irreversible step)
2. Isomerisation - Formation of fructose 6-phosphate catalysed by isomerase enzyme. The ring is rearranged from a six-membered ring form to a five-membered ring form. (reversible step)
3. Phosphorylation - Fructose 6-phosphate is phosphorylated to produce fructose 1,6 diphosphate (irreversible) and uses another ATP donor for the second phosphate addition, again requiring energy catalysed by kinase enzymes. This is also a point of feedback control to control the rate of glycolysis through the inhibition or activation of kinase enzymes. (irreversible)
4. Cleavage - fructose 1,6-diphosphate is cleaved into two molecules: two triose phosphates (3 carbon sugars). (reversible)
5. Isomerisation - interconversion of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The ketone is converted into glyceraldehyde 3-phosphate as it is acidic and affects the ability of blood to transfer oxygen. This is achieved by isomerisation through a rapid rearrangement reaction. Catalysed by isomerase enzyme (reversible)

Question 4

What occurs during stage 2 of glycolysis (5)

Answer 4

1. Oxidation & phosphorylation - conversion of glyceraldehyde 3-phosphate into 1,3-diphosphoglycerate. This is an oxidation reaction with an additional phosphorylation reaction to give the molecule a phosphate on the 1st carbon. (two molecules, so it happens twice). (reversible)
2. Transfer - of phosphate from 1,3-disphosphoglycerate to ADP to produce ATP, leaving 3-phosphoglycerate (2 molecules of ATP produced, making up for the loss in stage 1) (reversible)
3. Mutation - movement of phosphate from 3-phosphoglycerate, turning it into 2-phosphoglycerate. Catalysed by a mutase enzyme. (reversible)
4. Dehydration - of 2-phosphoglycerate introduces a double bond in the molecule, which makes this unstable and converts it into phosphoenolpyruvate. (reversible)
5. Conversion - of phosphoenolpyruvate into pyruvate (which is stable), when this phosphate group is lost, it is transferred to a molecule of ADP generating ATP. Catalysed by kinase enzymes. (Two molecules of pyruvate and two molecules of ATP) (irreversible)

Question 5

Where are the points of control in glycolysis (2)

Answer 5

1. step 3 - phosphorylation of fructose 6-phosphate through kinase enzymes
2. step 10 - Conversion of phosphoenolpyruvate into pyruvate through kinase enzymes

Question 6

How is acetyl-coa formed (6)

Answer 6

1. Pyruvate is transported from the cytoplasm into the mitochondria.
2. Pyruvate undergoes oxidative decarboxylation (loses the two terminal oxygens) by pyruvate dehydrogenase to form acetyl coenzyme A (acetyl-CoA) , which produces CO2 and NADH.
3. pyruvate + CoA-SH → acetyl S and CoA (acetyl-CoA)
4. NAD+ gains hydrogen from SH to form NADH.
5. NADH is then oxidised in the electron transport chain
6. Acetyle-CoA is the fuel for the Krebs Cycle.

Question 7

What happens during the Krebs Cycle stage 1 (4)

Answer 7

1. Condensation- of acetyl-CoA with oxaloacetate to produce citrate. (Point of feedback control)
2. Isomerisation - alcohol group of citrate is moved to form isocitrate. The alchol is changed from a tertiary alcohol to a secondary alcohol which can be oxidised
3. Oxidatative decarboxylation - alcohol is removed to from a ketone and CO2 is removed from isocitrate to form alpha-ketoglutarate (there is also a reduction of NAD+ to form NADH facilitated by gaining the hydrogen that was removed from the alcohol to form the ketone) (NADH drives the cell to make 3 ATP)
4. oxidative decarboxylation - of alpha-ketoglutarate to form succinyl-CoA (further reduction of NAD+ to from NADH)

Question 8

What happens during the Krebs Cycle stage 2 (4)

Answer 8

1. Cleavage of thioester bond in succinyl-CoA → succinate (side reaction: GTP + ADP → ATP + GDP)
2. Oxidation of succinate → fumarate
3. Hydration of fumarate → malate
4. Oxidation of malate → oxaloacetate