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

Glucose Disposalglucose

glucose G6P

Glycogen

F16BP

pyruvate

pyruvate acetyl-CoA

acetyl-CoA

Fatty Acids

CO2

GLUTs

GLYCOGENESIS

GLYCOLYSIS

KREBS CYCLE

LIPOGENESIS

Fat

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

glucose

glucose G6P

GLUTs GLYCOGENESIS

GLYCOLYSIS

glucose

insulin

Translocation

Vesicles in Golgi

PFK – phosphofructo kinase

GS – glycogen synthase

Muscle Glucose Uptake

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

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

[substrate]

Rate

Vmax

½ Vmax

Km S1 S2

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

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

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’

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