carb. metabolism glycolysis

• glycolysis


  .1.a complex series of cellular biochemical reactions, not requiring oxygen, that splits glucose, glycogen, or other carbohydrates into pyruvic, or lactic, acid while storing a relatively small amount of energy in ATP molecules

  2.The cellular degradation of the simple sugar, glucose, to yield ATP as an energy source
• Glycolysis – an overview


  .•Catabolic pathway that saves some potential energy from glucose/G-6-P by forming ATP through substrate level phosphorylation

  •It is essentially the only way that energy can be made from fuel molecules when cells lack O2 (exercising muscle) or mitochondria (RBCs)

  
• Gene sequencing has revealed that glycolysis is what?


  .•an ancient process, conserved through organisms

  -evolved before atmospheric O2 was plentiful

  -occurs in cytosol – no complex organelles required

  •It is therefore important to life and justifiably fills the central role in the metabolic pathways
• Glycolysis has two phases:


  .•For 1 Glc passing through the preparatory phase:

  -2 molecules of G-3-P formed to enter the payoff phase

  •For each Glc, 2 ATP are used in the preparatory phase and 4 ATP gained in the payoff phase

  •Imagine a car – it won’t work even with all of the petrol it contains unless it has a battery (2 ATP’s act as this initial energy to “kick-start” the glycolysis pathway)

  •Thus glycolysis gives a net gain of 2 ATP (and NADH) per Glc molecule

  
• (1) Phosphorylation of glucose


  .•Catalyst – hexokinase

  •Uses 1 ATP

  •ΔG = -16.7 kJ/mol – essentially irreversible

  
• (2) Conversion of G-6-P to F-6-P


  .•Catalyst – phosphohexose isomerase

  •ΔG = 1.7 kJ/mol – proceeds either way due to low free energy

  
• (3) Phosphorylation of F-6-P to F-1,6-bisP


  .•Catalyst – phosphofructokinase-1 (PFK-1)

  •Uses 1 ATP

  •ΔG = -14.2 kJ/mol – essentially irreversible

  1st “committed” step of glycolysis, because G-6-P and F-6-P can be used in other pathways, but   F-1,6-bisP is solelydestined for glycolysis

  
• (4) Cleavage of F-1,6-bisP


  .•Catalyst – fructose 1,6-bisphosphate aldolase (or aldolasefor short)

  •ΔG = 23.8 kJ/mol – under cellular conditions the actual free energy change is small so the reaction is readily reversible

  • This is the “splitting” part of glycolysis

  • One Glc (6 C’s) is converted to two different 3C triosesugars

  
• (5) Interconversion of triosesugars


  .•Catalyst – triose phosphate isomerase

  •ΔG = 7.5 kJ/mol – low, so readily reversible reaction

  •Only G-3-P can participate in glycolysis, so the other 3 C sugar produced (dihydroxyacetone phosphate) is rapidly converted to G-3-P, thus yielding two G-3-P molecules for every one Glc

  
• (6) Oxidation of G-3-P to 1,3-bisPG


  .•Catalyst – glyceraldehyde 3-phosphate dehydrogenase

  •2 NADH’s are produced

  ΔG = 6.3 kJ/mol – endergonic

  This is the first reaction in the “payoff” phase of glycolysis

  
• (7) P transfer from 1,3-bisPG to ADP

  .•Enzyme – phosphoglycerate kinase

  •2 ATP’s produced

  •ΔG = -18.5 kJ/mol – highly exergonic so spontaneous

  •Steps † and ‡are an energy-coupled process, so the overall ΔG is -12.2 kJ/mol

  •1,3-bisPG is the reaction intermediate between this coupled process

  •This is one of the substrate-level phosphorylation reactions in glycolysis (different than respiration-linked phosphorylation)

  Important distinction: “substrate-level” requires soluble enzymes and chemical intermediates, “respiration-linked” involves membrane bound enzymes and gradients of protons

  
• (8) Conversion of 3-PG to 2-PG


  .•Catalyst – phosphoglycerate mutase

  •ΔG = 4.4 kJ/mol – in cells this is even lower, so reaction is reversible

  
• (9) Dehydration of 2-PG to PEP


  . •Catalyst – enolase

  •ΔG = 7.5 kJ/mol – again, in cells this is low, so reversible reaction can occur

  
• (10) Transfer of P from PEP to ADP


  .•Catalyst – pyruvate kinase

  •2 ATP’s produced

  •ΔG = -31.4 kJ/mol – highly exergonic, so reaction is spontaneous

  •This final step produces pyruvate

  
• Glycolysis free energy balance sheet


  .

  
• NAD+ needs to be regenerated


  .•No NAD+ = no glycolysis

  •NAD+ limited in the cell – comes from niacin (essential vitamin)

  •Example opposite is for exercising muscle that produces lactate from the pyruvate, but pyruvate can have other fates

  •All of these fates will produce NAD+ to replenish the NAD+ required for reduction of various intermediate metabolites

  •This is termed redox balance

  
• What happens to pyruvate?


  .•it depends on what you need at any given time

  •The reactions that produce pyruvate from glucose are similar in most organisms

  •What happens to pyruvate next, is variable:

  •Ethanol

  •Lactate

  CO2

  
• Pyruvate → ethanol

  .•Yeast and several other microorganisms can generate ethanol from pyruvate

  •2-step process:

  •Pyruvate decarboxylase

  •Alcohol dehydrogenase

  
• Pyruvate → lactate


  .•In human cells lacking O2

  -Vigorously exercising muscle

  -RBC’s – lack mitochondria (see later lectures as to why mitochondria are important)

  •Pyruvate is reduced to lactate via fermentation

  •Oxidation of NADH drives the reduction of pyruvate to lactate, which in turn replenishes stores of NAD+ for further glycolysis

  
• Cori cycle


  .•When we sprint, muscles don’t receive O2 fast enough to make ATP via oxidative phosphorylation

  •Instead ATP is made via substrate-level phosphorylation, producing lactate

  •Lactate is converted to Glc in the liver by a process called gluconeogensis (see later lectures)

  •The liver repays the oxygen debt run up by the muscles

  •This interaction between the liver and muscle is called the Cori cycle

  
• Pyruvate → acetyl CoA


  .•In cells with access to O2 the pyruvate is oxidised to form acetyl coenzyme A (acetyl CoA)

  •This occurs within the mitochondria of cells

  NADH formed in this reaction will later give up it’s hydride ion (:H-) to the respiratory chain