Diabetes and Metabolism (Week 3) Flashcards
Why is glucose so essential and what happens if we don’t have enough?
Brain uses primarily glucose
Brain cannot use fatty acids, but liver can oxidize free fatty acids to ketones, and brain can use ketones for energy
What is the “fed state”
Anabolic
Hormone released: insulin, which causes anabolism to build up body’s resources
Nutrients absorbed from small intestine
Increased glucose, branched chain AAs and triglycerides in plasma
Decreased ketones and FFAs
Have glycogen synthesis/storage, AA uptake, protein synthesis and triglyceride formation
Liver uptakes glucose
What is the “fasting state”
Catabolic
Hormone released: glucagon, which breaks down glucose etc
Nutrients must be taken out of storage sites (liver, adipose tissue, muscle) once glycogen runs out
Glycogenolysis (breaking down glycogen to glucose) happens for 8-10 hours, but then get protolysis and gluconeogenesis (form glucose from AAs), and lipolysis and ketogenesis (oxidize fatty acids in liver to make ketones for brain to use)
Decrease in glucose, triglycerides
Increase in AA, FFAs, ketones
Liver releases glucose
If a diabetic has low blood sugar (vomiting, bowel obstruction, etc), should you stop giving insulin?
No, never stop giving insulin!
If you stop insulin, person will get lipolysis which will increase FFAs which will increase ketogenesis (via glucagon excess), which will cause ketoacidosis and electrolyte abnormalities. That can lead to cerebral edema, vascular thrombosis, infection, MI, cardiac arrythmia, death
Insulin causing glucose uptake in muscle and adipose tissue
Insulin in the bloodstream binds insulin receptor on cell surface –> cascade of events –> GLUT4 in vesicles translocated to membrane –> GLUT4 channels let glucose into cell via facilitated diffusion
How is it that any food we eat (carbs, proteins) can be stored as fat?
During the breakdown of amino acids, get alpha-ketoacids which go to fatty acids then triglycerides
During the breakdown of glucose, get fatty acids and alpha-glycerol phosphate, which go to to triglycerides
Glycogen
9 carbon rings?
When we have too much glucose, we store it as glycogen
Have glycogen reserves in cells, and this makes up 10% of the weight in the liver
Big picture of glucose and glycogen
Cell takes in glucose and turns it to pyruvate to make ATP (does GLYCOLYSIS!)
If enough ATP around, won’t do glycolysis (ATP inhibits glycolysis), and instead will store glucose as glycogen
Glycogenolysis
Glycogen –> glucose-6-phosphate (G6P)
(later G6P –> glucose)
(opposite of glycogen synthesis)
Anaerobic metabolism
People do this when not enough O2 (because low BP so no perfusion, and/or hemoglobin so low that O2 carrying capacity compromised, and/or burst of intense activity)
Pyruvate turns to lactate and get buildup of lactate then metabolic acidosis
If serum pH below 6.9 you’ll die because cellular processes won’t work (also can get cardiac arrhythmia)
GLUT transporters in which locations of the body have high and low affinity for glucose?
GLUT in brain has high affinity because always want glucose in the brain
GLUT in liver changes affinity (?) because only want to store glucose if you have extra glucose around
Are all glucose transporters dependent on insulin?
No!
Insulin independent: liver, pancreatic beta cell, brain
Insulin dependent: muscle (GLUT4)
Gluconeogenesis
Oxaloacetate –> glucose
Happens in liver during first part of fast
If person fasted many days cells in kidney would do this too, but still liver also
What happens right after you eat?
Glucose in lumen of intestine is transported (active transport) into intestinal cells and into blood –> glucose stimulates beta cells in pancreas to secrete insulin –> insulin causes liver to stop releasing glucose into blood AND causes muscles to take up glucose from blood (via GLUT4)
What happens when you’re fasting?
Low glucose in blood and high glucagon (how?) –> glucagon stimulates liver to release glucose into blood –> now tissues have source of energy?
Where do we store our energy?
Liver: 70g of glycogen (24 hour supply)
Muscle: 120g of glycogen (not available for export as glucose)
Adipose: 15,000g fat (MAJOR store)
Muscle: 6,000g protein (not preferred to use though)
Energy consumption of brain
Major consumer of energy (uses 20% of body’s energy at rest)
Requires continuous supply of glucose
Can use ketone bodies in prolonged fast and when newborn baby
Energy consumption/storage of muscle
Needs to generate energy for contraction
Can use glucose of fatty acids
Stores glycogen
Energy storage of adipose tissue
Stores energy as fat (triglycerides) in fed state
Releases glycerol and fatty acids in fasting state
Energy consumption/storage of liver
“Altruistic organ” attends to own needs when glucose high (fed state)
Releases glucose into blood so other organs can get it when glucose is low (fasting state)
Note: kidney does similar thing as liver
What does the pancreas secrete?
