Case 5- Carbohydrate Metabolism Flashcards
How Glucose enters the cell
By facilitated diffusion through transporters GLUT1-14. The number of transporters increase with insulin. GLUT1 is involves in glucose uptake in RBC. GLUT2 is in the liver and kidney. GLUT3 is in the neurons. GLUT4 is found in adipose tissue and skeletal muscle. A Na+ and glucose cotransporter (SGLT) is used in the intestine.
Where does Glycolysis occur
In the cytoplasm
The role of Glycolysis in different tissues
In the liver and adipose tissue the main function is to produce precursors for fat (triglyceride) synthesis. Skeletal muscles will produce ATP
What is NADH and FADH
Electron carriers
End products of aerobic Glycolysis
For each glucose molecules you get 2 Pyruvate molecules, 2 ATP molecules and 2 NADH molecules.
End products of anaerobic Glycolysis
For each glucose molecules you get 2 Pyruvate molecules, 2 ATP molecules and 2 NADH molecules.
Why does anaerobic glycolysis take place
When you have low O2, no mitochondria and low pH
Step 1 of Aerobic respiration
Energy investment (2 ATP molecules are used) this creates 2 ADP molecules and energy.
1) The phosphate from one ATP molecules converts Glucose into Glucose-6-phosphate.
2) This is then converted into Fructose-6-phosphate.
3) This is phosphorylated by another ATP molecule into Fructose 1,6-bisphosphate.
4) This is then converted into either two Glyceraldehyde-3-phosphate molecules or two Dihydroxyacetone (DHAP) molecules which can then be converted into 2 Glyceraldehyde-3-phosphate molecules. These end products are composed of 3 carbons whilst Glucose is composed of 6.
Ste 2 of aerobic respiration
Energy generation, 4 ATP molecules and 2NADH molecules are produced.
1) The 2 molecules of Glyceraldehyde-3-phosphate are converted into 2 molecules of 1,3-bis-phosphoglycerate by oxidising 2 molecules of NAD+ to form 2 molecules of NADH.
2) This is converted into 2 molecules of 3-phosphoglycerate by phosphorylating 2 ADP molecules to form 2 ATP molecules.
3) This is converted into 2 molecules of 2-phosphoglycerate
4) Which is converted into 2 molecules of 2-phosphoenolpyruvate.
5) This is finally converted into two Pyruvate molecules by phosphorylating two ADP molecules. All the molecules are 3 carbon intermediates, so one glucose molecule produces two Pyruvate molecules.
Substrate level phosphorylation
The transfer of phosphate from one molecule to another
Anaerobic Glycolysis
The Pyruvate is converted into lactate using the NADH produced earlier, ATP is still produced but not as much. Occurs in red blood cells and exercising cells. Lactate is converted to glucose in the liver but this requires ATP.
ATP
Used to provide energy for cellular processes
How is insulin released
It is secreted by beta cells of the islets of Langerhans in the Pancreas
How energy is generated from food
Metabolism coverts high energy products (fats, carbohydrates and proteins) into low energy products (CO2 and H2O). By oxidising these high energy products you generate a lot of electrons which can be used to generate ATP through substrate level phosphorylation. Energy stores can also be oxidised.
Oxidation
Removing electrons
Metabolism
The chemical processes by which cells produce the substances and energy needed to sustain life
Catabolism
Catabolic pathways are degradative pathways. They produce chemical energy from the break down of energy rich fuel molecules.
Anabolism
Anabolic pathways are bio-synthetic pathways, they combine small molecules to produce complex molecules using energy from ATP
The 3 stages of metabolism
Stage 1- Hydrolyses of complex molecules into building blocks. For example= carbohydrates to monosaccharides.
Stage 2= Conversion of building blocks into Acetyl CoA by oxidation.
Stage 3= Oxidation of acetyl CoA to produce ATP.
How is glycolysis controlled
The irreversible reactions are controlled by enzymes and the reversible reactions depend on the conditions of the cells. When energy is high glycolysis is inhibited, because in the liver their main function is to produce triglyceride Glycolysis will proceed when energy levels are high.
Hexokinase
Catalyse the first reaction of Glycolysis - (Glucose →Glucose-6-Phosphate). The enzyme used in all tissues but the liver. High affinity for glucose (Low Km Glucose), binds to Glucose even when there is a low concentration. Low Vmax Glucose, slow reaction. Inhibited by Glucose-6-phosphate (cell energy levels are high). Works more efficiently when glucose concentrations are low.
Glucokinase
Catalyse the first reaction of Glycolysis - (Glucose →Glucose-6-Phosphate), used in the liver. Low affinity for glucose (High Km Glucose). High Vmax Glucose. Stimulated by Glucose (Feed forward). Not inhibited by G6P, glycolysis proceeds even when cell energy levels are high. Works more efficiently when Glucose concentrations are high.
Phosphofructokinase 1 (PFK1)
Catalyses the third reaction of Glycolysis - (Fructose-6-P → Fructose-1,6-BP)
Most important regulatory enzyme of glycolysis it is the rate limiting step. Allosterically inhibited by ATP (cell energy levels high) and stimulated by AMP (cell energy levels low). Most potent allosteric activator of PFK1 is Fructose 2,6-bisphosphate, it activates PFK1 even when ATP levels are high. Stimulated by insulin
Pyruvate kinase
Catalyses the final reaction of Glycolysis - (Phosphoenolpyruvate → Pyruvate). It is used in the liver and is activated by Fructose-1,6-BP. The activity of PFK1 and Pyruvate kinase are linked.
