Carbohydrate metabolism and insulin synthesis Flashcards
What is the pentose phosphate pathway important for?
> Synthesis of fatty acids through the generation of NADPH, which is critical for reductive biosynthesis
> Nucleotides through the formation of ribose 5-phosphate
What is the committed step of the pentose phosphate pathway?
Dehydrogenation of glucose 6-phosphate
G-6-P dehydrogenase
Key branch point, relevant enzymes will be switched on/off to determine which pathway the G6P goes down
What is glycogen?
A polymer of glucose synthesised from glucose
Essentially the same as starch
How is glycogen stored?
Complexed with water
Carb-deficient diets will cause depletion of glycogen stores, and loss of water mass in the first week
Where is glycogen stored?
Majority:
> The liver - storage for blood glucose maintenance
> In muscle - storage for local energy production (only used by the muscle itself)
Most tissues can store some glycogen, but it may not be physiologically relevant as it’s such a small amount
How is glycogen synthesised?
Glycogenesis
Glucose -> G6P -> G1P -> UDP glucose -> glycogen
What is the key enzyme in glycogen synthesis?
Glycogen synthase (tightly regulated)
Catalyses reaction of UDP glucose -> Glycogen
How is glycogen broken down?
Glycogenolysis
Glycogen -> G1P -> G6P -> Fructose 6P -> Glycolysis
What is the function of the enzyme phosphoglucomutase?
G6P G1P
Catalyses both reactions
How is glycogenesis/glycogenolysis regulated?
Reciprocal regulation
Glycogen synthase and glycogen phosphorylase must be regulated inversely
- you would not want both to be active at the same time
What does hexokinase do
Catalyses reaction of glucose -> G6P
irreversible
What is gluconeogenesis?
synthesis of glucose from a noncarbohydrate (nonhexose) source
What are the non-hexose sources used in gluconeogenesis?
> Lactate
Pyruvate - less commonly found in the blood than lactate
Glycerol - from fat stores
- you cannot make glucose from fatty acids, but you can from glycerol
- fatty acids are used by peripheral tissues for feul
Certain amino acids (glucogenic amino acids)
- this only happens to a certain level when muscle mass is being lost (prolonged fasting or extreme starvation)
Where does gluconeogenesis occur?
Mainly in the liver but the kidneys can contribute with prolonged starvation
How does gluconeogenesis occur?
Essentially a reversal of glycolysis, but three reactions are not reversible as they all involve ATP (use/production)
> Hexokinase/glucokinase (in liver)
PFK
Pyruvate kinase
How are irreversible reactions in gluconeogenesis overcome?
Achieved by use of other enzymes
> Hexokinase bypassed by glucose 6-phosphatase
- principally expressed in the lover
- reciprocally regulated because otherwise it would waste ATP
PFK bypassed by fructose 1,6-bisphosphatase
Pyruvate kinase bypassed by two enzymes
- pyruvate carboxylase
- PEPCK = phosphoenolpyuvate carboxykinase
Where is glucose 6-phosphatase found?
In the lumen of the endoplasmic reticulum
Needs transporters for substrates & products to get in/out
- G6P has to be transported into the ER
How is glucose 6-phosphatase regulated?
At the level of expression
Expression is stimulated by adrenaline and glucocorticoids leading to increased transcription/translation of the enzyme
Suppression is by insulin - prevents gluconeogenesis in the fed state leading to reduced transcription/translation
What are the substrates for gluconeogenesis?
Lactate - pyruvate
Glycerol - dihydroxyacetone phosphate (DHAP)
Amino acids in various locations in TCA cycle and pryruvate
Why are fatty acids unable to contribute to gluconeogenesis?
Fatty acids are oxidised to acetyl CoA
2 CO2 produced
lost the carbons, therefore no contribution to glucose carbons
How is gluconeogenesis stimulated in the liver?
Through glucagon and adrenaline
- decrease glucokinase
- increases G6Pase and PEPCK activity
- i.e. -> increases gluconeogenesis
Effect is at level of gene expression
Which tissues are glucose dependent?
