Glycolysis and Gluconeogenesis
Glycolysis and Gluconeogenesis
Glycolysis
What is glycolysis? sequence of reactions that converts one
molecule of glucose to two molecules of pyruvate with the formation of two ATP molecules
anaerobic
Glycolysis
Why is glucose such a commonly used fuel? tends to exist in ring form, very stable,
doesn’t generally glycosylate proteins formed from formaldehyde under prebiotic
conditions
Glycolysis
What are the possible fates of glucose?
What’s the difference between a facultative anaerobe and an obligate anaerobe?
Can you give an example of habitat-dependent anaerobiosis?
What about activity-dependent anaerobiosis?
Glycolysis
Glycolysis
All the intermediates in glycolysis have either 3 or 6 carbon atoms
All of the reactions fall into one of 5 categories phosphoryl transfer phosphoryl shift isomerization dehydration aldol cleavage
Glycolysis
Entire reaction sequence may be divided into three stages glucose is trapped and destabilized six carbon molecule is split into two three
carbon molecules ATP is generated
Glycolysis – Stage 1
glucose converted to glucose-6-PO4
ATP is needed catalyzed by hexokinase or glucokinase induced fit G01= -4.0 kcal/mole
Glycolysis – Stage 1
phosphoglucoisomerase aldose is converted to ketose G01=+0.4 kcal/mole
Glycolysis – Stage 1
rate limiting enzyme – allosteric inhibited by high ATP, citric acid, long-chain fatty acids stimulated by ADP or AMP G01= - 3.4 kcal/mole
Glycolysis
Glycolysis – Stage 2
six carbon molecule split into 2- 3 carbon molecules aldose and ketose
G01=+ 5.73 kcal/mole
Glycolysis – Stage 3
At equilibrium most mixture exists as dihydroxyacetone phosphate
G01=+ 1.83 kcal/mole
Triose Phosphate Isomerase
Glycolysis – Stage 3
redox reaction energy from redox used to form acyl
phosphate G01= +1.5 kcal/mole
Glycolysis – Stage 3
Consists of two coupled processes
Glycolysis – Stage 3
formation of ATP – substrate level phosphorylation
Glycolysis – Stage 3
phosphoryl shift – uses 2,3 bisphosphoglycerate G01= +1.1 kcal/mole
dehydration G01 = +.44 kcal/mole phosphoryl transfer G01 = -7.5 kcal/mole
Glycolysis
Fate of Pyruvate
Alcoholic Fermentation
Which organisms carry out this process? yeast other microorganisms
PDC requires thiamine pyrophosphate as coenzyme NAD+ is regenerated
Lactic Acid Fermentation
Occurs in muscle cells, microorganisms Regenerates NAD+
NAD+ and Dehydrogenases
Various dehydrogenases have a similar binding domain for NAD+ showing their common origin Rossman fold
Glycolysis
How can fructose be used for energy?
Glycolysis
To use galactose it must be converted to glucose-6-PO4
Glycolysis
Glycolysis
What causes lactose intolerance?
Glycolysis
What is galactosemia? inability to metabolize galactose missing galactose 1-phosphate uridyl
transferase liver disease development of cataracts CNS malfunction
Control of Glycolysis
Of what value is glycolysis for cells? provides energy in form of ATP provides building blocks for synthetic reactions
Where are most control points found? enzymes that catalyze irreversible reactions
hexokinase phosphofructokinase pyruvate kinase
Phosphofructokinase
Most important control point in mammalian glycolytic pathway allosteric enzyme
activated by AMP and fructose 2,6 bisphosphate
inhibited by high levels of ATP, citrate, fatty acids
Phosphofructokinase
Hexokinase
Hexokinase is inhibited by its product glucose-6-PO4
glucose remains in blood
Glucokinase, an isozyme of hexokinase is not inhibited by glucose-6-PO4
found in liver has lower affinity for glucose
Pyruvate Kinase
Pyruvate kinase exists as isozymes L form – predominates in liver M form – mostly in muscle and brain
PK is an allosteric enzyme activated by fructose 1,6 bisphosphate inhibited by ATP, alanine
L form of PK influenced by covalent modification inhibited by phosphorylation
Pyruvate Kinase
Glucose Transport
What is the role of glucose transporters in animal cells? facilitate movement of glucose across cell
membrane
What kind of molecule is a transporter and where is it located? small protein embedded in plasma
membrane
Glucose Transport
mammalian glucose transporter
Glucose Transport
Glycolysis and Cancer
Why are rapidly growing tumor cells dependent upon glycolysis? insufficient oxygen supply
What is the function of HIF-1? hypoxia-inducible transcription factor
stimulates synthesis of many glycolytic enzymes and GLUT-1 and 3
also stimulates vascular endothelial growth factor
Gluconeogenesis
What is gluconeogenesis? synthesis of glucose from non-carbohydrate
precursors
Why is this an important pathway? What are some of the major precursors?
