Carbohydrate Metabolism Flashcards
Glucose Transporters
- GLUT 1
- GLUT 2 (found in specific cells and regulated)
- GLUT 3
- GLUT 4 (found in specific cells and regulated)
GLUT 2
Low Affinity Glucose transporter in Hepatocytes and Pancreatic Cells
- Captures excess glucose primarily for storage
- In Beta - Pancreatic cells GLUT 1 and Glucokinase serves as the glucose sensor for insulin release
- Follow 1st order kinetics
GLUT 4
Glucose transporter in Adipose tissue and muscle
- Responds to [glucose] in peripheral blood
- Rate of glucose transport in these tissues is increased by insulin
- Permit a constant rate of glucose influx (transporters are saturated when blood glucose levels are just a bit higher than normal)
- Follow 0 order kinetics
- Can increase glucose intake by increasing # of GLUT 4 Receptors on cell surface
DiHydroxyAcetone Phosphate (DHAP)
Used in hepatic / adipose tissue for triglycerol synthesis formed from fructose 1, 6 bisphosphate which can be isomerized to glycerol - 3 - phosphate which can be converted to glycerol the backbones of triacylglycerols
1, 3 BisPhosphoGlycerate (1, 3 BPG)
High energy intermediate used to generate ATP by substrate level phosphorylation
PhosphoEnolPyruvate (PEP)
High energy intermediate used to generate ATP by substrate level phosphorylation
Irreversible Enzymes in Glycolysis
- Glucokinase or Hexokinase
- PFK1
- Pyruvate Kinase
Phosphofructokinase-2 (PFK2)
Converts Fructose-6-P to Fructose-2,6-Bis-P which activates PFK1 allowing cells to override inhibition caused by ATP so glycolysis can continue
Phosphofructokinase
Converts Fructose-6-P to Fructose-1,6-Bis-P as a Rate Limiting Step in glycolysis; The control point
- Inhibited by ATP, Citrate and Glucagon
- Activated by Insulin and AMP
Hexokinase
Converts Glucose to Glucose-6-P to “trap” it in the cell so it cannot leak out
- Inhibited by glucose-6-P
Alsolase
Converts Fructose-1,6-Bis-P to Dihydrozyacetone-P (DHAP)
Glycerol-3-Phosphate Dehydrogenase
Converts Dihydroxyacetone-P (DHAP) to Glycerol-3-P which is involved in the electron shuttle
Glyceraldehyde-3-Phosphate
Converts Glycerladehyde-3-P to 1,3-Bisphosphoglycerate, a high energy intermediate
- Catalyzes an oxidation reaction and addition of an inorganic phosphate
- Reduces NAD+ to NADH
2,3-Bisphosphoglycerate Mutase
Converts 1,3-Bisphosphoglycerate to 2,3-Bisphosphoglycerate
2,3-Bisphosphoglycerate
Allosterically binds to the beta chains of hemoglobin A and decreases its affinity of O2 allowing unloading in tissues but 100% saturation in the lungs
Phosphoglycerate Kinase
Converts 1,3-Bisphosphoglycerate to 3-phosphoglycerate
- Converts ADP to ATP
Mutase
Converts 3-Phosphoglycerate to 2-Phosphoglycerate
Enolase
Converts 2-Phosphoglycerate to Phosphoenolpyruvate (PEP)
Pyruvate Kinase
Converts Phosphoenolpyruvate (PEP) to Pyruvate
- Catalyzes phosphorylation of ADP to ATP using PEP
- Activated by Fructose-1,6-Bisphosphate in a feed forward reaction where a product from earlier in the rxn stimulates a product later in the rxn
Pyruvate When O2 is Absent
Fermentation: Pyruvate is converted to Lactate with the help of Lactate Dehydrogenase (a rate limiting step)
- NADH is oxidized to NAD+ replenishing the oxidized coenzyme Glyceraldehyde-3-phosphate dehydrogenase
Pyruvate When O2 is Present
Pyruvate travels to the mitochondria where it is converted into Acetyl Co-A by Pyruvate Dehydrogenase
- Acetyl CoA can contribute to Fatty acid synthesis
- Acetyl CoA —> TCA —-> CO2 + ATP
Glycolysis Steps
- Glucose
- Glucose-6-phosphate
- Isomerase - Fructose-6-phosphate
- PFK-1 - Fructose-1,6-Bisphosphate
- Aldolase - Dihydroxyacetone-Phosphate (DHAP)
- Isomerase - Glyceraldehyde-3-Phosphate
