L6 Glycolysis

Learning Outcomes

  • Understand the importance of glycolysis, what substrates are involved & what the pathway is used for.
  • Describe the loc and reactions of glycolysis and the mech of E
  • Be able to give the reactions using and producing ATP and NADH and know their net overall yields
  • Explain the diff between reversible & irreversible reactions
    Understand the regulation of the glycolytic pathway, & which ezms are regulated

Glycolysis

  • Glycose = sweet, lysis = to break : conversion of glucose into pyruvate
  • Maj pathway of glucose catabolism
  • Unique: can function aerobically or anaerobically, depending on O2 availability & intact mitochond, thus allows tissues to survive in O2 presence or absence, e.g sk muscle
  • RBC lack mitochond, so reliant on glucose as fuel metabolised by anaerobic glycolysis
  • Provides E in form of ATP
  • In norm circumstances glucose is the only fuel the brain (CNS) can use
  • Glucose is also preferentially used by muscle

Phosphorylation - necessary for glucose to enter glycolytic pathway

  • Add a phosphate grp twice to that
  • Add a P grp & we've trapped tht glucose in the (C)
    ~ & traps glucose in (C), no transmembrane carriers & phosphorylated sugars can't cross (C) memb (too polars)
  • First stage of aerobic carbs metabolism
    ~ Glucose ⇒ pyruvate in cytosol
    ~ 10 step chain (linear pathway)
  • Glycolysis can occur in absence of O2 (anaerobic metabolism) glucose ⇒ lactate (later) if we don't have O2 around

2 Main Phases

  • Remember: st of pyruvate!
  • catabolic pathways = ATP is made
  • catabolicm: breaking down the food we eat, but direct reactons of ATP from ADP is anabolic, the total pathway of breaking down the food we eat is catabolic to produce ATP
  • 1) Preparatory: 6C glucose is split into 2x 3C triose-P
    ~ 5 step process (input)
  • 2) Payoff: Triose-P is oxidised to pyruvate
    ~ 5 step process (net ATP & NADH generate)

Phase 1

  • = Ezms:
  • Hexokinase ⭐
  • Phosphoglucose isomerase
  • Phosphofructokinase ⭐
  • ⭐ (where the process of ezm is regulated, irreversible, the rest of the ezms are driven in that reaction bc of these main 3)
  • Aldose
  • Triose phosphate isomerase
  • All subsequent reactions occur twice, have 2x Glyceraldehyde 3P, each 3C

Phase 2

  • =Ezms
  • Glyceraldehyde 3P dehydrogenase
  • Phosphoglycerate kinase
  • Phosphoglycerate mutase
  • Enolase
  • *Pyruvate kinase

Preparatory phase
Phosphorulation of glucose and its conversion to glyceraldehyde 3-(P)

  • 1) Hexokinase: Some of our (C) have an isoezm called a glucokinase: same function- one of the hexokinase
  • 2) Phosphohexose isomerase
  • 3) Phospho- fructokinase-1 : the main reg ezm in the whole pathway
  • 4) Aldolase
  • 5) Triose phosphate isomerase

Payoff Phase
Oxidative conversion of glyceraldehyde 3 (P) to pyruvate & the coupled formation of ATP and NADH

  • 6) Glyceraldehyde 2-phosphate dehydrogenase
  • 7) Phosphoglycerate kinase : can now be used to anabolically form ATP to ADP bc of those (P)s, the ADPstrips one of the (P) to give phosphohlycerate & 2ATP- now we've paid back to the (C) bc used 2x ATP & made 2
  • 8) Phosphoglycerate mutase
  • 9) Enolase- gaining a water molec in our cytoplasm
  • 10) Pyruvate kinase

Free E changes (∆G) in glycolysis (RBC)

  • All reactions are essentially reversible, except those catalysed by: 3 reactions
    1) # hexokinase
    2) oPhosphofructokinase
    3) pyruvate kinase
  • These 3 steps steps are essentially irreversible, large -ve ∆G, providing driving force for glycolysis
  • Reactions are never really equilibrium in our bodies
  • The other ones are kinda neutral
  • several isozymes of hexokinase, in liver & pancreatic islet (C)s have glucokinase

3 main points of glycolysis regulation

  • Hexokinase
    ~ -ve reduce the activity: turn off hexokinase: feedback inhibition: if we have high levels of glucose
    ~ if the (C) has lots of inorganic phosphates the (C) will have low ATP: can drive the activity of hexokinase
  • Phosphofructokinase
    ~ Since there is diff isozyme distribution, not all body tissues have all regulatory mechs shown
  • Pyruvate kinase
    ~ can be activated by feed forward syst
    ~ high lvl of ATP: switch off activity of pyruvate kinase
    ~ Bc role of glycolysis is to make ATP, if have enough, (C) won't waste time making more = control

Phosphofructokinase (PFK) = most important control site for our glycolytic pathway

  • PFK = tetramer
  • High [ATP] allosterically inhibit PFK (what?)
  • AMP reverses inhibits (low [ATP], when high [AMP])
  • pH drop also inhibits PFK- important in anaerobic metabolism (later)

Some key reaction types

ATP synthesis
-

Redox reactions - dehydrogenase (GAPDH)

  • GAPDH = glyceraldehyde 3P dehydrogenase
  • Can involve simply e- transfer or can involve transfer of H (as in NADH)

NADH

  • (C)ullar currency of reductive Ep & is produced during respiration... is a cosubstrate, a temporarily-bound coezm
  • In dehydrogenation: removing the H, 2e- plus that proton, to give NADH
  • Reaction proceeds in 2 steps, forming an intermediate linked to GAPDH by a thioester bond (high E), which preserves much of the oxidation rxn E for subsequent phosphorylation to give 1,3-BPG (coupling unfavourable & favourable rxns)...
  • Understand that process of oxidation

Substrate-lvl phosphorylation - Kinase
Phosphoglycerate kinase = PGK

  • 1,3-BPG is E-rich (essentially traps the E of GAPDH's oxidation rxn) w/ a high P-transfer potential, so can power ATP synth from ADp
  • This is substrate-lvl phosphorylation as 1,3-BPG (the substrate) has the P-tranfer potential

Respiration: Step 1 Glycolysis

  • complete breakdown of glucose into CO2 + H2O
  • can undergo glycolysis even if there's no O2
    -1x molec of glucose (6C) broken down to 2x molecs of pyruvate (3C)
  • E yield = 2x ATP & 2x NADH (e- carrier)
    ~ used to do (C)ullar work within the body

Glycolysis summary

All subsequent reactions occur twice

  • have 2x Glyceraldehyde 3(P), each 3C

PFK activity w/ changing [ATP]