2. Fed State Metabolism Flashcards
How are each of the following organs affected in the fed state: Brain, Muscle, Liver, Pancreas, Adipose tissues?
Brain: unaffected (oxidises glucose)
Muscle: glucose uptake, glycolysis, glycogenesis increase (insulin stimulated), FA oxidation decreases
Liver: glycolysis, glycogenesis, fatty acid synthesis increases (sense glucose increase)
Pancreas: releases insulin (senses increased [glucose])
Adipose Tissue: glucose uptake, FA synthesis, FA uptake increased (Insulin stimulus), FA release inhibited
How are fatty acids stored in the blood?
In lipoproteins
How does the pancreas sense the increased blood [glucose] and release insulin accordingly (7)?
- Glucose transported into cell through GLUT2
- Glucokinase phosphorylates it to G-6-P
- G-6-P undergoes glycolysis, producing ATP
- Increased ATP, decreased Mg-ADP stimulates K+ channel closure
- [K+] increases, cell depolarises
- Increase in cell voltage leads to opening of Ca2+ voltage dependent channels
- Ca2+ floods into cell, binds to insulin vesicles and causes insulin release
How does incretin increase amount of insulin released?
Metabolic hormone produced by the small intestine (GLP1) which increases the sensitivity of K+ transporters to inhibitory ATP
Where are GLUT1&3, GLUT2 and GLUT4 found in the body? How do their Km’s vary? Why?
GLUT1 & GLUT3: Brain, 1mM, low Km as glucose uptake wanted to be constant (not sensitive)
GLUT2: Liver, Pancreas, 20mM, high Km as glucose sensitivity, uptake varies considerably with respect to [glucose]
GLUT4: Muscle, Adipose tissue, 5mM, middle Km to keep glucose uptake ~constany but also respond to changes in insulin levels
How is the Km of HkIV different from HkI-III? Why?
HkIV/glucokinase: It is 8mM rather than 1mM as it used as a sensor protein
What is the expression of Glucokinase like in the pancreas? Why?
It is low, so this is the limiting step for glucose metabolism
What is the K_0.5 or a protein?
The Km if the allosteric enzyme does not follow the michaelis mention equation (sigmoidal not hyperbolic curve)
What is the curve shape for glucokinase? What is the curve shape for HkI-III?
HkIV/glucokinase has a sigmoidal curve rather than the rectangular hyperbola of HkI-III
What model of substrate binding does glucokinase follow?
The Mnemonical model - A form of cooperative binding where the activator binds at a different time rather than location
What is co-operative binding?
An interaction between molecules (where one bi/multivalent) where the binding of one ligand changes the binding affinity of the second ligand
What is MODY2? What is it caused by? What are it’s symptoms? How is it treated?
Maturity onset diabetes of the young
Caused by autosomal dominant mutations in glucokinase reducing substrate affinity, co-operativity and thermostability
Low insulin levels as pancreatic cells cannot detect [glucose] changes well
Treated by insulin injections or managed by diet
What disease occurs if there is a homozygous inherited defect in glucokinase?
Neonatal diabetes mellitus (this is bad)
What system does the liver use to regulate glucose uptake? How does this work?
GLUT2-GCK system
GKRP (glucokinase regulatory protein) binds to glucokinase (GCK) inhibiting it’s activity and localising it to the nucleus. This process is inhibited when glucose and fructose-1-p are present, allowing uptake of glucose into the cell.
How does the GLUT2-GCK system vary between the pancreas and the liver?
The liver expressed much higher amounts of glucokinase so it can take up large amounts of glucose from the plasma (rather than just sensing like the pancreas)
Which pathways are upregulated/inhibited during the transition from fasting to fed state (3)?
Glycogenesis upregulated, glycogenolysis inhibited
glycolysis upregulated, gluconeogenesis inhibited
FA synthesis upregulated, FA oxidation inhibited
What are the two most important enzymes involved in glycogen metabolism (glycogenesis/glycogenolysis)? Which of these are activated in the fed state?
Glycogen synthase (glycogenesis) - activated in fed state
Glycogen phosphorylase (glycogenolysis) - inhibited in fed state
What forms do GP and GS take?
Both dimers with two forms a (active) and b (less active)
Explain how glycogen phosphorylase changes between the fasting and fed state and what causes these changes.
In the fasting state GP is phosphorylated, active, R state and has PP1 bound. It is kept in this by phosphorylation from phosphorylase kinase (PK). PK is phosphorylated/ activated by PKA which is in turn stimulated by GLUCAGON.
In the fed state, glucagon levels decrease and insulin and glucose levels increase. Glucose binds to GP causing alpha helices to cover the PP1 binding site and PP1 is released (T–> R state). The PP1 then causes dephosphorylation of GP, inactivating it. PP1 also dephospharylates PK inactivating it. PKA is inc give due to low glucagon concentrations.