> Erythrocytes - no mitochondria, therefore cannot oxidise fuels, only energy from glycolysis
Brain - fatty acids cannot cross blood-brain barrier
Retina
Testes - testes blood barrier prevents entry of fatty acids - sperm use fructose, so have high no. GLUT5
What cell types are found in the islet of langerhans and what hormones do they produce?
alpha cells - secrete glucagon
beta cells - secrete insulin
delta cells - secrete somatostatin
Describe the pathology of the islet of langerhans
core of beta cells with a ring of alpha cells around the edge
How are insulin and glucagon synthesised?
As pro-hormones
How is glucokinase distinguished from other hexokinases?
> Glucokinase has a lower affinity for glucose than the other hexokinases
- Half-saturated at glucose conc. ~8mmol/L
Glucokinase is not inhibited by its product, G6P
- Allows cont. signal output (e.g. to trigger insulin release) amid significant amounts of its product
How does glucokinase change its activity?
- Glucokinase changes conformation and/or function in parallel with rising glucose conc. in the physiologically important range of 4-10mmol/L due to lower affinity for glucose
How is glucokinase regulated?
By a supply-driven metabolic pathway
The rate of reaction is driven by the supply of glucose, not by the demand for end products
How do beta cells secrete insulin?
Potassium channel is inhibited by ATP causing cell depolarisation
This opens the voltage-gated L-type Ca channels causing an influx of calcium that leads to exocytosis of vesicles containing insulin
How is insulin release regulated?
ATP production is a signal for glucose conc.
The more glucose enters the cell, the more ATP is made and the ATP:ADP ratio will go up
ATP will inhibit ATP-sensitive K+ channel
When glucose levels decrease, less ATP is produced, ATP:ADP ratio is decreased
Why do beta cells not convert glucose to glycogen?
Because of stimulus-secretion coupling - beta cells use ATP produced as a signal for glucose conc.
Can only be converted to ATP so that the correct amount of insulin can be produced for the body’s needs
How is insulin synthesised?
Preproinsulin = signal sequence - chain B - chain C - chain A
proinsulin = signal sequence cleaved
Insulin = chain C is cleaved
What is C peptide used for?
In insulin overdose, C peptide levels can be used to differentiate between exogenous and endogenous cause
As C peptide is secreted along with insulin
How is glucagon synthesised?
pre-proglucagon is only cleaved in a way to give glucagon in alpha cells
In cells in the GI tract, pre-proglucagon is cleaved to form glucagon-like peptide 1 and 2 and oxyntomodulin
How is glucagon excreted?
Mechanism poorly understood
Secreted in response to low glucose
> May be crosstalk between glucagon and insulin secretion in human islets (unconfirmed)
> ATP-sensitive K+ channel may also be important in alpha cells, but would have to work in reverse (? possible)
How does insulin signalling occur?
Insulin binds to the insulin receptor and initiates intracellular cascade
Where is the insulin receptor primarily found?
Liver, striated muscle, adipocytes
What kind of receptor is the insulin receptor?
Tyrosine kinase receptor
- Autophosphorylation of tyrosine residues on cytoplasmic side upon binding
- Initiates intracellular cascade
Which particular kinase is activated in response to insulin?
Protein kinase B (akt) is activated in response to insulin and is responsible for the metabolic effects of insulin
What are the metabolic effects of insulin?
Stimulation of anabolic processes
Inhibition of catabolic processes (inhibition of gluconeogenesis is one of the key roles of insulin)
Why does T1DM cause hyperglycaemia?
Gluconeogenesis in the liver is never inhibited, even during the fasted state
How does glucagon signalling occur?
Glucagon binds to the glucagon receptor, increasing clyclic AMP (cAMP), which stimulates cAMP-dependent protein kinases
What type of receptor is the glucagon receptor?
G-protein coupled receptor
Where is the glucagon receptor primarily found?
Only in hepatocytes as only the river will respond to glucagon