- Glyceradehyde-3-Phosphate Dehydrogenase - 1,3-Bisphosphoglycerate
- phosphoglycerate kinase - 3-Phosphoglycerate
- Mutase - 2-phosphoglycerate
- Enolase - Phosphoenolpyruvate (PEP)
- Pyruvate Kinase - Pyruvate
Galactose Metabolism Steps
- Lactose
- Lactase - Galactose + Glucose
- Galactose
- Galactokinase - Galactose-1-Phosphate
- Galactose-1-phosphate uridyltransferase - Glucsose-1-Phosphate
- Glucose-6-Phosphate + Glycogen
- Glucose + Glycolysis
Galactose Metabolism
- Galactose reaches the liver through the hepatic portal vein
- Once transported into tissues, galactose is phosphorylated by galactokinase trapping it in the cell
Fructose Metabolism Steps
- Sucrose (fruit, honey, etc…)
- Sucrase - Glucose + Fructose
- Fructose
- Fructokinase - Fructose-6-Phosphate
- Aldolase-B - DHAP + Glyceraldehyde
- Glyceraldehyde-3-Phosphate
- Glycolysis; Glycogenesis; Gluconeogenesis
Fructose Metabolism
- Fructose is found in honey / fruit (as sucrose) which is hydrolyzed by brushborder enzyme sucrase resulting in monosaccharides which are absorbed in the hepatic vein portal
- Liver phosphorylates fructose using fructokinase to trap it in the cell
Pyruvate Dehydrogenase
Pyruvate from Aerobic Glycolysis enters the mitochondria where it may be converted into Acetyl-CoA for entry into the Citric Acid Cycle if ATP is needed; Or it can be used in Fatty Acid Synthesis if there is sufficient ATP
- Irreversible process
Glycogen
A branching polymer of glucose (storage form); Glycogen synthesis and degradation occur primarily in liver and skeletal muscle
- Stored in the cytoplasm as granules with a central core protein and poly-glucose chains radiating outward to form a sphere
- Liver glycogen = broken down to maintain a constant level of glucose in the bloode
- Muscle Glycogen = broken down to provide glucose (an energy reserve) during muscle contraction
Glycogenolysis
Process of breaking down glycogen using glycogen phosphorylase to break bonds using an inorganic phosphate to produce glucose-6-phosphate and glycogen
Glycogenolysis Steps
- Glycogen
- Glycogen Phosphorylase + Debranching enzyme - Glucose - 1- phosphate
- UDP-Glucose
- Glycogen Synthase + Branching enzyme - Glycogen
- Glucose-1-Phosphate
- Glucose - 6 Phosphate
- Glucose-6-Phosphatase (liver) - Glucose
- Glucose-1-Phosphate
- Glucose-6-Phosphate
- Glycolysis (muscle) - Pyruvate
- Lactate OR CO2 + H2O
Branching Enzyme
- Glycogen Synthease makes a linear alpha-1,4-linked polyglucose chain
- Branching enzyme hydrolyzes an alpha-1,4-bond
- Branching enzyme transfers the oligoglucose unit and attaches it with an alpha-1,6-bond to create a branch
- Glycogen synthase extends both branches
- Introduces the 1,6 branch as the molecule grows to create branches
Debranching Enzyme
- Glycogen Phosphorylase releases glucose-1-phosphate from the periphery of the granule until it encounters the first branch point
- Debranching enzyme hydrolyzes the alpha-1,4-bond nearest the branch point
- Debranching enzyme (alpha-1,4-transferase) transfers the oligoglucose unit to the end of another chain
- Alpha-1,6-glucosidase hydrolyzes the alpha-1,6-bond releasing the single glucose from the former branch
- 2 Enzyme complex
Functions of NADPH
- NAD+ is a high energy electron acceptor - oxidizing agent (is reduced itself)
- NADPH is an electron donor - reducing agent (is oxidized itself)
- Contributes to biosynthesis of fatty acids and cholesterol
- Maintains gluthianone to protect against reactive O2 species
- Protect cells from free radical oxidative damage caused by peroxides (H2O2)
Glucogenic Amino Acids
All Amino Acids (except Leucine and Lysine) which can be converted into intermediates the feed gluconeogenesis