Explain how glycogen synthase changes between the fasting and fed state and what causes these changes.
In the fasting state, glycogen synthase (GS) is inactive, tense and phosphorylated. It is inhibited by phosphorylated GSK3.
In the fed state, glucose-6-p and insulin levels increase. G-6-P binds to GS causing T–> R state transition and PP1 then dephosphorylates it, causing activation. GSK3 is inhibited by insulin preventing deactivation of GS.
How is PP1 regulated to ensure minimal energy wastage?
10x less PP1 enzymes than GP so almost all of GP must be inactivated before PP1 activity restored. So most of GP switched off before GS is activated.
What are the 6 molecules in glycolysis (not enzymes)?
Glucose, Glucose-6-P, Fructose-6-P, Fructose-1,6-bisP, Phosphoenolpyruvate, Pyruvate
What three enzymes are responsible for catalysing glycolysis? What are each of these enzymes stimulated/inhibited by?
Glucokinase: Glucose->Glucose-6-P, activated by glucose
Phosphofructokinase: Fructose-6-P->Fructose-1,6-BisP, activated by F2,6-bisP (this is produced by PFK-2/FBPase-2 which is activated by insulin)
Pyruvate Kinase: Phosphoenolpyruvate->pyruvate, activated by Fructose-1,6-BisP, inhibited by [alanine] and [ATP]
Which four enzymes are responsible for catalysing gluconeogenesis? What are each of these enzymes stimulated/inhibited by?
Pyruvate carboxyase/ phosphoenolpyruvate carboxykinase: pyruvate->PEP, PC not affected, PEPCK transcription/mRNA stability decreased by insulin
Fructose bisphosphatase-1: Fructose-1,6-bisP->fructose-6-p, F2,6-bisphosphate inhibits this
Glucose-6-phosphatase: glucose-6-p->glucose, inhibited by insulin
What is the role of PFK-2/F2,6Pase? What does this enzyme complex catalyse? What inhibits/stimulates it?
Integral in glycolysis/gluconeogenesis controlling. Made up of phosphofructokinase-2 and fructose bisphosphatase-2.
PFK-2 catalyses Fructose-6-P–> Fructose-2,6-BisP (adds P from ATP->ADP)
F2,6Pase does the reverse reaction, releasing Phosphate
Insulin dephosphorylates complex, activating PFK-2 (which activates PFK-1 favouring glycolysis) and inhibiting FBPase-2 (inhibiting gluconeogenesis)
What two initial products may pyruvate be converted into?
Acetyl CoA or Lactate
What enzymes catalyses pyruvate to acetyl coA?
Pyruvate dehydrogenase
How is acetyl coA converted into fatty acids?
Some Acetyl coA converted to malonyl coA (using ATP), then malonyl and acetyl coA combined along with NADPH to produce palmitic acid. This is then elongated and modified to produce other fatty acids.
What does pyruvate dehydrogenase require to convert pyruvate to acetyl coA?
NAD and CoASH, releases NADH and CO2
How long is the pyruvate carbon chain? How long is the acetyl CoA carbon chain?
Pyruvate: 3 carbon
Acetyl CoA: 2 carbon
How is pyruvate dehydrogenase (PDH) regulated? What type of reaction does it undertake?
Inhibited by its products (NADH, Acetyl CoA) and stimulated by substrates (Pyruvate, ADP)
Irreversible reaction - destroys the gluconeogenic potential of pyruvate (2C compounds can’t be converted to glucose)
Why is pyruvate dehydrogenase only activated when glucose is plentiful?
Because reaction it catalyses destroys the gluconeogenic potential of the pyruvate (2C cannot be converted to glucose)
If there is excess acetyl CoA, how may it leave the mitochondria (3)?
As citrate - there are no ACA transporters in the mitochondria
- ACA converted to Citrate by citrate synthase
- Leaves cell via transporter on IMM
- Citrate converted to ACA by citrate lyase (releasing OAA)
What two ways may be used to make NADPH?
Conversion of OAA to pyruvate and the pentose phosphate pathway
What differing products do pyruvate carboxylase and pyruvate dehydrogenase produce?
PC makes OAA (requires ACA)
PD makes ACA
How is ACA converted to MCA (malonyl coA)? Share a favourite enzyme fact.
With Acetyl CoA carboxylase, requiring ATP and CO2. This adds on a COO group to ACA.
7 out of 8 ACA must be converted to MCA for FA synthesis
What causes activation of ACC (acetyl coA carboxylase)?
Citrate causes multimerisation of ACC leading to activation. Fatty acids cause dimerisation - inactivation. Starvation leads to low ACC transcription.
Other than being a FA synthesis substrate, how else does MCA affect FA synthesis?
Inhibits CPTI, thus preventing FA from entering the mitochondria for oxidation. Therefore MCA also up regulates FA synthesis.
How does FA synthesis occur?
