Glycolysis - Enzyme mechanisms - Creightonchemistry.creighton.edu/~jksoukup/lec16-1glySTUD.pdf ·...
Transcript of Glycolysis - Enzyme mechanisms - Creightonchemistry.creighton.edu/~jksoukup/lec16-1glySTUD.pdf ·...
Glycolysis - Enzyme mechanisms4. Regulating a metabolic pathway
Control of fluxMetabolic flux = amount of metabolites going through a pathwayper unit time
Flux through glycolysis varies by >100-fold in muscle dependingon the need for ATP
Maintaining homeostasisLevels of the glycolytic intermediates hardly change despite largechanges in flux
What determines flux and homeostasis? THERMODYNAMICS
Koelle, lec15, p18
Glycolysis - Enzyme mechanisms4. Regulating a metabolic pathwayPictorial analogy: water represents flux of metabolites, amount ofwater in flask represents amount of a particular intermediate, pipesbetween flasks are enzymes, vertical drop represents decrease infree energy
ΔG˚ = height difference between flask bottomsΔG = height difference between water levels
Koelle, lec15, p21
Glycolysis - Enzyme mechanisms4. Regulating a metabolic pathway
Control of fluxWhat limits flux through glycolysis?
A. Supply of accessible glucoseAnalogy: If less water is input than the system can carry, theamount of input will dictate the amount of flux
B. Capacity of glycolytic enzymes to process glcAnalogy: Nonequilibrium rxns are shown as skinny pipes that canlimit flow through the system
How regulate activity of nonequilibrium enzymes?• Allosteric effectors• Modifications (phosphorylation)• Altering amount of protein
Glycolysis - Enzyme mechanisms4. Regulating a metabolic pathwayMaintaining homeostasisA. Requires coordinately regulating all nonequilibrium rxnsAnalogy: narrowing just one skinny pipe causes water to back upin the system, narrowing all three does not
B. Intermediates must remain at a constant levelGlycolytic intermediates also involved in “feeding” otherpathways, altering levels interferes with control of other pathways
Koelle, lec15, p24
Glycolysis - Enzyme mechanisms4. Regulating a metabolic pathwayMaintaining homeostasisGlycolytic intermediates are fed in from other pathways
Koelle, lec15, p25
Glycolysis - Enzyme mechanisms4. Regulating a metabolic pathwayMaintaining homeostasisC. Accumulating certain metabolites can have severe consequences
Glycolysis - Regulation3 non-equilibrium reactions that limit fluxAll three must be coordinately regulated to control flux and maintainhomeostasis
Koelle, lec16, p2
Glycolysis - Regulation5. Regulating glycolysisA. Regulation by cellular energy chargeB. Regulation by hormoneC. Regulation to coordinate glycolysis with other pathways
Glycolysis - Regulation5. Regulating glycolysis
A. Regulation by cellular energy chargeFlux of glycolysis increases when [ATP] is needed
ATP, ADP, and AMP are interconverted
adenylate cyclase
ATP + AMP ⇔ 2 ADP
ATP and AMP allosterically regulate phosphofructokinase-1High physiological [ATP] ~ completely blocks PFK-1Inhibition relieved by [AMP][AMP]/[ATP] controls PFK-1 activity
Energy charge affects the Km of PFK-1 for its substrate (F6P)
Glycolysis - Regulation5. Regulating glycolysis
A. Regulation by cellular energy chargeEnergy charge affects the Km of PFK-1 for F6P
Koelle, lec16, p5
PFK-1 is an allosteric enzyme, allosteric effectors bind remote fromactive site and shift equilibrium between high and low affinityconformations
Glycolysis - Regulation5. Regulating glycolysis
A. Regulation by cellular energy chargeWhat about homeostasis?Hexokinase and pyruvate kinase are regulated to match PFK-1
Hexokinase is feedback inhibited by G6PPyruvate kinase is feed-forward activated by F1,6-BP
Koelle, lec16, p6
Glycolysis - Regulation5. Regulating glycolysis
A. Regulation by cellular energy chargeHexokinase versus Glucokinase (isozymes)HK - in most cells
- has a high affinity for glc (Km = 0.1 mM)- allosterically inhibited by its product, G6P
GK - in liver- affinity for glc (Km = 10 mM, blood [glc] = 5 mM)- efficient glc transporters maintain [glc] in liver at 5 mM- enzyme regulated by level of blood glc- after meals blood glc high, excess blood glc moved to liver- inhibited by F6P
Glycolysis - Regulation5. Regulating glycolysis
A. Regulation by cellular energy chargeWhy use PFK as the control point?
Koelle, lec16, p8
PFK is the first “committed step” - first nonequilibrium enzyme not alsoin other pathways
Glycolysis - Regulation5. Regulating glycolysis
B. Regulation by hormone (glucagon)Glucagon -
Glucagon inhibits PFK-2PFK-2 is an allosteric activator of PFK-1 and it synthesizes F-2,6-BPWhen glucagon is around, it binds its receptor on cell surface, activatesa protein kinase that phosphorylates and inactivates PFK-2
Glycolysis - Regulation5. Regulating glycolysis
C. Regulation to coordinate glycolysis with other pathways
Koelle, lec16, p10
Glycolysis - RegulationSummary
Koelle, lec16, p11
Glycolysis - RegulationWhat about NAD+? Why don’t we run out of it?
Koelle, lec16, p12