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"
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"
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
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
Glycogen --> glucose-6-phosphate (G6P)
(later G6P --> glucose)
(opposite of glycogen synthesis)
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?
Insulin independent: liver, pancreatic beta cell, brain
Insulin dependent: muscle (GLUT4)
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
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)
G6P --> pyruvate
8 steps, and 2 irreversible steps are regulated
Produces 2 molecules ATP per glucose oxidized
Occurs in cytosol (then pyruvate transported to mitochondria for TCA cycle)
Regulation by allosteric effects and hormone signaling
G6P --> glycogen
(opposite of glycogenolysis)
Fast mechanisms to regulate glucose metabolism (immediate changes)
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)
Response to diet and other environmental factors
Hormonal effects on gene expression
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?
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
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!
How glucagon inhibits glycolysis in the liver
Glucagon binds cell surface glucagon receptor --> G-protein coupled response --> increased cAMP --> PKA --> PKA phosphorylates PFK-2 --> phosphatase activity increases --> lower levels of F2,6BP (so won't stimlate PFK-1) --> inhibit glycolysis
Epinephrine stimulates the heart via beta receptors and PKA, so what does this do to glycolysis?
Increased PKA phosphorylates an ISOFORM of the PFK-2 enzyme --> in this case, phosphorylation actually increases kinase activity of this bifunctional enzyme --> increased level of F2,6BP --> stimulate PFK-1 --> increased glycolysis
This is good because when HR up, maybe running from a lion and want to have glycolysis in that case!
Remember, muscle bifunctional enzyme works opposite way of liver bifunctional enzyme (if it worked like liver's bifunctional enzyme, we'd get decreased glycolysis when running from lion--would be bad news!)
Does skeletal muscle use a bifunctional ("PFK-2-like") enzyme?
Yes, has a bifunctional enzyme but it is not phosphorylated by PKA
Skeletal muscle's bifunctional enzyme is regulated only by levels of substrate (F6P)
Turns phosphoenolpyruvate (PEP) --> pyruvate
Last step in glycolysis!
Inactivated by glucagon in the liver: glucagon binds receptor on cell membrane --> increase cAMP --> PKA --> PKA phosphorylates and thus inactivates pyruvate kinase
Pyruvate --> lactate (by lactate dehydrogenase) instead of going through TCA cycle
Cool side note: cancer cells do glycolysis then to lactate instead of TCA cycle even when there is O2 around and this can be used as a target for therapy
Aerobic metabolism and pyruvate dehydrogenase (PDH)
After glycolysis, pyruvate --> acetyl CoA (by pyruvate dehydrogenase (PDH))
PDH is a multi-enzyme complex that is inhibited by NADH, acetyl CoA and ATP
PDH active when NOT phosphorylated (Ca2+ during muscle contraction stimulates phosphatase)
When phosphorylated, PDH inactive
Regulation/control points of TCA cycle
1) Pyruvate dehydrogenase (PDH): turns pyruvate --> acetyl CoA; Inhibited by ATP, acetyl CoA, NADH; first step in TCA cycle
2) Isocitrate dehydrogenase: turns isocitrate --> alpha ketoglutarate; Stimulated by ADP; Inhibitied by ATP and NADH
3) alpha-ketoglutarate dehydrogenase: turns alpha-ketoglutarate --> succinyl CoA; Inhibited by ATP, succinyl CoA, NADH
Why is there no phosphorylation/dephosphorylation regulation of the TCA cycle?
TCA cycle takes place in mitochondria and phosphorylation signals (via hormones) start on cell surface and go to cytosol!
Instead, TCA cycle regulated by immediate (NADH) and eventual (ATP) end products
Is gluconeogenesis the reverse of glycolysis?
NO! Uses unique enzymes, and must go from pyruvate through oxaloacetate to get back to phosphoenolpyruvate
Also, requires 6 ATP (whereas glucose makes 2 ATP)
Regulation/control points of gluconeogenesis
1) Phosphoenol-pyruvate carboxykinase (PEPCK): turns oxaloacetate --> phosphoenolpyruvate; Regulated by transcription (has very complex promoter), so more slowly; Tx decreased (activity inhibited) by insulin; Tx increased (activity stimulated) by fasting, glucagon, glucocorticoids, thyroid hormone
2) Fructose 1,6-bisphosphatase: turns fructose 1,6-bisphosphate --> F6P; Stimulated by citrate; Inhibited by F2,6BP, AMP
3) Glucose-6-phosphatase: turns G-6P --> glucose
What step is the same but reversed in gluconeogenesis and glycolysis?
