BCHM-3050 Lecture Notes - Lecture 11: Hexokinase, Glycolysis, Gluconeogenesis
15 Jun 2018
School
Department
Course
Professor
Overview of carb metabolism
Glycolysis: breakdown of glucose to pyruvate
Universal for living cells
○
The regulation of glycolysis is well understood
○
It plays the central metabolic role in generating BOTH energy and
metabolic intermediates
○
•
Gluconeogenesis: synthesis of polysaccharides
Reverse of glycolysis
○
•
Catabolism: break apart
Energy is released
○
Oxidative
○
•
Anabolism: build up
Energy is required
○
Reductive
○
•
Three basic processes involved:
Process 1: add phosphoryl group to glucose
Yield compounds w low phosphoryl group transfer potential, a
process known as priming
§
○
Process 2: chemically converting low phosphoryl group transfer
potential intermediates into high phosphoryl group transfer potential
compounds
○
Process 3: chemically couple energy yielding hydrolysis to high
phosphoryl group transfer potential compounds
Help synthesis of ATP by direct transfer of phosphate groups
to ADP
§
○
•
Reactions 1-5: Energy investment
Reaction 1: Hexokinase (FIRST ENERGY INVESTMENT)
Glucose gets phosphorylated by ATP via hexokinase
Result: glucose-6-phosphate (G6P)
○
-OH group nucleophilic attacks the incoming phosphate group
○
•
The phosphate group is coordinated by magnesium
•
Hexokinase binding
Hexokinase has 4 isoforms (I,II,III, IV)
○
3/4 isoforms have high binding specificity (low Km)
Binds to fructose and mannose
§
○
Hexokinase isoform IV: used in reaction one for its low binding
specificity (high Km)
Used inside the liver
§
Allows liver to adjust its rate of glucose utilization in response
to variations in blood glucose lvls
§
○
•
Favorable reaction
Rxn is so favorable that it is irreversible
○
•
Reaction 2: Glucose-6-Phosphate isomerase
Glucose-6-phosphate becomes isomerized to fructose via glucose-6-
isomerase
Result: Fructose-6-phosphate
○
G6P is an aldose while F6P is a ketose
○
•
Barely unfavorable rxn
Rxn is reversible
○
•
Reaction 3: Phosphofructokinase-1 (MAJOR CONTROL POINT)
Fructose-6-phosphate is phosphorylated via PFK-1
Result: Fructose-1,6-bisphosphate
○
•
PFK-1 is a bifunctional enzyme
PFK-2 = favors glycolysis
○
Fructose-bisphosphatase = favor gluconeogenesis
○
•
How is this regulated?
In the presence of ATP, PEP or low pH the PFK-2 aspect of the
enzyme is inhibited and gluconeogenesis occurs
○
In the presence of AMP and Fructose-2,6-bisphosphate (activator)
glycolysis occurs
○
•
Favorable rxn
Rxn is irreversible
○
•
Reaction 4: fruct-1,6-bisphosphate aldolase aka aldolase
Fructose-1,6-bisphosphate is cleaved by Fruct-1,6-bisphosphate aldolase
Result: dihydroxyacetone phosphate (DHAP) and D-
glyceraldehyde-3-phosphate (GAP)
○
•
GAP is made first and moves to the second part of glycolysis followed by
DHAP
•
Catalysis mechanism for the process = covalent catalysis (to start) and
GABA (toward the middle)
•
Under biochemical conditions this rxn is unfavorable
Under intracellular conditions the rxn is favorable
○
•
Aldolase mechanism
Conversion of ketone to imine derived from lysine 1.
Deprotonation and cleavage to release GAP and enamine products 2.
Protonation of enamine 3.
Hydrolysis to release DHAP4.
Reaction 5: triose phosphate isomerase
DHAP is isomerized to GAP
Result: another GAP compound (end w 2 total)
○
•
The intermediate of this whole rxn is a enediol intermediate which is
extremely unstable
•
Slightly unfavorable
Rxn can be reversed
○
•
Reaction 6-10: The energy generation phase
Reaction 6: glyceraldehyde-3-phosphate dehydrogenase (FIRST ENERGY
RICH COMPOUND)
GAP is phosphorylated by an inorganic phosphate via glyceraldehyde-3-
phosphate dehydrogenase
Result: 1,3-biphosphglycerate (BPG)
○
•
Generates NADH
Energy rich compound
○
•
Not a true control point BUT if there is not enough phosphate then the rxn
will not go forward
•
Mechanism for enzyme req 2 electron transfer and the formation of
carboxylic phosphor. Acid anhydride or 1,3-bisphosphoglycerate
•
Slightly unfavorable
Rxn can be reversed
○
•
Reaction:
Attack by cysteine at carbonyl C of GAP1.