Insulin and glucagon
(key regulators of metabolism)
Glycolysis
G6P –> pyruvate
8 steps, and 2 irreversible steps are regulated
Produces 2 molecules ATP per glucose oxidized
Anaerobic
Occurs in cytosol (then pyruvate transported to mitochondria for TCA cycle)
Regulation by allosteric effects and hormone signaling
Glycogen synthesis
G6P –> glycogen
(opposite of glycogenolysis)
Fast mechanisms to regulate glucose metabolism (immediate changes)
Substrate concentration
Allosteric regulation (feedback or feed forward)
Signals originating from hormone action (phorphorylation or translocation within cell)
Several of these mechanisms acting together
Slow mechanisms to regulate metabolism (long-term changes)
Genetic regulation
Response to diet and other environmental factors
Hormonal effects on gene expression
Km
Substrate concentration at which the reaction rate is half of Vmax
(lower Km means higher affinity; lower Km means slower rate of reaction)
Km’s of different glucose transporters
GLUT2: in the liver and pancreas beta cells, highest Km (lowest affinity); linear concentration in physiological range means will take up glucose proportional to how much in blood
GLUT1: mid-range Km
GLUT3: in the brain; lower Km (higher affinity) and flatter curve in physiological range means uptake in brain independent of plasma concentration (constant, steady supply of glucose to brain)
GLUT4 regulation by insulin
Insulin binds its receptor on cell surface –> signals through PI3 kinase, etc –> GLUT4 that exists in vesicles translocates to cell membrane –> lets glucose in
Whenno insulin (fasting state), GLUT4 transporters internalized again
(this happens in muscle and adipose tissue)
Once you get glucose into the cell, you have to phosphorylate it–how do you do this?
Glucokinase: high Km, glucose sensor, present in liver and pancreatic beta cells
Hexokinase: low Km, saturated at low glucose concentration, present in most cells
GK (glucokinase) lives in the nucleus, bound to GKRP (glucokinase regulatory protein), when does it come out of the nucleus?
When lots of glucose in cytoplasm (let in by GLUT2), GK comes out to phosphorylate it to G6P –> as glucose converted to G6P, MORE glucose enters cell –> as G6P then F6P builds up, F6P tells GK to go back to nucleus (negative feedback)
Can you regulate a step of a process (a reaction) that is reversible?
Apparently not…
Ex: 6 steps of glycolysis are reversible so are not regulated
Regulation/control points in glycolysis
1) Phosphofructokinase (PFK-1) (turns F6P –> F1,6BP); Stimulated by F2,6BP and AMP; Inhibited by ATP, citrate, H+
2) Pyruvate kinase (turns phosphoenolpyruvate (PEP)–> pyruvate); Stimulated by F1,6BP; Inhibited by ATP, alanine; last step in glycolysis
Why isn’t the “first” step of glycolysis, using hexokinase or glucokinase a control point of glycolysis?
Because end product G6P has many functions! Starts glycolysis but also glycogen synthesis, pentose phosphate pathway
PFK-1 (phosphofructokinase-1) catalyzes first unique and irreversible reaction in glycolysis so that is the first regulated step
What molecule regulates PFK-1?
F2,6BP activates PFK-1
F2,6BP is made by taking a little F6P from the chain and using PFK-2 (kinase activity) to phosphorylate it to F2,6BP
F2,6BP is not part of the pathway, it is created solely to be a regulating molecule
PFK-2/FBPase-2
Bifunctional enzyme that can be kinase (PFK2) or phosphatase (FBPase-2)
Kinase PFK2: F6P –> F2,6BP; when NOT phosphorylated; during fed state; increased insulin; stimulates PFK1 to do glycolysis
Phosphatase FBPase-2: F2,6BP –> F6P; when phosphorylated; during fasting state; increased glucagon; does NOT stimulate PFK1 so does gluconeogenesis
Want PFK2 NOT phosphorylated so it WILL phosphorylate F6P to make F2,6P to activate PFK1 to do glycolysis
How insulin stimulates glycolysis in the liver
Complicated pathway –> dephosphorylates PFK-2 –> high levels of F2,6BP –> activate PFK-1 to continue glycolysis –> more glycolysis!