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Lecture Seventeen:
GLYCOLYSIS
(Figures in red are for the 7th Edition)
(Figures in Blue are for the 8th Edition)
 GLYCOLYSIS
[Based on
Chapter 16
of Berg,
Tymoczko,
& Stryer]
 The sequence of reactions that converts GLUCOSE
into PYRUVATE with the concomitant production
of a relatively small amount of ATP
 Glycolysis takes place in the cytosol
 GLYCOLYSIS is an “_______________________________
_____________________”
 OVERVIEW  Key Pathway Features
 The chemical intermediates in glycolysis are either
SIX-CARBON or THREE-CARBON UNITS
 ________________________ are derivatives of
Glucose or Fructose
 _________________________are derivatives of
Glyceraldehyde, Dihydroxyacetone,
Glycerate or Pyruvate
 All intermediates are PHOSPHORYLATED with the
phosphoryl groups linked as either Esters or Anhydrides
 Phosphorylation _______________ these intermediates
 THE STAGES OF GLYCOLYSIS
 Glycolysis can be considered to occur in
THREE STAGES
 Figure 16-2, page 436 (16-2, page 472) (16-2, page 452)
 Stage one: ‘trapping’ of glucose, and its destabilisation
 Stage two: breakdown of a six-carbon unit to create
two three-carbon units
 Stage three: generates ATP
 THE FIRST STAGE
[ Figure, Page 435 ] [Figure, Page 471] [Figure, Page 451combined 1 & 2]
 This stage traps glucose in the cell and forms a
compound easily broken down into phosphorylated
three-carbon units
 Hexokinase adds ____________
 Glucose becomes glucose-6-phosphate
 Trapped: It cannot diffuse out of the cell
 Phosphate addition ______________ glucose
 Enables further reactions to take place
 Formation of fructose 1,6-bisphosphate from
glucose-6-phosphate
 There are two steps in this process
 An _______________
 A further phosphorylation
 Step 1: Isomerisation of glucose-6-phosphate to
fructose-6-phosphate
 Catalysed by phosphoglucose isomerase
 The enzyme:
 Opens the six-membered ring
 Catalyses the isomerisation
 Promotes the formation of a five-membered ring
 Converts a six-membered ring to a five-membered
ring BUT still containing overall six carbons
 One carbon now is a side group
 Step 2: A second phosphorylation of
fructose-6-phosphate to fructose-1,6-bisphosphate
 Catalysed by phosphofructokinase
 FRUCTOSE-1,6-BISPHOSPHATE is a molecule
that is easily cleaved into two three-carbon units
 THE SECOND STAGE [ONLY TWO STAGES in 8th Edition]
[ Figure, Page 438 ] [Figure, Page 471] [Figure, Page 451combined 1 & 2]
 This stage produces two different three-carbon units
BUT these two are ________________
 Formation of glyceraldehyde 3-phosphate
 An aldol cleavage of fructose-1,6-bisphosphate into
glyceraldehyde 3-phosphate and dihydroxyacetone
phosphate
 Catalysed by aldolase
 Glyceraldehyde 3-phosphate ________ the glycolytic
pathway BUT dihydroxyacetone phosphate ___________
 Half of glucose intake could be lost at this step in the
pathway
 BUT glyceraldehyde 3-phosphate and
dihydroxyacetone phosphate can be
interconverted in an isomerisation process
 Catalysed by triose phosphate isomerase
 Note: Structure => a TIM-BARREL
 Now ________ glyceraldehyde 3-phosphates continue the
glycolytic pathway
 THE THIRD STAGE [ONLY TWO STAGES in 8th Edition]
[ Figure, Page 441 ] [Figure, Page 477] [Figure, page 457 – "Stage 2" here]
 This stage generates ATP
 There are five steps in this third stage
 Step 1: Oxidation of glyceraldehyde 3-phosphate
to 1,3-bisphosphoglycerate (1,3-BPG)
 Catalysed by glyceraldehyde 3-phosphate
dehydrogenase
+
 This requires a reduction of NAD to NADH
 NOTE: 1,3-BPG is a high-potential phosphorylated
product
 More energy released when losing phosphoryl group
than in creating bond to make _______________
 Step 2: A phosphoryl group is transferred from
1,3-BPG to ADP, forming ATP and 3-phosphoglycerate
 Catalysed by phosphoglycerate kinase
 Step 3: A phosphoryl shift occurs in the conversion of
