Glycolysis

• Feedback Inhibition occurs when formation of product inhibits its continued production
• • Allosteric: Greek allo + steric, other shape
• Allosteric enzyme: an oligomer whose biological activity is affected byother substances binding to it

  • these substances change the enzyme’s activity by altering the conformation(s) of its 4°structure
• Allosteric effector: a substance that modifies the behavior of an allosteric enzyme; may be an

  • allosteric inhibitor
  • allosteric activator
• The side chain -OH groups of Ser, Thr, and Tyr can form phosphate esters. They use protein kinases to catalyze the transfer of phosphate almost always from ATP
• Zymogen:Inactiveprecursorofanenzymewherecleavageofone or more covalent bonds transforms it into the active enzyme
• Chymotrypsinogen: a zymogen synthesized and stored in the pancreas; it is a single polypeptide chain of 245 amino acid residues cross linked by five disulfide (-S-S-) bonds.

  • when secreted into the small intestine, the digestive enzyme trypsin cleaves a 15 unit polypeptide from the N-terminal end to give π-chymotrypsin
• Chymotrypsin prefers an aromatic side chain on the residue whose carbonyl carbon is part of the peptide bond to be cleaved

  Trypsin prefers a positively charges Lys or Arg residue at this point
• Glycolysis is the first stage of glucose metabolism; it plays a key role in the way organisms extract energy from nutrients
• • One molecule of glucose is converted to fructose-1,6-bisphosphate, which gives rise to two molecules of pyruvate (has several fates)
• • Phosphorylation of glucose to give glucose-6-phosphate
• • Isomerization of glucose-6-phosphate to givefructose-6-phosphate
• Phosphorylation of fructose-6-phosphate to yield fructose-1,6-bisphosphate
• Cleavage of fructose-1,6,-bisphosphate to give glyceraldehyde-3-phosphate and dihyroxyacetone phosphate
• Isomerization of dihyroxyacetone phosphate to give glyceraldehyde-3-phosphate
• Oxidation of glyceraldehyde-3-phosphate to give 1,3-bisphosphoglycerate
• Transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP to give 3-phosphoglycerate
• Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate
• Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate
• Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate
• The conversion of glucose to glyceraldehyde-3-phosphate is also called the “Preparation Phase” of Glycolysis
• The “Preparation Phase” of Glycolysis makes use of the free energy from the hydrolysis of ATP and are all reduction reactions.
• STEP 1: glucose is phosphorylated to give glucose-6-phosphate

  • The reaction is endergonic, as it is driven by the free energy of hydrolysis of ATP
  • enzyme: Hexokinase (with cofactor Mg2+)
• Kinase -> energy dependent enzymes that transfer phosphate group from ATP to substrate
• Glucose-6-phosphate inhibits the activity of hexokinase; this is a control point in the pathway.
• Isozyme - two or more enzymes with identical function but different structure (ex. hexokinase)

  • Hexokinase does not necessarily only target glucose, it can act on fructose and mannose too
• STEP 2: Isomerization of glucose-6-phosphate to fructose-6-phosphate

  • The C-1 aldehyde of glucose-6-phosphate is reduced to a hydroxyl group
  • The C-2 hydroxyl group is oxidized to give the ketone group of fructose-6-phosphate
  • There is no net redox reaction
  • enzyme: glucosephosphate isomerase
• STEP 3: Fructose-6-phosphate is then phosphorylated again to generate fructose-1,6-bisphosphate

  • second reaction to be coupled to ATP hydrolysis
  • enzyme: phosphofructokinase (with cofactor Mg2+)
• Phosphofructokinase (PFK):

  • Exists as a tetramer and subject to allosteric feedback
  • The tetramer is composed of L and M subunits & combinations of these subunits are called isozymes
  • Muscles are rich in M4; the liver is rich in L4
  • ATP is an allosteric effector; high levels inhibit the enzyme, low levels activate it
  • Fructose-1,6-bisphosphate is also an allosteric effector
• STEP 4: Fructose-1,6-bisphosphate is split into two 3-carbon fragments -> Dihydroxyacetone phosphate & D-Glyceraldehyde-3-phosphate

  • enzyme: aldolase
  • Side chains of an essential Lys and Cys (has thiol) play key roles in catalysis
• STEP 5: dihydroxyacetone phosphate (DHAP) is converted to glyceraldehyde-3-phosphate

  • enzyme: triosephosphate isomerase
• In the preparation phase, 2 ATP is used
• The second phase of Glycolysis, also called as the 'payoff phase', the the conversion of Glyceraldehyde-3-Phosphate to Pyruvate
• 1 glucose molecule = 2 glyceraldehyde-3-phosphate molecules
• STEP 6: Oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate

  • this reaction involves addition of a phosphate group, as well as an electron transfer
  • The oxidizing agent, NAD+, is reduced to NADH
  • enzyme: Glyceraldehyde-3-phosphate-dehydrogenase
• STEP 7: 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate via phosphorylation of ADP to ATP

  • 1,3-bisphosphoglycerate transfers a phosphate group to ADP. This is known as substrate-level phosphorylation
  • phosphate group is transferred from carbon 3 to carbon 2 of the glyceric acid backbone
  • enzyme: phosphoglycerate kinase (with cofactor Mg2+)
  • produces 1 ATP
• STEP 8: isomerization of 3-phosphoglycerate to 2-phosphoglycerate

  • enzyme: phosphoglyceromutase (with cofactor Mg2+)
• STEP 9: 2-phosphoglycerate loses one molecule of water,

  producing phosphenolpyruvate

  • enzyme: enolase (with cofactor Mg2+ which binds to the water)
  • produces 1 molecule of water
  • Phosphoenolpyruvate contains a high energy bond
• STEP 10: Phosphenolpyruvate (PEP) transfers its phosphate group to ADP, producing ATP and pyruvate

  • enzyme: pyruvate kinase
  • produces 1 ATP
• 1 molecule of glucose produces 4 ATP in glycolysis

  • net gain is only 2 as it uses 2 ATP in its preparation phase