Fructose-6-phosphate <--> Fructose-1,6-bisphosphate
Citrate stimulates fructose 1,6-bisphosphatsase (to make F6P) and inhibits PFK-1
AMP and F2,6BP inhibit fructose 1,6-bisphosphatsase and stimulate PFK-1 (to make F1,6BP)
How and where is G6P dephosphorylated to make glucose?
This happens in the liver and kidney in order to provide glucose to other tissues (cannot happen anywhere else, this is why the liver has to be nice/altruistic and give glucose to everyone!)
G-6-P enters the ER through G-6-P transporter --> SP is a Ca-binding stabilizing protein that stabilizes glucose-6-phosphatase enzyme --> glucose-6-phosphatase enzyme turns G6P to glucose --> Pi is transported out Pi transporter and glucose is transported out glucose transporter back into cytoplasm
Van Gierke disease
Liver can't produce glucose for the rest of the body and get severe hypoglycemia during fasting
Glucose-6-phosphate deficiency; glycogen storage disease (can either have messed up G6P transporter or messed up G-6-Pase)
Treatment: IV glucose (necessary for nighttime) or just can eat uncooked starch!
What are the substrates for gluconeogenesis and where do they come from?
Alanine, lactate, glycerol (more?)
Come from peripheral tissues (muscle, RBCs, renal medulla, adipose tissue)
How are glycogen synthesis and breakdown regulated regarding phosphorylation?
Glycogen --> glucose-1-phosphate (by glycogen phosphorylase-P)
UDP-glucose --> glycogen (by glycogen synthase)
Glycogen phosphorylase is active when phosphorylated
Glycogen synthase is active when NOT phosphorylated
Phosphorylation stimulated by glucagon and epi (during fasting state)
Dephosphorylation catalyzed by protein phosphatase 1 and stimulated by insulin (during fed state)
What factors stimulate the synthesis of glycogen and the breakdown of glycogen?
Synthesis of glycogen (glycogenesis): G6P, ATP, glucose (in liver)
Breakdown of glycogen (glycogenolysis): Ca2+, AMP (in muscle)
(Makes sense because you want glucose (do glycogenolysis) only when you need it, not when you have plenty of G6P, ATP and glucose!)
What is the rate-limiting step of fatty acid synthesis?
Acetyl CoA --> Malonyl CoA (by Acetyl CoA carboxylase)
Allosteric regulation of ACC: Acetyl CoA carboxylase exists in inactive dimers --> citrate stimulates formation into acetyl CoA carboxylase polymers (OR --> long-chain fatty acyl CoA inhibits formation into active polymer (just negative feedback??))
Regulation of ACC by hormones and phos/dephosphorylation: Insulin stimulates protein phosphatase to dephosphorylate acetyl CoA carboxylate and activate it; Glucagon and epi stimulate cAMP-dependent protein kinase to phosphorylate acetyl CoA carboxylate and inactivate it
Want to make fatty acids (which then you make into triglycerides to store) when you've just eaten
Hormone sensitive lipase (HSL)
First enzyme of lipolysis (breaking down TGs to free fatty acids + glycerol so you can oxidize them to ketones and acetyl CoA to get more energy when you can't do glycolysis!)
Glucagon binds receptor --> cAMP activates protein kinase that phosphorylates and activates HSL --> HSL turns fat (triglycerides) into fatty acids and glycerol
When glucose is low, glycerol is released into circulation and can be sent to liver and made back into glucose
Fatty acids released, bind to albumin and are taken up by tissues for activation to acyl CoA and then beta-oxidation ("fatty acid degradation")
HSL active during fasting
Want to break down fat (triglycerides) into fatty acids and glycerol when fasting to give you more energy!
What do glucagon and epinephrine do in general?
Via cAMP and PKA
What does insulin do in general?
Via protein phosphatase?
