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 glucose G6P GLYCOGENESIS 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  Vmax Rate ½ Vmax Km S1 S2 [substrate]
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