Glucose Metabolism Flashcards
What are the different glucose transporter isoforms, what are their functions, and where are they expressed?
- GLUT1 = uptake of basal glucose to sustain respiration in adult cells (especially erythrocytes and endothelial cells of barrier tissues); fetal tissues. Regulated by cellular glucose levels and upregulated in tumors. Class I.
- GLUT2 = bidirectional, high-frequency, low-affinity transporter. Renal tubular cells, liver cells (in = glycolysis / out = gluconeogenesis), pancreatic beta cells (flow of glucose allows gauging of serum glucose), basolateral membrane of small intestinal epithelium. Class I.
- GLUT3 = high-affinity (allowing transport even when [glu] is low). Neurons and placenta. Class I.
- GLUT4 = Adipose tissues and striated muscle; insulin regulated. Class I.
- GLUT14 = Testicular optic canals and similar to GLUT3. Class I.
- GLUT5,7,9,11 = Class II. GLUT5 = fructose transport in enterocytes. GLUT7 = intestinal epithelium.
- GLUT6,8,10,12,13 = Class III.
Describe the function of hexokinase and the different hexokinase isoforms.
- Phosphorylates glucose to form G6P, trapping it in the cell (charged molecule cannot recross membrane)
- HKI = Mitochondrial surface, involved in glycolysis, regulated by G6P (inh) and Pi (act)
- HKII = Mitochondrial surface, involved in glycogen synthesis and PPP, regulated by G6P (inh) and Pi (inh)
- HKIII = Perinuclear, glycogen synthesis and PPP, regulated by G6P (inh) and Pi (inh)
- HKIV (glucokinase) = Cytoplasmic and nuclear, glycolysis, not inhibited
What are the possible fates of G6P?
glycolysis, PPP, glycogen synthesis, glycoproteins/glycolipids, proteoglycans, glucuronides
What is the purpose of glycolysis and where does it occur?
- Generates energy aerobically or anaerobically from glucose: 1 mol glucose -> 2 mol pyruvates = 2 ATP + 4 electrons
- Cytosol
What are the two phases of glycolysis?
- Preparative phase: generates high energy molecule (fructose-1,6-BP) from glucose
- Energy rearrangement phase
What are the steps of glycolysis?
- Glucose => G6P: phosphorylation by HK, uses 1 ATP
- G6P => F6P: rearrangement by isomerase
- F6P => F-1,6-BP: phosphorylation by PFK-1, uses 1 ATP
- F-1,6-BP => DHAP + GAP (both 3C molecules)
- DHAP <=> GAP: rearrangement isomerase, equilibrium favors GAP because GAP rapidly removed by GAPDH
- GAP => 1,3-BPG: oxidation by GAPDH, generates 1 NADH
- 1,3-BPG => 3-PG: dephosphorylation by kinase, generates 1 ATP via substrate level phosphorylation
- 3-PG => 2-PG: rearrangement by mutase
- 2-PG => PEP: rearrangement by enolase
- PEP => pyruvate: dephosphorylation by pyruvate kinase, generates 1 ATP
How is energy generated from NADH in aerobic glycolysis?
- Transferred by G3P and malate-aspartate shuttles into mitochondria
- oxidative phosphorylation in ETC regenerates NAD+
How is energy generated from NADH in anaerobic glycolysis?
- Pyruvate is reduced to lactate by LDH, requiring the oxidation of 1 NADH
- Lactate re-oxidized to pyruvate in the liver
- Occurs in cells without mitochondria (kidney medulla, RBCs)
What are the regulatory points in glycolysis?
- HK inhibited by G6P
- PFK-1 is allosterically regulated. Activated by AMP and F-2,6-BP (which is regulated by insulin/glucagon ratio). Inhibited by ATP and citrate
- Liver PEP isozyme is activated by F-1,6-BP (feed forward) and inhibited by ATP
How can fructose enter glycolysis?
- Conversion to sorbitol and then glucose
- Conversion to G3P
- Fructose => F6P: phosphorylation by fructokinase, requires 1 ATP
- F6P => DHAP + glyceraldehyde: cleavage by aldolase B (liver) or aldolase A (muscle)
- Glyceraldehyde => GAP: phosphorylation triose kinase, requires 1 ATP
- GAP and DHAP can enter glycolysis
What diseases are associated with fructose metabolism?
- Too much dietary fructose => sorbitol buildup => osmotic issues
- Essential fructosuria: Autosomal recessive, mutation to fructokinase. Fructose builds up and is excreted in urine
- Inherited fructose intolerance: aldolase B mutation. Can cause depletion of liver ATP
How can galactose enter glycolysis?
- Galactose => Gal1P: phosphorylation by galactokinase (GALK), requires 1 ATP
- Gal1P => Glu1P: galactose-1-P-uridylyltransferase (GALT), exchange reaction with UDP-glucose => UDP-galactose.
- UDP-gal => UDP-glu: epimerization via GALE
- Glu1P => Glu6P via polyol pathway
Which enzymes are inhibited in galactosemias?
- Non-classical galactosemia = GALK (step 1)
- Classical galactosemia = GALT (step 2)
- Neither clinically severe
What is the consequence of erythrocytic pyruvate kinase deficiency?
Hemolytic anemia: insufficient ATP production to suppose Na-K ATPase activity in plasma membrane
What are the symptoms of F-1,6-bisphosphatase deficiency?
Compromised gluconeogenesis, fasting hypoglycemia, ketosis, acidosis
What is the Warburg Effect?
