GlycolysisGlycolysis
GluconeogenesisGluconeogenesis
Glycolysis - OverviewOne of best characterized pathways
Characterized in the first half of 20th century
Glucose --> 2 pyruvates + energy
Strategy
add phosphoryl groups to glucose
convert phosphorylated intermediates into compounds with high phosphate group-transfer potentials
couple the subsequent hydrolysis of reactive substances to ATP synthesis
Glucose + 2NAD+ + 2 ADP + 2Pi -->
2NADH + 2 pyruvates + 2ATP + 2H2O + 4H+
Overview of Glycolysis
The Embden-Meyerhof (Warburg) Pathway
• Essentially all cells carry out glycolysis
• Ten reactions - similar in most cells - but rates differ
• Two phases:
– First phase converts glucose to two G-3-P
– Second phase produces two pyruvates
• Products are pyruvate, ATP and NADH
• NADH must be recycled
• Three possible fates for pyruvate
Glycolysis
Glycolysis
Fate of pyruvate
Decarboxylation to acetaldehyde
Reduction to ethanol
Reduction to lactate
Mitochondrial oxidation
1 NADH --> ~3 ATP
Enzymes of glycolysisCatalyzed reactions
and properties
Enzymes of glycolysisCatalyzed reactions
and properties
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
Fructose-1,6-biphosphate
Glyceraldehyde-3-phosphate
Hexokinase, glucokinase
Phosphoglucoisomerase
Phosphofructokinase
Aldolase
Triose phosphate isomerase
Dihydroxyacetone phosphate
First Phase of GlycolysisThe first reaction - phosphorylation of glucose
• Hexokinase or glucokinase • This is a priming reaction - ATP is consumed here
in order to get more later • ATP makes the phosphorylation of glucose
spontaneous
Hexokinase 1st step in glycolysis; G large, negative
• Hexokinase (and glucokinase) act to phosphorylate glucose and keep it in the cell
• Km for glucose is 0.1 mM; cell has 4 mM glucose
• So hexokinase is normally active!
• Glucokinase (Kmglucose = 10 mM) only turns on when
cell is rich in glucose• Hexokinase is regulated - allosterically inhibited by
(product) glucose-6-P -
Hexokinase
• First step in glycolysis• Large negative deltaG • Hexokinase is regulated - allosterically inhibited by
(product) glucose-6-P• Corresponding reverse reaction (Gluconeogenesis) is
catalyzed by a different enzyme (glucose-6-phosphatase)
• Is it the committed step in glycolysis ?
Glucose
Fructose-6-P
Glucose-6-P
Glyceraldehyde-3-P
Pyruvate
ATP
Glycogen Ribose-5-P + NADPH
Nucleic acidsynthesis
Reducingpower
Glucose-6-P dehydrogenase
Rx 2: Phosphoglucoisomerase
Glucose-6-P to Fructose-6-P
Rx 3: PhosphofructokinasePFK is the committed step in glycolysis!
• The second priming reaction of glycolysis • Committed step and large, neg delta G - means PFK is
highly regulated • ATP inhibits, AMP reverses inhibition • Citrate is also an allosteric inhibitor • Fructose-2,6-bisphosphate is allosteric activator • PFK increases activity when energy status is low • PFK decreases activity when energy status is high
Glycolysis - Second Phase
Metabolic energy produces 4 ATP
• Net ATP yield for glycolysis is two ATP
• Second phase involves two very high energy phosphate intermediates
• .
– 1,3 BPG
– Phosphoenolpyruvate
Glyceraldehyde-3-phosphate
1,3-biphosphoglycerate
3-phosphoglycerate
2-phosphoglycerate
phosphoenolpyruvate
pyruvate
Glyceraldehyde-3-phosphate dehydrogenase
Phosphoglycerate kinase
Phosphoglycerate mutase
Enolase
Pyruvate kinase
Rx 10: Pyruvate Kinase
PEP to Pyruvate makes ATP
• These two ATP (from one glucose) can be viewed as the "payoff" of glycolysis
• Large, negative G - regulation!
• Allosterically activated by AMP, F-1,6-bisP
• Allosterically inhibited by ATP and acetyl-CoA
The Fate of NADH and PyruvateAerobic or anaerobic??
• NADH is energy - two possible fates: – If O2 is available, NADH is re-oxidized in the
electron transport pathway, making ATP in oxidative phosphorylation
– In anaerobic conditions, NADH is re-oxidized by lactate dehydrogenase (LDH), providing additional NAD+ for more glycolysis
The Fate of NADH and PyAerobic or anaerobic??
• Pyruvate is also energy - two possible fates: – aerobic: citric acid cycle
– anaerobic: LDH makes lactate
The elegant evidence of regulation!
• Standard state G values are scattered: + and -
G in cells is revealing:
• Most values near zero
• 3 of 10 reactions have large, negative G
• Large negative G reactions are sites of regulation!
Energetics of Glycolysis
Gluconeogenesis
Synthesis of "new glucose" from common metabolites
• Humans consume 160 g of glucose per day
• 75% of that is in the brain
• Body fluids contain only 20 g of glucose
• Glycogen stores yield 180-200 g of glucose
• So the body must be able to make its own glucose
Comparison of glycolysis and gluconeogenesis pathways
Substrates for Gluconeogenesis
Pyruvate, lactate, glycerol, amino acids and all TCA intermediates can be utilized
• Fatty acids cannot!
• Most fatty acids yield only acetyl-CoA
• Acetyl-CoA (through TCA cycle) cannot provide for net synthesis of sugars
Gluconeogenesis I
• Occurs mainly in liver and kidneys
• Not the mere reversal of glycolysis for 2 reasons:– Energetics must change to make
gluconeogenesis favorable (delta G of glycolysis = -74 kJ/mol
– Reciprocal regulation must turn one on and the other off - this requires something new!
Energetics of Glycolysis
The elegant evidence of regulation!
G in cells is revealing:
• Most values near zero
• 3 of 10 reactions have large, negative G
• Large negative G reactions are sites of regulation!
• Reactions 1, 3 and 10 should be different to go into opposite direction
Gluconeogenesis II Something Borrowed, Something New
• Seven steps of glycolysis are retained:– Steps 2 and 4-9
• Three steps are replaced:– Steps 1, 3, and 10 (the regulated steps!)
• The new reactions provide for a spontaneous pathway (G negative in the direction of sugar synthesis), and they provide new mechanisms of regulation
Top Related