Ketogenic AMino Acids
Amino acids that can be converted into ketone bodies
Gluthianone
Reducing agent (is oxidized) to help reverse radical formation before damage is done to the cell
Pentose Phosphate Pathway (PPP)
Hexose Monophosphate Shunt (HMP)
- Occurs in the cytoplasm of cells; Functions to produce NADPH and Ribose-5-phosphate (for nucleotide synthesis)
Part 1: PPP Begins with glucose-6-phosphate and ends with ribulose -5-phosphate (Irreversible)
Part 2: Begins with ribulose-5-phosphate and has a series of reversible reactions that produce an equillibrated pool of sugars for biosynthesis including Ribose-5-phosphate for nucleotide sythesis
Glucose-6-dehydrogenase
converts Flucose-6-phosphate to 6-phosphogluconate in the PPP as a rate limiting step
- Induced by insulin
Pyruvate Dehydrogenase (PDH)
Enzyme in the Pyruvate Dehydrogenase Complex:
Pyruvate is oxidized yielding CO2 while the remaining 2C molecule binds covalently to Thiamin Pyrophosphate (TPP), a coenzyme held by noncovalent bonds to PDH; Mg2+ is also needed
Dihydropropyl Transacetylase
Enzyme in the Pyruvate Dehydrogenase Complex:
2C molecule is oxidized; transferred from TPP to Lipoic acid whose disulfide group acts as an oxidizing agent creating the acetyl group; This group bonds to the lipoic acid via thioester linkage so dihydropropyl transacetylase can catalyze the CoA-SH interaction with thioester link transferring acetyl group to form acetyl-CoA
Dihydropropyl Dehydrogenase
Enzyme in the Pyruvate Dehydrogenase Complex:
FAD (coenzyme) reoxidizes Lipoic Acid to be reused; FAD is reduced to FADH2 which in subsequent reactions is reoxidized to FAD while NAD+ is reduced to NADH
Beta Oxidation
Occurs in the inner membrane space of the mitochondria
- Fatty Acid
- Fatty Acid-CoA (Activation causes a thioester bond to form between the carboxyl group of the Fatty Acid and CoA
- Fatty Acid Carnitine (transported to the inner membrane of the mitochondria via Carnitine transfering the fatty acyl group to a mitochondrial Co-A-SH via transesterification Rxn)
- Fatty Acid-CoA -
- Acetyl-CoA (Once in the matrix; Beta oxidation removes the 2C fragment from the craboxyl end resulting in Acetyl-CoA)
Other Ways To Form Acetyl-CoA
- Amino Acid Catabolism: amino acids must lose their amino group via transamination and their carbon skeletons can form ketones bodies. When the pyruvate dehydrogenase complex reverses ketones can then produces Acetyl-CoA
- Alcohol: can be converted into Acetyl-CoA by alcohol dehydrogenase and acetalaldehyde dehydrogenase accompanied by NADH build up and generally used to synthesize fatty acids (then goes into Beta oxidation)
Citric Acid Cycle
Functions to oxidize Acetyl-CoA to CO2 + H2O and to produce the high energy electron carrying molecules NADH ad FADH2
- Pyruvate
- Pyruvate Dehydrogenase - Acetyl-CoA
- Citrate Synthase - Citrate
- Cis-Aconitase - Isocitrate
- Isocitrate Dehydrogenase - Alpha-Ketoglutarate
- Alpha-Ketoglutarate Dehydrogenase - Succinyl-CoA
- Succinyl-CoA Synthetase - Succinate
- Succinate Dehydrogenase (Complex II) - Fumarate
- Fumarase - Malate
- Malate Dehydrogenase - Oxolaoacetate
- Citrate Synthase - Citrate
Synthetases
Create new covalent bonds with input of energy
Synthases
Enzymes that form new covalent bonds without needing significant energy
Citrate Formation in Krebs Cycle
Acetyl-CoA and oxaloacetate undergo a condensation reaction to form Citryl-CoA an intermediate which is hydrolyzed to yield citrate and Co-A-SH
Catalyzed by Citrate Synthase
Citrate Isomerized to Isocitrate in Krebs Cycle