- Tumor cells need lots of ATP and R5P => increased rate of glycolysis and PPP
- High glycolysis rate works well in anaerobic environment
What biosynthetic pathways might glycolytic intermediates enter?
- G6P: 5 carbon sugars, triglycerides
- 1,3-BPG: 2,3-BPG
- 3-PG: serine, 2,3-BPG
- Pyruvate: alanine, acetyl coA -> TCA cycle -> amino acids, acetyl coA -> FAs
Describe the steps of the polyol pathway.
- Glucose => sorbitol. Reduction by aldose reductase, requires 1 NADPH
- Sorbitol => fructose. Oxidation by sorbitol dehydrogenase, requires 1 NAD+
- Same steps with galactose (intermediate = galactitol)
What is glycation?
- Nonenzymatic addition of reducing sugars to proteins
- Sugars can spontaneously covalently bond with certain amino acids
- Leads to production of advanced glycation end products (AGEs)
- Very slow process
- AGEs involved in cross-linking with ECM, oxidative stress, neovascularization, inflammation
When and where does the PPP occur?
- Cytoplasm
- Well fed, low energy consumption, growth, need for reducing agents
What are the main products of the PPP?
- 2 NADPH => FA synthesis, glutathione (antioxidant) reduction, drug detox
- R5P => nucleotide biosynthesis
Describe the oxidative phase of the PPP.
- G6P => 6-phosphogluconolactone: Oxidation by G6PD, generates 1 NADPH, requires Ca or Mg ions
- 6-phosphogluconolactone => 6-phosphogluconate: hydration by gluconolactone hydrolase, requires water and Mg, Mn, or Ca ions
- 6-phosphogluconate => Ribulose-5-phosphate: oxidative decarboxylation by 6-phosphogluconate dehydrogenase, generates 1 NADPH and CO2, requires Mg, Mn, or Ca ions
Net: G6P + 2 NADP+ + water -> R5P + 2NADPH + 2H+ + CO2
What diseases are associated with the PPP?
G6PD is the most common inherited enzyme deficiency
When and where does gluconeogenesis occur? What are the energy requirements?
- Mitochondria and cytoplasm of liver, kidney, intestine
- Fasting, starvation, low-carb diet, intense exercise
- 4 ATP + 2 GTP per glucose
At what points does gluconeogenesis differ from glycolysis in reverse?
- G6P => glucose: HK replaced by glucose-6-phosphatase
- F-1,6-BP => F6P: PFK-1 replaced by fructose bisphosphatase
- Pyruvate => PEP: pyruvate carboxylated to OAA by pyruvate carboxylase then OAA decarboxylated and phosphorylated to PEP by PEP carboxykinase requiring 1 GTP
What are the glucogenic amino acids and what gluconeogenic intermediates do they form?
- pyruvate: thr, gly, trp, ala, ser, cys
- OAA: asp, asn
- fumarate: asp, tyr, phe
- propionyl CoA (succinyl CoA precursor): val, thr, ile, met
- a-ketoglutarate: arg, his, gln, pro, glu
How is pyruvate converted to PEP?
- Pyruvate => OAA: carboxylation by pyruvate carboxylase in the mitochondrion, requiring 1 Acetyl CoA + 1 ATP + biotin cofactor
- OAA converted to malate (via malate DH, requiring 1 NADH) or aspartate (via transamination) and transported to cytosol via malate/aspartate shuttles.
- Conversion back to OAA via reverse mechanism (if malate, will generate 1 NADH)
- Phosphorylation and decarboxylation by PEPCK, requiring 1 GTP
What additional enzymes are regulated during gluconeogenesis?
- Pyruvate kinase is inhibited under gluconeogenesis conditions (glucagon -> phosphorylation of PK -> inactivation)
- PDH inhibited by NADH coming from B-oxidation
- Pyruvate carboxylase activated by acetyl CoA from B-oxidation
- PEPCK is induced transcriptionally
- Fructose-1,6,-bisphosphatase is activated and induced
- glucose-6-phosphatase induced
How are opposing enzymes in gluconeogenesis/glycolysis regulated?
- Glucokinase vs glucose-6-phosphatase: GK has a high Km
- PFK-1 vs Fructose-1,6-bisphosphatase: PFK-1 is inhibited by ATP and alloseterically activated by AMP and F-2,6-P. Fructose-1,6-bisphosphatase is allosterically inhibited by AMP (and in the liver, by F-2,6-BP) and allosterically activated by ATP.
- Pyruvate kinase vs pyruvate carboxylase/PEPCK: PK deactivated by phosphorylation from cAMP signaling
Describe the structure of glycogen.
- Branching glucose polymer with linear a-1,4 and branching a-1,6 linkages. More branching allows for more storage and degradation in parallel.
- Non-reducing (-OH) ends exposed and attached to glycogenin and reducing ends
Describe the process of glycogen synthesis.
Same process at all non-reducing ends:
- G6P => G1P: phosphoglucomutase rearranges
- G1P => UDP-Glu: addition of UDP
- Glycogen synthase transfers glucose from UDP to glycogen chain with a-1,4 link
- every 6 additions, 4:6 transferase (branching enzyme) moves a chain of 6 glucoses to the main chain and links via a-1,6 linkage
Describe the process of glycogen degradation.
- Glycogen phosphorylase uses Pi to remove G1P’s
- When branch length = 4, 4:4 transferase moves 3 from the branch to the main chain.
- a-1,6 linked glucose is removed by a-1,6-glucosidase
