Carb Metabolism Flashcards
Key Reactions, Enzymes and Transport Proteins in Carb Digestion/absorption
Saliva (salivary amylase - breaks alpha 1,4 glucosidic bonds) —> 2 phases in SI (see below) —> absorbed at brush border (GLUT5 and SGLT1) —> exit basolateral membrane into portal circulation (GLUT 2)
2 Phases of Carb Digestion (+ where each occurs)
- Pancreatic Phase- LUMEN
- Secretin- stim release of pancreatic juice w/ bicarb to neutralize stomach chyme
- CCK- stim release of pancreatic alpha-amylase
- Products = maltose, glucose and limit dextrins (glucose still connected by alpha 1,6 bonds)
- Disaccharide phase - BRUSH BORDER
- DIsaccharidases (maltase, lactase, isomaltase/sucrase double-headed enzyme)
- Dextrinase cleaves alpha 1,6 bonds in limit dextrins
Glucose Transporters in Small Intestine (3)
Brush border
- SGLT1- Brings glucose and fructose into cell using potential energy of Na+ gradient so requires energy
- High affinity for glucose
- GLUT5- only works if sugars are moving down a conc gradient (so higher conc of sugars outside cell) so does not require energy
Basolateral membrane
- GLUT 2 -transports glucose out of cell and into portal circulation; also only works when glucose moves down a conc gradient so does not require energy
- Low affinity for glucose
Which tissues use insulin-dep glucose uptake?
fat cells, skeletal muscle, heart muscle
Which GLUT transporter is used in liver and why?
GLUT 2
- Because it has a low affinity for glucose, it is more contingent on the concentration of glucose; allows differences b/n fed and fasted states
Which GLUT transporter is used in brain and why?
GLUT 3 (neurons)
- HIGHEST affinity for glucose so highest rate of glucose transport - Ensures that brain gets glucose even in low-glucose state
Which GLUT transporter is used in muscle and why?
GLUT 4
- Insulin stimulates it to incorporate into membrane (insulin- dep because do not need glucose all the time)
Effects of hyperglycemia in diabetic patients
Chronic high blood glucose —> inc glucose uptake in cells that are insulin-independent (lens, retina, kidney and nerve cells) —> reacts w/ aldol reductase —> sorbitol
Sorbitol accumulation leads to tissue damage in these cells (blindness, cataracts, kidney problems and neuropathy)
Glycogen Structure
- Chains held together by alpha 1,4 glycosidic bonds then branches every 8-10 glucose molecules with alpha 1,6 glycosidic bonds
- Glycogenin protein core (autocatalytic)
Function of Glycogen in Muscle v Liver
Liver- maintain blood glucose levels (mobilize when blood glucose is low and store when blood glucose is high)
Muscle- for immediate energy needs during exercise (does not contribute to blood glucose in fasting state)
Glycogen Synthesis Reactions
Gluc —> glucose-6-P (+ glucose-1,6-bisphos intermediate) glucose-1-P —> UDP-glucose (“activated glucose”) —> added to glycogen
Glycogen Synthesis Enzymes
- Hexoinase (muscle) or glucokinase (liver)
- Phosphoglucomutase
- UGPase
- Glycogen Synthase (regulated enzyme/step)
Glycogen Synthesis Energetics
Step 1 requires 1 phosphate bond; step 2 is in equilibrium so no energy cost; step 3 requires -2 phosphate bonds and last step GAINS 1 phosphate bond
Net = -2
Synthesis of “Activated Glucose”
Synthesis: Glucose-1-P + UTP —> UDP-glucose + pyrophosphate
Pyrophosphate almost immediately turns into 2P so the above reaction occurs despite low deltaG b/c conc of pyrophosphate is so low )reacts immediately) that reaction is driven to the right
cAMP and Glycogen Synthesis v Mobilization
Synthesis: glucagon —> cAMP levels inc —> activate protein kinase A —> phosphorylates glycogen synthase which DEACTIVATES it (cAMP also inhibits protein phosphatase which would normally dephosphorylate glycogen synthase)
Mobilization: Glucagon/epinephrine —> inc cAMP — >act protein kinase A —> phosphorylates phosphorylase kinase A (active) —> phosphorylates glycogen phosphorylase
What happens to glycogen synthase after a high carb meal?
