BIOL 301 Chapter Notes - Chapter 14: Hexose, Alcohol Dehydrogenase, Ribulose
Chap 14: Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway
• 14.1 Glycolysis
• What is glycolysis?
• Molecule of glucose is degraded in a series of enzyme-catalyzed
reactions to yield 2 molecules of the 3-carbon compound pyruvate
• Some of free energy released from glucose conserved in form of ATP and
NADH
• Glycolytic breakdown of glucose is sole source of metabolic energy in
some mammalian tissue and cell types
• Fermentation = general term for anaerobic degradation of glucose or
other organic nutrients to obtain energy, conserved as ATP
• Overview of glycolysis: 2 pieces
• 10 steps- first 5 are preparatory phase (SHOULD WE KNOW EACH
STEP???)
• Step 1- glucose phosphorylated at hydroxyl group on C-6
• Step 2- D-glucose 6-phosphate thus formed → D-fructose 6-
phosphate
• Step 3- it is again phosphorylated (at C-1 this time) → D-fructose
1,6- biphosphate
• Step 4- It is split → 2 three-carbon molecules, dihydroxyacetone
phosphate (this is the -lysis step)
• Step 5- it is isomerized to a second molecule of glyceraldehyde 3-
phosphate
• To summarize: prep phase- energy of ATP is invested, raising the free-
energy content of the intermediates
• Second phase: payoff phase
• Step 6- each molecule of the glyceraldehyde 3-phosphate is
oxidized and phosphorylated by inorganic phosphate → 1,3-
biphosphoglycerate
• Step 7-10 energy released as the 2 molecules of 1,3-
biphosphoglycerate are converted to 2 molecules of pyruvate
• Facts of pyruvate
• Pyruvate formed from glycolysis is further metabolized from one of
3 catabolic routes (glycolysis is only the first stage in complete
degradation of glucose)
• Option 1: aerobic conditions: Pyruvate oxidized to yield
acetyl coA→ then oxidized completely to CO2 by citric
acid cycle
• Option 2: lactic acid fermentation
• In hypoxia→ NADH cannot be re-oxidized to NAD+
but NAD+ is required as electron acceptor for
further oxidation of pyruvate
• So-- pyruvate is reduced to lactate and accepts
electrons from NADH→ regenerating NAD+
necessary for glycolysis to continue
• Option 3: pyruvate converted under hypoxic or anaerobic
conditions to ethanol and CO2 -- process called ethanol
(alcohol) fermentation
• Pyruvate has anabolic fates as well- it can provide the
carbon skeleton for the synthesis of fatty acids
• ATP and NADH formation coupled to glycolysis
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• For each molecule of glucose degraded to pyruvat, 2 molecules of
ATP are generated from ADP and Pi, and 2 molecules of NADH
are produced by reduction of NAD+
• Glycolysis is an essentially irreversible process driven to
completion by a large net decrease in free energy
• Energy remaining in pyruvate
• Glycolysis releases only a small fraction of the total available
energy of the glucose molecule; the two molecules of pyruvate
formed by glycolysis still contain most of chemical potential energy
of glucose
• Importance of phosphorylated intermediates - 3 functions
• 1. After initial phosphorylation, no further energy is necessary to
retain phosphorylated intermediates in the cell bc they cannot
leave the cell (b/c plasma membrane does not have transporters
for phosphorylated sugars)
• 2. Phosphoryl groups are essential in conserving metabolic
energy
• 3. Binding energy from binding of phosphate groups to active sites
of enzymes lowers the activation energy and increases specificity
of enzymatic reactions
• Prep phase of glycolysis requires ATP
• 2 molecules of ATP are invested and hexose chain is cleaved into 2 triose
phosphates
• Experiments→ investigations on role of organic esters and anhydrides of
phosphate in biochem, leading to current understanding of the central role
of phosphoryl group transfer
• Phosphoryl group transfer- role
• 1) phosphorylation of glucose
• Catalyzed by hexokinase which requires Mg2+ for its
activity and is present in nearly all organisms
• 2 or more enzymes that catalyze the same reaction but are
encoded by different genes are called isozymes
• 2) conversion of glucose 6-phosphate to fructose 6-phosphate
• Enzyme phosphohexose isomerase (phosphoglucose
isomerase) catalyzes the reversible isomerization of
glucose 6-phosphate, an aldose, to fructose 6-
phosphate, a ketose
• 3) phosphorylation of fructose 6-phosphate to fructose 1,6-
biphosphate
• PFK-1 catalyzes this phosphoryl group transfer
• 4) cleavage of fructose 1,6-biphosphate
• Enzyme fructose 1,6-biphosphate aldoase (aldoase)
catalyzes reversible aldol condensation
• 5) interconversion of the triose phosphates
• The payoff phase of glycolysis yields ATP and NADH
• 6) oxidation of glyceraldehyde 3-phosphate to 1,3-
biphosphoglycerate
• First step in payoff phase is oxidation of glyceraldehyde 3-
phosphate to 1,3-biphosphoglcerate, catalyzed by
glyceraldehyde 3-phosphate dehydrogenase
• The overall balance sheet shows a net gain of ATP
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Document Summary
Step???: step 1- glucose phosphorylated at hydroxyl group on c-6, step 2- d-glucose 6-phosphate thus formed d-fructose 6- phosphate, step 3- it is again phosphorylated (at c-1 this time) d-fructose. 3 catabolic routes (glycolysis is only the first stage in complete degradation of glucose: option 1: aerobic conditions: pyruvate oxidized to yield acetyl coa then oxidized completely to co2 by citric acid cycle, option 2: lactic acid fermentation. Importance of phosphorylated intermediates - 3 functions: 1. After initial phosphorylation, no further energy is necessary to retain phosphorylated intermediates in the cell bc they cannot leave the cell (b/c plasma membrane does not have transporters for phosphorylated sugars: 2. Phosphoryl groups are essential in conserving metabolic energy: 3. 14. 2 feeder pathways for glycolysis: dietary polysaccharides and disaccharides undergo hydrolysis to monosaccharides, endogenous glycogen and starch are degraded by phosphorolysis, phosphorolytic reaction- catalyzed by glycogen phosphorylase (starch phosphorylase in plants)