Achiral Citrate is isomerized to one of 4 possible isomers of isocitrate; First citrate binds at 3 points to the enzyme aconitase, then H2O islost to form Cis-Aconitate. Finally H2O is added back to form isocitrate
Alpha-Ketoglutarate and CO2 Formation in Krebs Cycle
Isocitrate is oxidized too oxalosuccinated by isocitrate dehydrogenase (rate limiting enzyme); Then oxalosuccinate is decarboxylated to produce an alpha-ketoglutarate and CO2. One carbon from Acetyl-CoA is lost and NADH is produced
Succinyl Co-A and CO2 Formation in Krebs Cycle
Alpha-ketoglutarase dehydrogenase coplex begins formation of Co-A, Succinyl-CoA and Alpha-Ketoglutarate coming together. A molecule of CO2 is produced as the Carbon is lost from Acetyl-CoA reducing NAD+ to NADH
Succinate Formation in Krebs Cycle
Hydrolysis of the thioester bond on succinyl-CoA yields succinate and CoA-SH which is coupled with the phosphorylate of GDP to GTP
- Enzyme used is succinul CoA Synthetase; ATP is produced directly
Fumarate Formation in Krebs Cycle
In Inner Mitochondrial Membrane instead of the matrix*
Succinate undergoes oxidation to yield fumarate catalyzed by succinate dehydrogenase, a flavoprotien because it is covalently bonded to FAD.
As Succinate is oxidized, FAD is reduced to FADH2 which passes the electrons it carries to the electron transport chains
Malate Formation in Krebs Cycle
Enzyme fumarase catalyzes the hydrolysis of the alkene bond in fumarate producing malate
**Only L-malate will form
Oxaloacetate is Formed Anew in Krebs Cycle
Enzyme malate dehydrogenase catalyzes the oxiation of malate to oxalaoacetate, ready for another cycle and NAD+ is reduced to NADH
Citric Acid Cycle Regulation
- Inhibited by ATP and NADH
- Promoted by ADP and NAD+
Pyruvate Dehydrogenase Complex Regulation
- Inhibited by phosphorylation of Pyruvate Dehydrogenase facilliatated by enzyme Pyruvate Dehydrogenase Kinase to inhibit Acetyl-CoA production
- Activated by enzyme Pyruvate Dehydrogenase Phosphatase to high levels of ADP; Removing phosphate from Pyruvate Dehydnrogenase reactivates Acetyl-CoA production
Net Result of Glycolysis
2ATP + 2NADH (~5ATP) = 7ATP
Net Result of Pyruvate Dehydrogenase
Pyruvate + CoA-SH + NAD+ ——-> Acetyl-CoA + CO2+ H+ + NADH
NADH ~ 2.5ATP = 2.5ATP
Net Result of Citric Acid Cycle
Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
Results in:
2CO2+ CoA-SH + 3H+ + 3NADH + FADH2 + GTP
3NADH (~ 7.5 ATP) + FADH (~ 1.5ATP) + GTP (~1ATP) = 10 ATP
Total ATP Production Per Glucose Molecule
30-32 ATP
Electron Transport Chain (ETC)
- Proton gradient that ultimately produces ATP
- Series of Redox Reactions
- Aerobic components executed in mitochondrial cristae
- Inner mitochondrial membrane generates ATP using proton motive force; electrochemical gradient generated by complexes of the ETC
Complex I of ETC
NADH-CoQ Oxidoreductase
- Transfers electrons from NADH to Coenzyme Q (CoQ) is catalyzed in first complex
- Flavoprotein has a coenzyme called flavin mononucleotide (FMN) covalently bound to H
- NADH transfers its electron to FMN becoming oxidized to NAD+ and FMN Reduced to FMNH2
- Flavoproteins becomes reoxideized while the Fe-S sub-unit is reduced donating the electron it receives to CoQ (ubiquinone)
- CoQ becomes CoQH2
- 4 Protons are moved to the intermembrane space
Complex II of ETC
Succinate-CoQ Oxidoreductase
- Complex II receives electrons from succinate (Citric acid cycle intermediate)
- Succinate is oxidized converting the FAD bound to complex II to FADH2 which is reoxidized to FAD as it reduces the Fe-S protein
- Final step reoxidizes Fe-S protein to reduce COQ to COQH2
Result: Succinate + CoQ + 2H+ —-> Fumarate + CoQH2
Complex III of ETC
CoQH2-Cytochrome C Oxidoreductase