Inc insulin/glycogen ratio —> activates protein phosphatase 1 (PP1) —> dephos glycogen synthase (NOW ACTIVE) —> glycogen is formed
STORAGE
Glycogen Mobilization Enzymes
glycogen phosphorylase
Bifunctional Debranching Enzyme
Muscle Glycogen Phosphorylase v Liver Glycogen Phosphorylase
Muscle- responds to epinephrine; reg by energy state and cAMP and Ca++
Liver- responds to both epinephrine and glucagon; reg by glucose state of cell and cAMP; also inhibited by ATP though
Ca++ and Muscle Cells
Coordinates muscle contraction with glycogen mobilization b/c Ca++ contracts cell and activates phosphorylase kinase –> activates glycogen phosphorylase
Epinephrine alpha v beta receptors
Alpha- inc Ca++ (also inc IP3 in liver)
Beta- inc cAMP –> cascade
Von Gierke’s Disease
Deficiency in glucose-6-phosphatase …glucose cannot leave cell so hypoglycemia —> always hungry, fatigued, maybe stunted growth
McArdle’s Disease
Deficiency in muscle glycogen phosphorylase… muscle weakness, muscle cramps when exercising, excessive muscle glycogen stores
Treat by high protein diet and avoiding strenuous exercise
Glycogen Phosphorylase Deficiency in Liver v Muscle
- In liver glycogen phosphorylase …hypoglycemia, excessive liver glycogen stores so enlarged liver (Hers Disease)
- In muscle glycogen phosphorylase… muscle weakness, muscle cramps when exercising, excessive muscle glycogen stores (McArdle’s Disease)
Which tissues use insulin-indep glucose uptake?
All tissues except adipose and muscle
Where are glycolytic enzymes located in cell?
All in cytosol
Kinetics of Glucokinase v Hexokinase
- Glucokinase- specific to liver and pancreatic beta cells
- Lower affinity/higher Km AND higher Vmax …so more responsive to change in [glucose] in blood
- Hexokinase- all others (including muscle)
- Higher affinity/lower Km for glucose AND lower Vmax …so always saturated in brain/RBCs that need constant supply
Role of Glycolysis in Muscle, Liver and RBCs
- Muscle- supply energy for muscle contraction
- Liver- if fed then path for eventual conversion of extra glucose to fat (ANABOLIC)…if fasting then reversed for gluconeogenesis
- RBCs- all ATP derived from glycolysis b/c no mito so no oxidative metabolism
Which glycolysis enzymes use ATP? Generate ATP? Generate NADH?
Glucokinase/hexokinase uses 1 ATP
PFK-1 uses 1 ATP
P-glycerate kinase and pyruvate kinase each make 1 ATP per G3P
G3P dehydrogenase generates 1 NADH
What is the primary site of regulation in glycolysis?
PFK-1
- Activated by AMP in muscle (inc affinity for F-6-P)
- Activated by fructose-2,6-P in liver (inc affinity for F-6-P)
- Inhibited by ATP and citrate (dec affinity for F-6-P)
What are the secondary sites of regulation in glycolysis?
- Pyruvate kinase
- Liver influenced by phosphorylation and allosteric
- Inc glucagon —> inc cAMP —> phosphorylate pyruvate kinase (inactive)
- Muscle only influenced allosterically
- Liver influenced by phosphorylation and allosteric
- Hexokinase
- ONLY IN NON-LIVER TISSUES
- Inhibited by glucose-6-phosphate (original substrate for step I)
G3P DH v Lactate DH
In Anaerobic Conditions…
G3P dehydrogenase —> NADH …if anaerobic it does NOT enter ETC so…reduced to lactate via lactate dehydrogenase
Pyruvate + NADH —> lactate + NAD+
Products of Aerobic v Anaerobic Glycolysis
Aerobic: 2 ATP per glucose
glucose + 2NAD+ + 2ADP + 2P —> 2 pyruvate + 2 NADH + 2ATP
Anaerobic: 2 ATP per glucose
glucose + 2ADP + 2P —> 2 lactate + 2 ATP
Name 3 Glycolysis Intermediates that are Branch Points
- Glucose-1-P can be made into glycogen
- 3-phosphoglycerate and pyruvate used to make non-essential AAs
Which glycolysis enzyme deficiency leads to hemolytic anemia?
Mutation in pyruvate kinase —> impaired ATP synthesis —> accumulate 2,3-BPG which dec efficiency of Hb to bind O2
Name two types of cells that used anaerobic glycolysis
Activate skeletal muscle and RBCs
Dietary Sources of galactose and fructose
Galactose- dairy
Fructose- honey and fruit; now also high fructose corn syrup
Name 4 Compounds Whose Synthesis Requires Galactose
glycolipids, glycoproteins, proteoglycans and lactose (in moms)
Unique Galactose Metabolism Enzymes (3) + Where are they expressed?
