Carbohydrate metabolism Flashcards
Why is the brain most vulnerable to hypoglycaemia?
(3 points)
- It cannot store or synthesise glucose in significant quantities
- It can metabolise only glucose and ketone bodies as substrates
- It cannot extract enough glucose from ECF at low concentrations (because not hormone driven entry)
Location and role of GLUT1 transporter?
Constitutive, constant basal uptake, high affinity.
Most tissues including RBCs, muscles, brain etc
Location and role of GLUT2 transporter?
Liver and pancreatic beta cells.
Low affinity, high capacity: Normally low uptake but deals with massive intake at high blood glucose concentration after meals. (15-20mM)
Low affinity important to prioritise other tissues like brain and muscle at low glucose concentrations.
Role and location of **GLUT3 **transporters?
Found in brain and other high demand tissues
Supplements GLUT1 in these tissues
Location and role of GLUT4 transporters?
GLUT4 is Insulin induced! Affinity 5mM so controlled by plasma glucose conc.
Found on skeletal muscle and adipose tissue.
What is GLUT5?
A misnomer, it is a fructose transporter found in the small intestine.
Glycogenolysis produces what, how?
difference between liver and muscle?
Produces G1P (‘high energy form of glucose’) by phosphorolysis by phosphorylase.
Muscle stores most glycogen in body but cannot export it as glucose (for brain) because lacks G6P phosphatase (unlike LIVER). Hence just enters glycolysis.
Liver cell metabolism in fed state?
(high 0.5:1 insulin to glucagon ratio)
Glucose enters cell (through GLUT2 low affinity)
Most goes to glycogen or TAGs (exported as VLDLs)
Some used for energy via TCA cycle
Excess amino acids enter cell –> Pyruvate/AcCoA –> TCA cycle or TAGs
Returning Lactate and Glycerol also converted to TAGs or go in TCA
Muscle cell metabolism in FED state?
high insulin to glucagon ratio
Glucose enters cell (GLUT4) –> glycogen (or glycolysis, TCA)
Fatty acids from gut via Chylomicrons, or VLDL (from liver)
LPL extracts and Beta-oxidation converts to Acetyl-CoA
Amino-acids incorporated into proteins.
Metabolism in adipose tissue in fed state?
(high insulin:glucagon ratio)
And brain tissue?
Adipose tissue: Glucose enters via GLUT4
Glucose converted to fatty acids and finally TAGs for storage. (via glycolysis and PDH to AcCoA)
Fatty acids enter from VLDL (from liver) or Chylomicrons (from gut) –>TAGs
Glycerol released from TAGs in lipoproteins –> (by LPL) goes back to liver
Brain: simple: takes up glucose by GLUT1 and 3. –> glycolysis and oxidative metabolism for energy
What are the roles of fatty acids in the early and late fasting stages?
Mobilised from Adipocyte TAG stores –> fatty acids into bloodstream
Cannot be used for gluconeogenesis as acetylCoA is 2 carbon.
They are burnt in liver, muscle, and other peripheral tissues (by B-oxidation in mitochondrial matrix) to AcCoA –> TCA cycle. In the liver this energy supports gluconeogenesis, but in the late fasting stage excess AcCoA is converted to Ketone bodies. (Acetoacetate, B-hydroxybutyrate)
AcCoA and Citrate inhibit glycolysis to spare glucose for brain!
What is the glucose-fatty acid cycle?
Randle cycle, and why important for brain?
Oxidation of FAs –> sparing of glucose
In fasted state oxidation of fatty acids produces AcetylCoA, excess of which is converted to citrate.
AcCoA Inhibits pyruvate dehydrogenase PDH from converting pyruvate (from glycolysis) to further AcetylCoA.
Citrate inhibits PFK-1 (from phosphorylating F6P to F1,6BP) leading to build up of F6P and G6P. G6P negative feedback inhibition of Hexokinase leads to glucose build up which stops net diffusion of glucose into cell and spares blood glucose concentration for brain!
Where is the pancreas? where are the beta cells?
Pancreas is a diffuse gland found inferior to the stomach.
Beta cells are in centre of islets, with a high blood flow and good autonomic innervation.
(other cells in periphery)
Different processing of proglucagon?
In pancreatic alpha cells proteases cut it into GRPP, Glucagon and Major Proglucagon fragment
In small intestine, different proteases cut it into Glycentin and GLP1 (incretins) and GLP2.
