BIOC 203 - glycolysis & krebs chemical reasoning

glycolysis - step 1

phosphorylation of glucose

• traps glucose inside the cell

• keeps internal [glucose] low, promotes further glucose transport into cell

glycolysis - step 2

isomerization of G6P

• conversion of aldose to ketose

• later cleavage occurs next to carbonyl group (C2), which is easier for a ketone than an aldehyde

glycolysis - step 3

phosphorylation of F6P

• adds a second negative charge, destabilzing molecule for cleavage

• rate limiting and commitment step

glycolysis - step 4

cleavage of F1,6BP

• destabilization in step 3 allows molecule to split in half

glycolysis - step 5

isomerization of DHAP

• only GAP can proceed through next steps

• isomerase ensures both 3-carbon molecules are utilized

glycolysis - step 6

oxidation and phosphorylation of GAP

• redox reaction is exergonic

• provides energy to add Pi without using ATP

• forms a high energy phosphate bond

glycolysis - step 7

substrate level phosphorylation

• 1,3-BPG transfers phosphate group to ADP → ATP

• high energy phosphate bond on 1,3-BPG makes reaction highly exergonic

  • allows it to drive ATP synthesis

glycolysis - step 8

phosphate shift

• rearranges structure to less stable form

  • prepares molecules for dehydration and creation of a high energy phosphate bond in next step

glycolysis - step 9

dehydration of 2-PG

• removal of water increases internal energy of molecule by creating C=C

• traps phosphate in a high-energy, unstable enol position

glycolysis - step 10

substrate level phosphorylation

• PEP transfers its phosphate group to ADP → ATP + pyruvate

• reaction is irreversible, pushes pathway to completion

krebs - step 1

condensation

• driven by hydrolysis of high-energy thioester bond in acetyl-CoA

krebs - step 2

isomerization

• citrate is a tertiary alcohol, difficult to oxidize

• isocitrate is a secondary alcohol, easier to oxidize

krebs - step 3

oxidative decarboxylation

• oxidation of the secondary alcohol to ketone allows for removal of carbon atom as CO₂

• releases high-energy electrons captured by NAD⁺

• rate-limiting step

krebs - step 4

oxidative decarboxylation

• releases energy to bond remaining succinyl group to coenzyme A

• forms high-energy thioester bond

krebs - step 5

substrate level phosphorylation

• cleavage of high-energy thioester bond drives phosphorylation of GDP

krebs - step 6

oxidation

• oxidation of C-C to C=C requires FAD (reduced FAD to FADH₂)

krebs - step 7

hydration

• addition of water to the double bond forms a hydroxyl group

• prepares molecule for final oxidation step

krebs - step 8

oxidation

• oxidizes hydroxyl group into ketone

• prepares 4-carbon molecule to accept new acetyl group for next cycle