Enzymes active in phosphorylated form
Phosphatase domain of PFK-2 (actually called FBPase2)
Hormone sensitive lipase
These are all phosphorylated by glucagon or epi, and are active during fasting (want to decrease glycolysis, break down glycogen (give glucose to rest of body?) and break down fat when fasting)
Enzymes active in dephosphorylated form
Kinase domain of PFK-2
Acetyl CoA carboxylase
These are all dephosphorylated by insulin (want to break down glucose (do glycolysis), synthesize glycogen and synthesize fat when fed)
Glycogen storage disease
Defect in glycogen synthesis or breakdown
If can't break down glycogen stored in liver, it accumulates and damages cells, can get enlarged liver
If can't get glucose out into blood, get low blood sugar
Which tissues have glucose-6-phosphatase?
Glucose-6-phosphatase converts G6P --> glucose
Liver (and kidney) ONLY produce glucose to send out to other tissues
Muscle does NOT have this enzyme so they can't create their own glucose!
Why do patients with liver failure get hypoglycemia?
Because liver can no longer put glucose into blood
(remember muscles can't put glucose into blood)
What can and cannot be converted to glucose?
Converted to glucose: lactate, amino acids, glycerol
Cannot be converted to glucose: fatty acids (bc chopped into 2 carbon chains and need 3 carbon chains to do gluconeogenesis)
In Type 1 Diabetes, do all your beta cells get destroyed?
Eventually (I think), but at the beginning of the disease, some beta cells destroyed and others still surviving
What is one reason we see Type 1 diabetes during teenage years?
Body is getting bigger, growing a lot but not much beta cell growth so don't have enough insulin?
Used to distinguish between Type 1 and Type 2 Diabetes
C peptide connects the A and B chains of proinsulin and gets cleaved when insulin is formed, so level of C peptide tells you how much insulin you yourself are producing
Type 1 diabetes: low C peptide because not secreting much insulin on your own
Type 2 diabetes: high C peptide because when you secrete insulin you secrete same amt C peptide
To test for Type 1 diabetes
C peptide: will be low
Anti-GAD antibodies or insulin autoantibodies: positive
What does the liver do when there is no insulin around (Type 1 Diabetes)?
Insulin usually signals the liver to stop glycogenolysis, to stop gluconeogenesis, and to inhibit the release of glucose from the liver, but now insulin not there so liver puts out a ton of glucose into blood!
Lots of glycogenolysis so glycogen stores depleted
Lots of gluconeogenesis so lots of glucose made and dumped out into circulation
Lots of proteolysis and lipolysis to get more glucose made for rest of the body
What do muscle cells do when there is no insulin around (Type 1 Diabetes)?
Insulin usually tells muscle cells to take up glucose from blood, but now no insulin around
Glucose can't get into muscle cells because GLUT4 transporters not on cell surface --> cell deprived of energy --> Na/K ATPase pump fails with no ATP around --> K+ leaks out into blood --> K+ excreted in urine --> hypokalemia (whole body K+ depletion)
Also, muscle is broken down for energy since muscle can't uptake glucose --> AAs used to make glucose instead and get negative nitrogen balance
Remember: Type 1 Diabetes used to be a WASTING disease
What happens with the kidney when you have too much glucose in the blood?
Renal threshold is 180, and after this point, glucose cannot all be reabsorbed and is passed through kidney tubules with the rest of filtrate
Glucose in filtrate in loop of henle causes water and other ions to be secreted to maintain osmotic pressure
Lose water (polyuria thus polydipsia, dehydration), lose salts (NaCl, K), lose energy because losing glucose
What happens to the beta cells that are still left in the pancreas in a patient with Type 1 Diabetes and thus high blood glucose?
Beta cells don't function well in high glucose environment
So even if you have beta cells left, they won't be secreting insulin well because glucose toxicity caused them to not function well
What happens to the brain in a patient with high blood glucose (Type 1 Diabetes)?
Brain downregulates (to 10% of normal) glucose transporters to prevent glucose toxicity
Important when treating patient, because when you decrease blood glucose, brain thinks there's not enough glucose around since it now has fewer transporters
What happens to fat when you have no insulin (Type 1 Diabetes)?
Usually, insulin inhibits breakdown of triglycerides to free fatty acid and glycerol (lipolysis and ketogenesis)
Fat is all broken down into FFA and glycerol (lipolysis) --> also have no insulin, so presenting liver with lots of FFAs --> liver converts FFAs to ketones if no insulin around --> diabetic ketoacidosis
What happens if you don't treat Type 1 Diabetes?