Glucagon is single chain peptide, 29aa.
Interaction between glucagon and insulin secretion?
Glucagon promotes insulin secretion! (like its incretin sister GLP1)
But insulin inhibits glucagon secretion.
Difference between liver and isoenzymes of Glycogen phosphorylase?
Muscle isozyme responds strongly to allosteric activation (R form stabilisation) by AMP and Pi, and inactivation (T form stabilisation) by ATP and G6P. (AMP prevents dephos by PP-1)
Whereas liver form only inactivated by Glucose and activated by phosphorylation (serine 14 by phosphorylaseb kinase)
Muscle phosphorylase responds to energy status (AMP over insulin, PPase1)
Liver phosphorylase responds to negative feedback from glucose, and glucagon,adrenaline, insulin etc
Mechanisms of insulin action overview:
Opposing glucagon by: Increasing phosphodiesterase activity to reduce cAMP (via PKB)
Increasing tyrosine kinase activity
Increasing Protein-phosphatase-1 (serine/threonine phosphatase) activity (via MAPK, ISPK, PPase1(GM))
IRS, PI3K, PIP3, PKB (PDK1) leading to GSK3 inactivation, Glut4 translocation, protein synthesis and cell survival, PDE activity
Regulation of glycogen synthase:
Phosphorylation deactivates (to synthase D, dependent on energy status: activated by G-6-P, increases Vmax ?deactivated by ATP, UTP and Pi)
Dephosphorylation (to synthase I [by insulin,MAPK,PPase1], independent of energy status)
How does glucose activate synthase and deactivate phosphorylase in liver?
why only in liver?
Phosphorylasea binds PPase1 strongly.
Glucose binds Phosphorylasea, stabilising T-conformation, exposing p-serine 14 for dephosphorylation.
Phosphorylaseb has weak affinity for PPase1, releases it.
PPase1 dephosphorylates Glycogen synthase D, activating it to synthase I.
Only occurs in liver because GLUT2 and Glucokinase (allows high intracellular [glucose])
Both insulin and glucagon result in phosphorylation of PP-1, but have opposite effects, how does this occur?
Insulin (RTK,IRS,GRB2,GEF,Ras,MAPK,ISPK) ISPK phosphorylates site 1 of Glycogen binding regulatory subunit of PP-1, activating it, causing dephos of Phosphorylasea, Phos.kinase and Glycogen synthase D.
PKA in response to Beta adrenoceptor or glucagon receptor stimulation phosphorylates site 2, which conversely causes PP-1 to dissociate from glycogen metabolism enzymes. This allows it to bind (phosphorylated) Protein Inhibitor-1, PI-1, and become inactivated.
Basic regulation of PFK-1 (glycolysis “committed step”) by energy status and “nutrients”?
Energy status: ATP inhibits PFK-1, AMP activates PFK-1
(H+ ions indicating anoxia, lactic acid build-up, AMP overrides their inhibitory effect in the heart)
Nutrients: F6P and F2,6BP activate (indicate high glucose)
Citrate inhibits (indicates excess AcetylCoA from FA B-oxidation or overload)
Regulation of PFK-2, F26BPase in liver?
(F26BP most potent allosteric activator of PFK1 and inhibitor of F1,6,BP)
PFK-2 regulation similar to PFK-1 except not sensitive to ATP concentration, but instead inhibited by PEP.
Phosphorylation of the tandem enzyme (by PKA, from glucagon) inhibits PFK2, and activates F2,6BPase (promoting gluconeogenesis)
Insulin (via PP-1) and pentose phosphate pathway product Xylulose-5-Phosphate (via PP-2A) stimulate desphorylation of tandem enzyme. (more glycolysis)
AMP activates PFK-1 and inhibits F2,6BPase. Citrate and PEP inhibit (indicating FA oxidation or gluconeogenesis etc)
Allosteric regulation of liver pyruvate kinase? (L-type)
Active –> Glycolysis and FA synthesis
Inactive –> Gluconeogenesis
Switched off (increased Km) by: alanine (starvation signal) and ATP (gluconeogenic substrate, and high energy status). And by phosphorylation by PKA etc.(adrenaline+glucagon)
Switched on by: insulin (dephos, via PP-1?), F1,6BP (high glucose indicator, overrides all!)