Galactokinase (Galactose —> Galactose 1-P)
Uridyl Transferase (Galactose-1-P –> Glucose-1-P and UDP-galactose)
UDP-4-gluc-epimerase (UDP-galactose —> UDP-glucose)
**All expressed in liver
2 Causes of Galactosemia
Uridyl-transferase def…galactose accumulates in cell —> reacts w/ aldose; effects liver glucose metabolism (liver damage, failure to thrive)
Galactokinase def…more rare and does NOT affect glucose liver metabolism; galactose does NOT accumulate inside cell
What reaction leads to galactose toxicity?
Galactose –> galactitol via aldose enzyme
Unique Fructose Metabolism Enzymes (3) +Where are they located?
Fructokinase (Fructose —> fructose-1-P)
Aldolase B (fructose-1-P –> glyceraldehyde + DHAP)
Triose Kinase/Glyceraldehyde Kinase (glyceraldehyde —> G3P)
**All located in liver
Essential Fructosuria
If fructokinase deficient…essential fructosuria …fructose accumulates in blood —> such a high conc that it is taken up by hexokinase (in non-liver tissue) and converted to fructose-6-P
HFI (Hereditary Fructose Intolerance)
If aldolase B deficient…hereditary fructose inolerance (HFI)…severe damage to lier and kidney because phosphate is trapped w/in high amounts of fructose-1-P —> no available phosphates for glucose-1-P so dec in amount of ATP
Enzymes Needed to Metabolize Ethanol + Toxic Intermediate
ADH + ALDH
Toxic intermediate = acetaldehyde
Disulfiram and Fomepizole
Disulfarim - competitive inhibitor of ALDH (inc acetylaldehyde if ethanol consumed –> inc symptoms)
Fomepizole - competitive inhibitor of ADH (used if someone accidentally consumes ethylene glycol)
Pyruvate Utilization (4)
- Converted to lactate via lactate dehydrogenase (in cells that use anaerobic glycolysis)
- Converted to alanine by alanine aminotransferase
- Converted to acetyl CoA by pyruvate dehydrogenase (in mito matrix- need to shuttle pyruvate from cytoplasm)
- Converted to OAA by pyruvate carboxylase (in mito matrix -need to shuttle pyruvate from cytoplasm)
Pyruvate Path When Fed
Liver- PDH activated —> acetyl CoA —> FA synthesis
Muscle has permission to use glucose as energy source
Pyruvate Path When Fasted
Liver- PDH inactivated —> OAA —> gluconeogenesis
Muscle does not have permission to use glucose as energy source
What reaction links glycolysis to TCA cycle?
pyruvate + CoASH + NAD+ —> Acetyl CoA + CO2 + NADH
- Uses 3 enzymes …E1 E2 E3
- Uses 5 Coenzymes… thiamine pyrophosphate, lipoid acid, CoASH, NAD+, FAD
- In mito matrix
Regulation of Pyruvate DH v Pyruvate Carboxylase
- Acetyl CoA accumulation —> favors gluconeogenesis
- Acetyl CoA does this by activating pyruvate carboxylase while inhibiting PDH
- Acetyl CoA consumption (low levels) —> favors TCA cycle
- Acetyl CoA no longer inhibiting PDH
- Reflects energy state of cell b/c high acetyl CoA means high energy state (do not need more TCA cycle—> more ATP)
Why would thiamin or biotin deficiency result in lactic acidosis?
PDH no longer works w/o thiamine cofactor and pyruvate is instead converted to lactose via lactate dehydrogenase
Pyruvate carboxylase uses biotin cofactor so deficiency leads to lactic acidosis (pyruvate converted into lactate by lactate dehydrogenase instead)
Why might alanine levels be elevated in blood if pyruvate carboxylase deficiency?
pyruvate now converted to alanine by alanine aminotransferase instead
Acquired v Genetic PDH Deficiency
- Deficiency in enzyme itself can be hereditary (rare mutation in one of the pDH genes) OR acquired (thiamin def from diet or ethanol consumption)
- “Wet brain”
- Beri Beri - eat polished rice
Arsenic Poisoning Mechanism
PDH inhibitor- arsenate forms non-functional complex w/ lipoid acid
3 Functions of Pentose Phosphate Pathway
- Source of NADPH
- Source of ribose-5-P for nucleotide synthesis
- Recycle excess pentose phosphates back to G3P and glycolysis (esp in RBCs)
Oxidative Branch of PPP
- G-6-P DH (Glucose-6-P —> 6-P-gluconolactone + NADPH
- 6-phosphogluconate DH (6-P-gluconate —> ribulose-5-P + NADPH)
Oxidative Branch of PPP Regulation
- G-6-P DH is REGULATED STEP
- Inhibited by NADPH and fatty acyl-CoA (end products)
- Inhibition reversed by oxidized glutathione
4 Tissues Enriched in PPP Enzymes + Why?
- Liver and adrenal glands- hydroxylation reactions, FA/cholesterol/bile/steroid synthesis
- RBCs- need NADPH for reactions against superoxide and hydrogen peroxide
- Phagocytes- need NADPH to produce superoxide to kill bacteria
How can knowledge of the PPP help you diagnose thiamine deficiency?
