Euglycemia

2. Euglycemia • Importance of keeping blood [glucose] at 5 mM • Hypoglycemia • [glucose] < 2 mM leads to coma • Brain has obligatory requirement for glucose • Hyperglycemia • Glucose is a reactive molecule • Glycosyates proteins • Reaction with amine residues • The greater the glycemia and the longer the exposure, the more the glycoslation • Glycosylated proteins tend to be dysfunctional • Problem particularly affects tissues in direct contact with blood • Kidneys – nephropathy • Retina – retinopathy • Blood vessels – endothelial cells – vascular disease • After a carbohydrate meal, priority is to dispose of glucose • Uptake into tissus, conversion into glycogen, fat or carbon dioxide • Liver has first look at the glucose • Direct contact to gut via hepatic portal vein • Hyperglycemia ellicits insulin secretion • Insulin will stimulate glucose uptake and storage/oxidation of glucose

3. Glucose Disposal glucose GLUTs Glycogen GLYCOGENESIS glucose G6P Fat F16BP Fatty Acids GLYCOLYSIS LIPOGENESIS pyruvate acetyl-CoA pyruvate acetyl-CoA KREBS CYCLE CO2

4. Glucose Transporters • GLUT-1 • Present in all cells at all times in constant amounts • Catalyze basal transport • GLUT-4 • Insulin dependent • Present in muscle and WAT only • Translocation and fusion – in response to insulin, vesicles that contain GLUT-4 move from Golgi Apparatus and fuse with cell membrane • Translocation is stimulated when insulin binds to its receptor or in response to exercise

5. Muscle Glucose Uptake glucose GLUTs GLYCOGENESIS GS – glycogen synthase glucose G6P PFK – phosphofructo kinase GLYCOLYSIS glucose Translocation Vesicles in Golgi insulin

6. Rate Limiting Enzymes • The slowest enzyme in the metabolic pathway determines the overall speed • Rate-limiting step • Flux generating step • Properties of these enzymes • Irreversible • Need alternative enzymes to ‘go back’ • Not ‘equilibrium’ under physiological conditions • Committed steps • Saturated with substrate • Low Km or [S] >> Km • Working at Vmax • Key points of regulation

7. Enzyme kinetics  • At high [substrate], minor changes in [substrate] will not affect the rate of reaction • Doubling or halving the [S] isn’t even going to affect the rate Vmax Rate ½ Vmax Km S1 S2 [substrate]

8. Redfern Station Analogy • Imagine a railway station at peak hour with just one barrier operating • This step will soon become ‘saturated’ with people • It is the ‘rate limiting’ step • The point of regulation of the rate of the people moving pathway! • There are 3 major ways to regulate this (and metabolic!) pathways • Change the intrinsic activity of the step • Make ticket-reading & gate-opening happen faster • Akin to Allostery • molecules bind to allosteric site of an enzyme and influence the activity of the active site • Make more gates open • Switch them from being ‘off’ to ‘on’ • Or change the direction from ‘in’ to out • Akin to Covalent Modification and reversible phosphorylation • transporters working  more activated enzymes • Make and destroy gates according to need • Akin to making more enzymes (and then degrading them later!) • This very energy consuming and seemingly inefficient, involving • Transcription of genes • Translation of mRNA

9. Glycogen Synthase • Catalyses the addition of ‘activated’ glucose onto an existing glycogen molecule • UDP-glucose + glycogenn UDP + glycogenn+1 • Regulated by reversible phosphorylation (covalent modification) • Active when dephosphorylated, inactive when phosphorylated • Phosphorylation happens on a serine residue • Dephosphorylation catalysed by phosphatases (specifically protein phosphatase I) • Phosphorylation catalysed by kinases (specifically glycogen synthase kinase) • Insulin stimulates PPI • And so causes GS to be dephosphorylated and active • So insulin effectively stimulates GS

10. Phosphofructokinase • Catalyses the second ‘energy investment’ stage of glycolysis • Fructose 6-phosphate + ATP  fructose 1,6 bisphosphate + ADP • Regulated allosterically • Simulated by concentration changes that reflect a low energy charge • An increase in ADP/AMP and a decrease in ATP • These molecules bind at a site away from the active site – the allosteric binding sites. • Many other molecules affect PFK allosterically but all are effectively indicators of ‘energy charge’

11. Coupling (again!) • The stimulation of glycogen synthesis by insulin creates an ‘energy demand’ • Glycogenesis is anabolic • The activation of glucose prior to incorporation into glycogen requires ATP • This drops the cellular [ATP] and increases the [ADP] • This drop in ‘energy charge’ is reflected by a stimulation of PFK • A good example of how an anabolic pathway requires energy from a catabolic pathway • Insulin has ‘indirectly’ stimulated PFK and glucose oxidation even though it does not have any direct lines of communication to this enzyme