Transfer of hydride to NAD+2.
Attack by phosphate 3.
Release of cysteine as a free thiol 4.
Reaction 7: phosphoglycerate kinase
1,3-biphosphglycerate is dephosphorylated using phosphoglycerate kinase
•
Result: 3-phosphoglycerate
•
First instance of substrate level phosphorylation
The phosphate pops off of 1,3-bisphosphoglycerate and pops onto
ADP
○
•
This reaction is coupled with reaction 6 to yield an overall favorable rxn
•
Reaction 8: phosphoglycerate mutase
3-phosphoglycerate is phosphorylated via phosphoglycerate mutase
Result: 2-phosphoglycerate
○
•
The phosphate group seems to be transferred from carbon one to carbon
two BUT it actually has something to do with the enzyme
The enzyme has a n-phosphohistidine residue which swaps
phosphates with 3-phosphoglycerate
○
•
This helps prep for the synth of the next "high energy compound"
•
Reaction 9: Enolase (2ND HIGH ENERGY COMPOUND)
2-phosphoglycerate gains another double bond via enolase
Alpha, beta elimination
○
Result: phosphenolpyruvate (PEP)
○
•
Creation of an enol group is very unstable
•
The whole reaction is slightly favorable
•
Reaction 10: pyruvate kinase
The phosphate group on phosphenolpyruvate pops onto ADP creating
ATP through substrate level phosphorylation
Result: pyruvate
○
•
The overall rxn is exergonic
•
Fates of Glycolysis products
NADH: oxidized in citric acid cycle
Need to oxidize for glycolysis to happen again
○
May also be converted back to NAD+ via anaerobic means
○
•
Common anaerobic fates
Homolactic (euk)
○
Alcoholic (prok/yeast)
○
•
•
Chapter 12: Part 1
Monday, June 11, 2018
3:14 PM
Overview of carb metabolism
Glycolysis: breakdown of glucose to pyruvate
Universal for living cells
○
The regulation of glycolysis is well understood
○
It plays the central metabolic role in generating BOTH energy and
metabolic intermediates
○
•
Gluconeogenesis: synthesis of polysaccharides
Reverse of glycolysis
○
•
Catabolism: break apart
Energy is released
○
Oxidative
○
•
Anabolism: build up
Energy is required
○
Reductive
○
•
Three basic processes involved:
Process 1: add phosphoryl group to glucose
Yield compounds w low phosphoryl group transfer potential, a
process known as priming
§
○
Process 2: chemically converting low phosphoryl group transfer
potential intermediates into high phosphoryl group transfer potential
compounds
○
Process 3: chemically couple energy yielding hydrolysis to high
phosphoryl group transfer potential compounds
Help synthesis of ATP by direct transfer of phosphate groups
to ADP
§
○
•
Reactions 1-5: Energy investment
Reaction 1: Hexokinase (FIRST ENERGY INVESTMENT)
Glucose gets phosphorylated by ATP via hexokinase
Result: glucose-6-phosphate (G6P)
○
-OH group nucleophilic attacks the incoming phosphate group
○
•
The phosphate group is coordinated by magnesium
•
Hexokinase binding
Hexokinase has 4 isoforms (I,II,III, IV)
○
3/4 isoforms have high binding specificity (low Km)
Binds to fructose and mannose
§
○
Hexokinase isoform IV: used in reaction one for its low binding
specificity (high Km)
Used inside the liver
§
Allows liver to adjust its rate of glucose utilization in response
to variations in blood glucose lvls
§
○
•
Favorable reaction
Rxn is so favorable that it is irreversible
○
•
Reaction 2: Glucose-6-Phosphate isomerase
Glucose-6-phosphate becomes isomerized to fructose via glucose-6-
isomerase
Result: Fructose-6-phosphate
○
G6P is an aldose while F6P is a ketose
○
•
Barely unfavorable rxn
Rxn is reversible
○
•
Reaction 3: Phosphofructokinase-1 (MAJOR CONTROL POINT)
Fructose-6-phosphate is phosphorylated via PFK-1
Result: Fructose-1,6-bisphosphate
○
•
PFK-1 is a bifunctional enzyme
PFK-2 = favors glycolysis
○
Fructose-bisphosphatase = favor gluconeogenesis
○
•
How is this regulated?