3-phosphoglycerate to 2-phosphoglycerate
 Catalysed by phosphoglycerate mutase
 2-phosphoglycerate less stable
 Step 4: A dehydration converts 2-phosphoglycerate
to phosphoenolpyruvate
 Catalysed by enolase
 Note: Phosphoenolpyruvate is another high-potential
phosphorylated compound
 Step 5: A phosphoryl group is transferred from
phosphoenolpyruvate to ADP, forming ATP and
pyruvate
 Losing phosphoryl group leaves pyruvate in an
unstable enol form
 Rearranges to Pyruvate
 Catalysed by pyruvate kinase
 This is virtually an irreversible reaction
 This last step produces a second ATP in the third stage
of glycolysis
 BUT remember: This stage is REPEATED TWICE
 Remember
 Two three-carbon compounds are formed from
one molecule of fructose-1,6-bisphosphate
 Energy yield in the conversion of glucose into pyruvate
 The net reaction in the transformation of glucose into
pyruvate is:
Glucose + 2Pi + 2ADP + 2NAD
+

+
2 Pyruvate + 2ATP + 2NADH + 2H +2H2O
 NOTE: One glucose molecule generates two molecules
of ATP and two molecules of pyruvate
 Figure 16-2, page 436 (16-2, page 472) (3-38, page 95)
 NOTE:
 The reactions of glycolysis (energetically
favourable) are coupled to the synthesis of
ATP (energetically unfavourable) via shared
chemical intermediates
 These are at the two positions where ATP is formed
 1: The 1,3-Bisphosphoglycerate to 3-Phosphoglycerate
 The 1,3-BPG passes a phosphate to ADP
 This is known as substrate-level phosphorylation
 2: The phosphoenolpyruvate to pyruvate
 The loss of the phosphate creates pyruvate in an
unstable enol form
 The free-energy released on the rearrangement of
pyruvate to its more stable ketone form is more than
is needed to produce ATP
 REGULATION OF GLYCOLYSIS
 Glycolysis regulation reflects its dual role in:
 Degrading glucose to make ATP
 Providing building blocks for biosynthetic
reactions (i.e. formation long-chain fatty acids)
 NOTE:
 In metabolic pathways, enzymes catalysing
essentially irreversible reactions are potential
sites of control/regulation
 Reactions catalysed by:
 Phosphofructokinase
 Hexokinase
 Pyruvate kinase
 ARE __________________
 Each serves as a control site, with their activities regulated
by:
 Reversible allosteric control (______________) i.e. by
feedback inhibition
 Regulation by reversible covalent modification (in
seconds) i.e. phosphorylation
 Transcriptional control (______________)
 The most important controlling element in the glycolytic
pathway of mammals is PHOSPHOFRUCTOKINASE
 Two features of the phosphofructokinase enzyme
 One: Regulation of ATP production
 Allosteric inhibition by high levels of ATP
 Allosteric activation by high levels of AMP
 So glycolysis is stimulated as the
ENERGY CHARGE falls
 To prevent excess formation of lactate the enzyme is
+
also inhibited by H (low pH)
 The enzyme is stimulated by fructose
2,6-bisphosphate
 A molecule produced only when
glucose is abundant
 Two: Regulation of provision of building blocks
 The enzyme is inhibited by citrate, an early
intermediate in the citric acid cycle
 Hexokinase and pyruvate kinase
 Hexokinase is inhibited by increased levels of glucose
6-phosphate (i.e when phosphofructokinase is
inactive)
 Fructose 1,6-bisphosphate activates pyruvate kinase
 ATP allosterically inhibits pyruvate kinase
 Summary of Lecture Seventeen
 Glycolysis converts glucose to pyruvate and creates ATP
 Occurs in the cytosol
 Involves six-carbon and three-carbon molecules
 All intermediates molecules are phosphorylated
 At a critical SECOND STAGE in the process TWO
three-carbon molecules are produced
 Both can get used in the next stage of the
glycolysis pathway
 Because of triose phosphate isomerase
 The THIRD STAGE therefore runs through TWICE
from one glucose molecule
 It is in this stage that TWO molecules of ATP are
produced
 Hence glycolysis produces two molecules of ATP used as
the energy currency of metabolism