Facilitates the transfer of electrons from CoQ to Cytochrome C involving the oxidation and reduction of cytochromes proteins with heme groups in which iron is reduced to Fe2+ and reoxidized to Fe3+’
Result: CoQH2 + Cytochrome C [with Fe3+] —-> CoQ + 2 Cytochrome C [with Fe2+] + 2H+
Q Cycle in ETC
Two electrons are shuttled from a molecule of ubiquinol (CoQH2) near the intermembrane space to a molecule of ubiquinone (CoQ) near the mitochondrial matrix
- Increases the gradient of the proton motive force across the inner mitochondrial membrane
- 4 Protons are moved to the inner membrane space
Complex IV of ETC
Cytorchrome C Oxidase
Facilitates the culminating step transfer of electrons from cytochrome A and A3 to make up the cytochrome oxidase which gets oxidized as ocygen becomes reduced to form H2O
- Proton pumping moves 2 more proteins across the membrane
Result: 2 Cytochrome C [with Fe2+] + 2H+ + 1/2 O2 —> 2 Cytochrome C [with Fe3+] + H2O
Proton Motive Force
As [H+} increase in the intermembrane space:
- pH drops in the intermembrane space
- voltage difference between the intermembrane space and the matrix increases due to proton pumping
Contributes to the electrochemical gradient having both electrostatic and chemical properties (ATP Synthase will harness this to make ATP)
NADH Shuttles
NADH cannot directly cross into the mitochondrial matrix a shuttle mechanism transfers the high energy electrons of NADH to a carrier that can cross the inner mitochondrial matrix
Glycerol-3-Phosphate Shuttle
- Glycerol-3-phosphate dehydrogenase oxidizes cystolic NADH to NAD+ while forming glycerol-3-phosphate from dihydroxyacetone phosphate (DHAP)
- On the outer surface of the inner mitochondrial matrix, another glycerol-3-phosphate dehydrogenase uses FAD as the oxidizing agent, being reduced to FADH2; once reduce, it can transfer its electrons to the ETC via Complex II generating 1.5ATP for every cystolic NADH
Malate-Asparate Shuttle
- Cystolic oxaloacetate is reduced to malate by using cystolic malate dehydrogenase; accompanying this reduction is the oxidation of NADH to NAD+
- Once malate crosses into the matrix, mitochondrial malate dehydrogenase reverse the reaction to form NADH which can now pass its electrons to the ETC via Compex I generating 2.5ATP
- Recycling malate requires oxidation to oxaloacetate which can be transaminated to form aspartate which can cross freely back into the cytosol and being the cycle anew.
Oxidative Phosphorylation
ATP Synthesis where ATP Synthase Spans the entire mitochondrial inner membrane and protrudes into the matrix.
Fo Section of ATP Synthase in Oxidative Phosphorylation
The proton motive force interacts with Fo which functions as an ion channel so H+ can travel along their gradient into the matrix
Chemiosmotic Coupling in Oxidative Phosphorylation
Allows the chemical energy of the proton gradient to be harnessed as a means of phosphorylating ADP to ATP
F1 Section of ATP Synthase in Oxidative Phosphorylation
Utilizes the energy releases from the electrochemical gradient to phosphorylate ADP to ATP
Conformation Coupling in Oxidative Phosphorylation
Relationship between the proton gradient and ATP synthesis is indirect; ATP is released by the synthase as a result of conformational change in the gradient.
- The F1 portion is like a turbine spinning within a stationary compartment to facilitate the harnessing of gradient energy for chemical bonding
Regulation of Oxidative Phosphorylation
- Is O2 is limited, oxidative phosphorylation will decrease and [NADH], [FADH2] will both increase
- Respiratory control = coordination of regulation of the above pathways
- With adequate O2, oxidative phosphorylation id dependent on the availability of ADP