[also insulin induces enzyme in the longer term]
Pyruvate carboxylase regulation:
Pyruvate to OAA (using ATP and CO2/HCO3-)
Requires biotin for CO2 transfer. Mg2+ and H+
Stimulated by AcetylCoA (cf PDH, inhibition increases Pyruvate supply), by ATP, [glucagon and adrenaline increasing AcCoA and Pyruvate?]
Inhibited by: Glutamate and Ca2+…
Long term induction by: Glucagon, Thyroid hormones, glucocorticoids, +diabetes
Inhibition by deinduction by insulin (and decreased FA B-oxidation)
Expression regulation of PEPCK?
Function?
Induced by Glucagon, cAMP, Glucocorticoids
Overriding de-induction by insulin!
Converts OAA to PEP (Using GTP, releasing CO2)
Components of PDH complex?
(found in mitochondrial matrix)
E1: pyruvate decarboxylase: removes CO2, adds acetyl group to TPP coenzyme (Thiamine Pyrophosphate).
E2: Dihydrolipoate transacetylase accepts Acetyl group from TPP, transfers it to CoA.
E3: Dihydrolipoate dehydrogenase catalyses reduction of NAD+ to NADH + H+, (using FAD prosthetic group), re-oxidising 2 thiol groups of E2.
Regulation of PDH (pyruvate dehydrogenase, complex)
Simple: End product competitive inhibition of E2 by Acetyl CoA and E3 by NADH (both also from FA oxidation)
Own unique kinase and phosphatase (inhibitory phosphorylation of alpha-subunit of E1, decarboxylase)
AcetylCoA, NADH, ATP all activate PDkinase –> inhibit PDH.
Kinase inactivated by opposite (CoASH, NAD+, ADP) and pyruvate, TPP, Ca2+
PDphosphatase activated (thereby activating PDH) by Ca2+ (e.g. from muscle contraction) and Mg2+
Starvation increases fat utilisation by increased PDKinase, and decreased PDphosphatase expression. (insulin does opposite)
Triglyceride lipase regulation:
Simple: cAMPPK phosphorylation increases activity. (glucagon, adrenaline [Beta], ACTH, sympathetic stimulation)
Insulin dependent dephos by PP1 (MAPK, ISPK pathway) decreases triglyceride lipase –> less lipolysis
Reciprocal regulation of fatty acid synthesis and breakdown?
Synthesis inhibiting breakdown: MalonylCoA (produced by AcetylCoA carboxylase as first step in fatty acid synthesis) inhibits CarnitinePalmitoylTransferase1 (CPT1), preventing creation of Fatty Acyl carnitine for transport across mitochondrial membranes, so B-oxidation in mitochondria cannot occur.
Breakdown inhibiting synthesis: Fatty Acyl-CoA (palmitoyl Co-A) inhibits Acetyl-CoA Carboxylase (ACC) protomer aggregation and activation. (inhibiting fatty acid synthesis)
Fatty Acyl-CoA also inhibits Citrate and ATP export from mitochondria, leading to PDH inhibition (PDH found in matrix)
[also lack of Citrate in cytoplasm prevents ACC activation]
Hormonal regulation of ACC?
Occurs via regulation of PP1 by PKA (PI-1 and site 2 phosphorylation) and ISPK(site 1, activation).
Dephosphorylation by PP-1 activates!
Phosphorylation by AMPPK deactivates, (to a lesser degree also cAMPPK)
Possible mechanisms of insulin resistance in type 2 diabetes mellitus?
Increase number of adipocytes, not enough insulin to suppress FA release after meal –> increased blood FAs and TAGs (which may activate PKC and Ceramide dependent protein kinases, phosphorylate IRS1)
White adipose tisse WAT as endocrine organ:
IL-6 and TNFa inflammatory cytokines lead to serine phosphorylation on IRS-1 (blocking tyrosine phosphorylation and pathway activation)
[Also inhibits adiponectin secretion.]
Resistin: found in fat mice, may promote insulin resistance in humans, currently unclear.
Alcohol induced hypoglycaemia:
symptoms and mechanism
Alcohol metabolism in liver (to acetaldehyde, to acetic acid) produces excess NADH which drives reversible gluconeogenesis reactions backwards (e.g. pyruvate to lactate)
Therefore hypoglycaemia (<2.2mM) after glycogen stores depleted.
Acetaldehyde also toxic (forms adducts)
Stress response in attempt to raise blood glucose, also rapid breathing from acidosis from blood lactate)
[Plus fatty liver from excess AcetylCoA, from Acetate, –> fatty acids]