Polyuria and polydipsia
Volume depletion (postural hypertension, tachycardia)
Loss of body fat and muscle (weight loss and muscle weakness)
Deep shallow breathing, breath of acetone (DKA)
Unexplained abdominal pain
Labs: high blood glucose, low bicarb (DKA, buffering acids), low pH, normal or high K+ even though hypokalemia
How do we treat Type 1 Diabetes?
Now have insulin pumps, or shots that don't hurt
Must test glucose level ~6 times per day
How many people in the US have Type 2 Diabetes?
What are some problems Type 2 Diabetes causes?
Most common cause of early death (heart attack, stroke), blindness, kidney disease, leg amputation
Greatest cost to health care system in USA
Risk factors for Type 2 Diabetes
Genetics (Asian, AA, Latina, Pacific Islander, Native American)
Obesity (or any other cause of insulin resistance)
Low birth weight/fetal malnutrition
Relationship between BMI and insulin required per day
The fatter you are (higher BMI), the more insulin you need per day
Because fat people not very sensitive to insulin, so need a lot of it to suppress hepatic glucose release
Why are obese people insulin resistant?
Have lots of adipocytes and these recruit inflammatory cells that release cytokines
Cytokines contribute to insulin resistance. Interleukins (ILs) oppose action of insulin
Note: because of this, can measure C-reactive protein (increased in inflammation) to predict risk for type 2 diabetes
Why do you become insulin resistant when you're pregnant?
Want baby to grow more than mother
Placenta makes hormones to tell glucose not to go to mother but to go to baby instead (placental lactogen)
What happens to 80% of people who becomes insulin resistant?
They just increase insulin secretion to ensure that glucose is not released from liver too much and is taken up by muscles enough (don't get diabetes even if they're really obese)
Other 20% of those who become insulin resistant actually decrease insulin secretion (get Type 2 Diabetes)
What causes glucagon levels to change?
We have baseline of glucagon in our blood (probably), but glucose suppresses glucagon so after we eat, glucagon decreases
What levels of glucose, insulin and glucagon do we see in a type 2 diabetic after a meal?
No drop in glucagon
How do genetics and obesity combine to bring about Type 2 Diabetes?
If you have a hereditary component you'll start losing beta cells early in life so can create less insulin. Then as you get older and/or more obese, you become insulin resistant and need more insulin. But you can't make any more! Get Type 2 Diabetes.
Treatment for Type 2 Diabetes
Lifestyle (diet and exercise)
Insulin and other medications
Drug treatments for Type 2 Diabetes
Metformin: makes liver more sensitive to insulin (enhances how insulin causes liver to inhibit gluconeogenesis and glycogenolysis); causes people to lose weight; decreases risk of cancer
Sulphonylurea: lower blood sugar by making beta cells secrete more insulin
GLP-1: also lower blood sugar by making beta cells secrete more insulin (secrete this naturally from bowel when you eat); increases risk of pancreatic cancer
Glitazones: act on muscle to enhance how insulin causes muscle to bring glucose into muscle cell
How many people in the US have Type 1 Diabetes?
1,500,000 people (half as many as Type 2)
Not as heritable as Type 2, but more in caucasians
Microvascular complications of diabetes
Tiny blood vessels exposed to high sugar causes damage to endothelium (glycosylation)
Diabetic nephropathy (renal failure)
Diabetic retinopathy (blindness)
Diabetic neuropathy (amputations, impotence, severe hypoglycemia)
Macrovascular complications of diabetes
Coronary artery disease (MI)
Cerebrovascular disease (CVA/stroke)
Peripheral vascular disease (amputations)
What is important to control in diabetics to prevent death and other disease?
Most diabetics die of cardiovascular disease (80%)!
Control cholesterol (use statins)
Use anti-platelet drugs to prevent clots (aspirin)
Keep BP low to prevent athreosclerotic complications
Keep blood sugar normal/low to prevent micro- and macro-vascular disease
Type 2 must lose weight--might even not need medications this way
When someone is in diabetic ketoacidosis, you give insulin and this allows K+ to enter the cell--how?
Glucose enters the cell because insulin is around now --> glucose does glycolysis and TCA and electron transport to create ATP --> now have ATP for Na/K ATPase that wasn't working before --> pump works to bring K+ into the cell
Note that you should still give K+ IV too!