- MCA combines with ACA - CO2 released
- Reduction, dehydration, reduction process occurs (removes second =O)
- Process continues until carbon chain = 16
- then hydrolysis of FA from ACP
What are the roles of fatty acids?
Storage of energy and structural roles
How would you increase the fluidity of a membrane?
By increasing the number of double bonds in the phospholipid FAs
What are essential FAs and how are they obtained?
n-3 and n-6 series of fatty acids, may only be obtained through the diet or by modification of dietary uptake FAs
What are n-6 FAs made by? What is the name of the most common FA? What is the name of the most important n-6 FA? Why is it important?
Made by terrestrial plants
Most common is linoleic acid
Most important is arachidonic acid
Arachidonic acid is important for inositol phosphoglycerides and may be converted into signalling molecules (eicosanoids)
What two things may arachidonic acid be converted into? Which enzymes can do this?
Leukotriene - by 5-lipoxygenase
Prostaglandin - by cyclooxygenase
What are n-3 FAs made by? What is the name of the most important n-3 FAs? Why are they important?
Made by cold water plants
DHA: enriched in brain/retinal phospholipids their flexibility facilitates membrane embedded protein function
EPA: anti-thrombotic/inflammatory
What are PUFAs? What are they involved in? How?
Polyunsaturated Fatty Acids
Involved in gene regulation
Repress expression of lipogenic genes and up regulate lipid oxidation genes (favour lipid breakdown) by binding nuclear receptors which then bind DNA response elements, enhancing transcription
What are the three FA naming systems? How do they work?
If FA is 20 C’s long, has 4 double bonds at 5, 8, 11, 14 (carbon number starting from the COO carbon)
Delta numbering system: 20:4^delta 5,8,11,14
Miller system: 20:4n-6
omega system: 20:4omega6
What is a methylene interruption in a FA?
When two double bonds of a FA are separated by a methylene (CH2) bridge
Where are fatty acids elongated? How does this occur (6)?
Elongated on the endoplasmic reticulum.
Catalysed by Fatty acid elongases (ELOVL1-5)
- Fatty acid with CoA combines with malonyl CoA
- Condensation reaction releasing CO2 & CoASH
- Reduction reaction using NADPH
- Dehydration reaction releasing H2O
- Reduction reaction using NADPH
- FA elongated by one carbon
What is arachidonic acid made from?
Arachidonic acid (20:4) is made from linoleic acid (18:2) from the diet
What is the benefit of being able to elongate fatty acids?
This means that you can store excess fatty acids from a series (e.g. x:4) until needed, then can synthesise whatever is required
What enzymes carry out FA desaturation and where? What is the role of fatty acid desaturation?
Desaturases, on the smooth endoplasmic reticulum.
2 Hydrogen atoms are removed, creating a C=C double bond, used to generate components of bodily cells
How many desaturases are there? What are they? How do they differ?
Delta 9: create C=C at position 9 & 10
Delta 6: IF C=C at 9/10 then create C=C at 6/7
Delta 5: IF C=C at 8/9 then create C=C at 5/6
What occurs in the retroconversion of fatty acids? Why is this done?
Two carbons (ACA) are removed from the alpha end (COOH end) of the FA, occurs in peroxisomes.
This process allows cells to store excess FA as 2 or more versions and cut them back down as needed
Process is similar to beta-oxidation in mitochondria but FAD is regenerated by O2 directly producing H2O2 (broken down by catalase). ACA and NADH exported from peroxisome
What 5 process would you undertake to get from palmitic acid (16:0) to 22:4n-9?
- Elongation - 18:0
- delta9 desaturation - 18:1n-9
- delta6 desaturation - 18:2n-9
- elongation - 20:2n-9
- delta5 desaturation - 20:3n-9
During the fed state, how are TAGs synthesised? Where are they synthesised?
TAGs are synthesised via the glycerol phosphate pathway from fatty acids from the plasma or via de novo synthesis. These are then packaged into VLDL (very low density lipoproteins ) and released into the plasma.
TAG synthesis occurs in the liver.
Does TAG synthesis change in the fed or fasted state?
No, it’s continuous
How does the small intestine introduce fatty acids into the bloodstream?
Packages the digested FA via the monoacylglycerol pathway (glycerol from glycolysis used) into chylomicrons (lipoprotein)
What are the products of TAG synthesis?
2-monoacylglycerol and fatty acids (monoacylglycerol pathway)
What occurs in the adipose tissue during the fed state (4)? What stimulates this?
- Glucose uptake/glycolysis enhanced - supplies energy/stuff for FA and TAG synthesis
- FA synthesis enhanced
- TAG synthesis enhanced
- FA taken up from blood for storage (via lipoprotein lipase)
This is all stimulated by insulin
How is glucose uptake controlled in adipose tissue?
GLUT4 is translocated to membrane (stimulated by insulin) to allow glucose uptake.