In the presence of ATP, PEP or low pH the PFK-2 aspect of the
enzyme is inhibited and gluconeogenesis occurs
○
In the presence of AMP and Fructose-2,6-bisphosphate (activator)
glycolysis occurs
○
•
Favorable rxn
Rxn is irreversible
○
•
Reaction 4: fruct-1,6-bisphosphate aldolase aka aldolase
Fructose-1,6-bisphosphate is cleaved by Fruct-1,6-bisphosphate aldolase
Result: dihydroxyacetone phosphate (DHAP) and D-
glyceraldehyde-3-phosphate (GAP)
○
•
GAP is made first and moves to the second part of glycolysis followed by
DHAP
•
Catalysis mechanism for the process = covalent catalysis (to start) and
GABA (toward the middle)
•
Under biochemical conditions this rxn is unfavorable
Under intracellular conditions the rxn is favorable
○
•
Aldolase mechanism
Conversion of ketone to imine derived from lysine 1.
Deprotonation and cleavage to release GAP and enamine products 2.
Protonation of enamine 3.
Hydrolysis to release DHAP4.
Reaction 5: triose phosphate isomerase
DHAP is isomerized to GAP
Result: another GAP compound (end w 2 total)
○
•
The intermediate of this whole rxn is a enediol intermediate which is
extremely unstable
•
Slightly unfavorable
Rxn can be reversed
○
•
Reaction 6-10: The energy generation phase
Reaction 6: glyceraldehyde-3-phosphate dehydrogenase (FIRST ENERGY
RICH COMPOUND)
GAP is phosphorylated by an inorganic phosphate via glyceraldehyde-3-
phosphate dehydrogenase
Result: 1,3-biphosphglycerate (BPG)
○
•
Generates NADH
Energy rich compound
○
•
Not a true control point BUT if there is not enough phosphate then the rxn
will not go forward
•
Mechanism for enzyme req 2 electron transfer and the formation of
carboxylic phosphor. Acid anhydride or 1,3-bisphosphoglycerate
•
Slightly unfavorable
Rxn can be reversed
○
•
Reaction:
Attack by cysteine at carbonyl C of GAP1.
Transfer of hydride to NAD+2.
Attack by phosphate 3.
Release of cysteine as a free thiol 4.
Reaction 7: phosphoglycerate kinase
1,3-biphosphglycerate is dephosphorylated using phosphoglycerate kinase
•
Result: 3-phosphoglycerate
•
First instance of substrate level phosphorylation
The phosphate pops off of 1,3-bisphosphoglycerate and pops onto
ADP
○
•
This reaction is coupled with reaction 6 to yield an overall favorable rxn
•
Reaction 8: phosphoglycerate mutase
3-phosphoglycerate is phosphorylated via phosphoglycerate mutase
Result: 2-phosphoglycerate
○
•
The phosphate group seems to be transferred from carbon one to carbon
two BUT it actually has something to do with the enzyme
The enzyme has a n-phosphohistidine residue which swaps
phosphates with 3-phosphoglycerate
○
•
This helps prep for the synth of the next "high energy compound"
•
Reaction 9: Enolase (2ND HIGH ENERGY COMPOUND)
2-phosphoglycerate gains another double bond via enolase
Alpha, beta elimination
○
Result: phosphenolpyruvate (PEP)
○
•
Creation of an enol group is very unstable
•
The whole reaction is slightly favorable
•
Reaction 10: pyruvate kinase
The phosphate group on phosphenolpyruvate pops onto ADP creating
ATP through substrate level phosphorylation
Result: pyruvate
○
•
The overall rxn is exergonic
•
Fates of Glycolysis products
NADH: oxidized in citric acid cycle
Need to oxidize for glycolysis to happen again
○
May also be converted back to NAD+ via anaerobic means
○
•
Common anaerobic fates
Homolactic (euk)
○
Alcoholic (prok/yeast)
○
•
•
Chapter 12: Part 1
Monday, June 11, 2018 3:14 PM
Document Summary
It plays the central metabolic role in generating both energy and metabolic intermediates. Yield compounds w low phosphoryl group transfer potential, a process known as priming. Process 2: chemically converting low phosphoryl group transfer potential intermediates into high phosphoryl group transfer potential compounds. Process 3: chemically couple energy yielding hydrolysis to high phosphoryl group transfer potential compounds. Help synthesis of atp by direct transfer of phosphate groups to adp. Oh group nucleophilic attacks the incoming phosphate group. 3/4 isoforms have high binding specificity (low km) Hexokinase isoform iv: used in reaction one for its low binding specificity (high km) Allows liver to adjust its rate of glucose utilization in response to variations in blood glucose lvls. Rxn is so favorable that it is irreversible. Glucose-6-phosphate becomes isomerized to fructose via glucose-6- isomerase. G6p is an aldose while f6p is a ketose.