Transketolase requires thiamin so you can look at transketolase activity in lyses RBCs to clinically assess thiamin deficiency (AKA this enzyme will not be active if thiamine def)
Why do people w/ gluc-6-P DH deficiency have adverse drug reaction to antimalarial pills?
Antimalarial drugs inc oxidative stress —> if inadequate NADPH supply —> RBC lysis —> anemia
2 Ways PPP Protects RBCs from Oxygen Toxicity
- MetHB Reductase uses NADPH
- Glutathione peroxidase disposes of hydrogen peroxide but needs NADPH for glutathione reductase action
Which TCA enzymes produce NADH?
Isocitrate DH, alpha-ketogluterate DH, malate DH
Which TCA enzymes produce CO2?
Isocitrate DH and alpha-ketogluterate DH
Which TCA enzyme produces FADH2?
Succinate dehydrogenase (complex II)
Which TCA enzyme produces GTP by substrate-level phosphorylation?
Succinate thiokinase
Primary Site of TCA Cycle Regulation
Isocitrate DH
- inhibited by ATP and NADH - Act by ADP and Ca++
Secondary Sites of TCO Cycle Regulation
Alpha-Keto DH inhibited by NADH and succinyl CoA; activated by Ca++
Citrate accumulation (from inhibition of isocitrate DH)—> inhibits citrate synthase
How many ATP molecules per TCA cycle?
- 3 NADH (3x3) = 9 ATP
- 1 FADH2 (1 x 2) = 2 ATP
- 1 GTP
12!
How many ATP molecules per fully oxidized glucose?
- TCA cycle = 12 ATP (x2 because 2 acetyl CoA per glucose) = 24 ATP
- PDH run of pyruvate —> acetyl CoA yields 1 NADH (x2 pyruvate per glucose) = 6 ATP
- Either 8 or 6 ATP from glycolysis (depends on which shuttle used)
36-38!
What is an anaplerotic reaction and an example?
Reaction that replaces TCA cycle intermediates
- Ex) pyruvate —> OAA by pyruvate carboxylase)
What conditions would lead to accumulation of citrate?
High energy state —> inhibits isocitrate DH —> citrate accumulation —> inhibits citrate synthase
How is gluconeogenesis dependent on glycolysis, FA oxidation and TCA cycle?
- Fatty acid oxidation - energy source
- TCA cycle and glycolysis - carbon source from intermediates
4 Unique Gluconeogenesis Enzymes + Regulation
- Pyruvate carboxylase
- Pyruvate + ATP + CO2 +H2O—> OAA + ADP + P
- Need acetyl CoA, biotin and ATP for activation
- PEPCK
- OAA + GTP —> PEP + GDP + P + CO2
- Regulated at level of transcription (cAMP —> inc transcription of PEPCK gene)
- Act by glucagon
- FBPase-1
- Fruc-1,6-bisphos + H20 —> Fruc-6-P + P
- Act by ATP and glucagon and deact by AMP and signaling molecule
- G-6-Pase
- Gluc-6-P + H20 —> Glucose + P
- Synthesis is dec by insulin and inc by glucagon
Location of Gluconeogenesis
pyruvate —OAA occurs in mitochondria then all others in cytosol…so must use malate -OAA shuttle
3 Sources of Carbon Skeletons for Gluconeogenesis
- Glyercol
- Lactate
- AAs (esp alanine)
Cori Cycle
- Lactate —> pyruvate —> glucose (in liver)
- Can recycle lactate from RBCs and muscles to make glucose (transported from RBCs and muscle to liver as lactate in blood)
How does signaling molecule affect gluconeogenesis?
Signaling molecule stim glycolysis and inhibits gluconeogenesis
- HIGH INSULIN - HIGH SIGNALING MOLECULE - PFK2 active (fed so make fat) - HIGH GLUCAGON - LOW SIGNALING MOLECULE - FBPase-2 (or FBPase-1) phosphorylated and active (fasted so make glucose via gluconeogenesis)
Bifunctional Enzyme
2 sep kinase domains
- phosphofructokinase-2 (PFK-2) - fructose bisphosphatase (FBPase-2)
Signaling molecule activates PFK-2 and inhibits FBPase-2