lactate, amino acids, glycerol
Where does this process occur? liver, kidney
Gluconeogenesis
If gluconeogenesis involves the conversion of pyruvate to glucose why is it not simply the reverse of glycolysis? glycolysis contains several irreversible reactions
Which reactions in glycolysis are irreversible? phosphoenolpyruvate to pyruvate fructose 6-phosphate to fructose 1,6-
bisphosphate glucose to glucose 6-phosphate
Gluconeogenesis
What is the first reaction?
CH3 CCO2-
O
CH2 CCO2-
O
CO2-
+ CO2+ ATP
+ ADP + Pi
Pyruvate
Oxaloacetate
biotin
pyruvatecarboxylase
Gluconeogenesis
Why is pyruvate carboxylase of special interest? structural properties
contains ATP-grasp domain at N-terminal end contains biotin-binding domain at C-terminal
end
Gluconeogenesis
What is the role of biotin in this reaction? prosthetic group lined to -amino group of lysine
residue carrier of activated carbon dioxide
Gluconeogenesis
Pyruvate carboxylase is an allosteric enzyme activated by acetyl
CoA needed to form
carboxybiotin
Gluconeogenesis
Carboxylation of pyruvate occurs in the mitocondria but next step in reaction sequence occurs in cytosol
Gluconeogenesis
Decarboxylation of oxaloacetate is coupled with
phosphorylation by GTP
enzyme is phosphoenolpyruvate carboxykinase
CH2 = CCO2-
OPO32 -
CH2 CCO2-
O
CO2-
+ CO2
+ GTP
Phosphoenol pyruvate
Oxaloacetate + GDP
Gluconeogenesis
Which other steps in glycolysis are irreversible? conversion of fructose 1,6-bisphosphate to
fructose 6-phosphate conversion of glucose 6-phosphate to
glucose
Gluconeogenesis
Fructose-6-phosphate
C
CH2 OP O32 -
O
HHO
OHH
OHH
CH2 OH
Fructose-1,6-bisphosphate
C
CH2 OP O32 -
O
HHO
OHH
OHH
CH2 OP O32 -
fructose-1,6-bis-phosphatase
H2 O P i
G° = -16.7 kJ mol-1
fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and activated by ATP
Gluconeogenesis
Enzyme that catalyzes last reaction not found in all tissues liver and kidney cortex
Gluconeogenesis
Is gluconeogenesis an energetically favorable reaction in the cell?
What drives this reaction?
Are glycolysis and gluconeogenesis active at the same time?
Regulation of Glycolysis and Gluconeogenesis What are some of the factors that
ensure the reciprocal regulation of these processes? allosteric regulators of key enzymes energy charge fructose 2,6-bisphosphate hormones
Regulation of Glycolysis and Gluconeogenesis
Regulation of Glycolysis and Gluconeogenesis fructose 2,6-bisphosphate stimulates
PFK and inhibits fructose 1,6-bisphosphase controlled by insulin and glucagon and
reflects the nutritional status of the cell
Regulation of Glycolysis and Gluconeogenesis How do hormones influence the
enzymes associated with these processes? influence gene expression
change transcription rate influence degradation of m-RNA
– insulin PFK, PK– glucagon PEPCK, fructose 1,6-bisphosphatase
Regulation of Glycolysis and Gluconeogenesis What are substrate
cycles and why are they important? can amplify
metabolic signals can generate heat
Regulation of Glycolysis and Gluconeogenesis What is the Cori cycle and why is it
important?
Regulation of Glycolysis and Gluconeogenesis