Fructose
A component of sucrose, does not cause Insulin secretion and is mostly processed in the liver, gets converted to Pyruvate
Fructose metabolism
Fructose is converted to Fructose-1-phosphate, this is then converted into either two molecules of Glyceraldehyde or two molecules of Dihydroxyacetone (DHAP). Both will be converted into two molecules of Glyceraldehyde-3-phosphate. This is a 3-carbon molecule and can enter the second stage of Glycolysis. Alternatively, the 2 molecules of Glyceraldehyde can be converted into two molecules of Glycerol. This can be converted into Glucose or used to make fatty acids.
Fructokinase
The enzyme which catalyses Fructose to make Fructose-1-phosphate. If there is a deficiency of this enzyme you get essential fructosoria, you get accumulation of fructose in urine. It is a benign condition. Very rare autosomal recessive conditions
Aldolase B
The enzyme which converts Fructose-1-phosphate into either 2 molecules of Glyceraldehyde or two molecules of Dihydroxyacetone.
HFI (Hereditary Fructose Intolerance)
Deficiency in Aldolase B. Can cause severe illness and liver damage. It is an autosomal recessive condition, dietery management reduces the symptoms. During weaning the symptoms appear, these are nausea, vomiting, abdominal distress and failure to thrive. Blood tests show hypoglycaemia, liver function tests show liver damage. The weaning child is exposed to fructose for the first time.
Role of ATP
Provides energy for the metabolic processes in the body
How is ATP generated
Substrate level phosphorylation of food sources by oxidation
ATP-ADP cycle
ATP is converted to ADP for energy utilisation whist ADP is converted into ATP for energy production.
Oxidative decarboxylation
Catalysed by the enzyme Pyruvate dehydrogenase (PDH) complex. Pyruvate is converted into Acetyl CoA.
Pyruvate + CoA + NAD+ -> Acetyl CoA + CO2 + NADH
Regulation of PDH
PDH is inactivated by PDH kinase when ATP, Acetyl CoA and NADH levels are high. Cell energy levels are high so oxidative decarboxylation is not required. PDH is activated by PDH phosphatase, an enzyme which removes a phosphate group from PDH when intracelular Ca+ levels are high. Which indicates that cell energy levels are low.
Where does the krebs cycle occur
Mitochondrial matrix
Regulation of Krebs cycle
It speeds up if there is a high concentration of Ca+2 and ADP (low energy). It slows down if there are high concentrations of NADH and ATP (high energy)
Krebs cycle reactions
- Acetyl CoA will combine with Oxaloacetate to form COA and Citrate
- The Citrate converts to Isocitrate
- Isocitrate converts to alpha-Ketoglurate by oxidation, reducing NAD+ to NADH.
- Alpha-Ketoglurate converts into Succinyl CoA by oxidation, reducing NAD+ to NADH.
- Succinyl CoA converts into Succinate by phosphorylating GDP to GTP.
- Succinate converts into Fumarate by oxidation by reducing FAD to FADH2
- Fumarate converts into Malate
- Malate converts into Oxalocetate by oxidation, reducing NAD+ to NADH.
What is created in the krebs cycle for each Glucose molecule used
2 x Acetyl CoA, 6 x NADH, 2 x FADH2, 2 x GTP, 2 x ADP
How much ATP is generated in the krebs cycle per Glucose molecule
24 molecules of ATP and 2 GDP molecules
How much energy is generated per NADH molecule
3 ATP molecules
How much energy is generated per FADH2 molecule
2 ATP molecules
Oxidative phosphorylation
Electron donation from NADH and FADH2 to O2 via the electron transport chain which is a series of proteins that accept/donate electrons. ATP is generated
Complex 1- oxidative phosphorylation
NADH dehydrogenase - NADH transfers electrons to this protein complex
Complex 2-oxidative phosphorylation
Succinate dehydrogenase - FADH2 transfers electrosn to this protein complex
Complex 3-oxidative phosphorylation
Cytochgrame bc1
Complex 4-oxidative phosphorylation
Cytochrome C oxidase
Whats O2 role is oxidative phosphorylation
Final electron acceptor
Proton motive force
In oxidative phosphorylation the protons are pumped into the intermembrane space, they re-enter the matrix of the mitochondria via complex 5 (ATP synthase) which couples oxidation to phosphorylation of ADP to ATP as the electrons pass through.
Steps of oxidative phosphorylation
- NADH transfers 2 electrons to complex 1 resulting in 4 H+ ions being pumped across the inner membrane, NADH is oxidised to NAD+ which is recycled back to the Krebs cycle. Electrons are carried to complex 3
- FADH2 transfers electrons to complex 2 which are carried to complex 3. As FADH2 bypasses complex 1 fewer protons are transferred.
- The passage of electrons to complex 3 transports 4 more H+ ions across the inner membrane. Electrons are transported to complex 4
- In complex 4 two more H+ ions are pumped across the inner membrane. The electrons in the elctron transfer chain are passed to an oxygen molecules to form two molecules of water.
- There is now a high concentration of protons in the intermembrane space. The protons will move down their concentration gradient through complex 5 and provide the energy the energy to generate ATP from ADP
How the proton motive force and oxidative phosphorylation are linked
As the electrons move along the electron transport chain they provide the energy to transport protons into the intermembrane space. The proton generate an electrochemical gradient which is used by complex 5 to generate ATP form ADP.
How much ATP is produced from one Glucose molecules
38- net 36
Cyanide and carbon monoxide effect on ATP production
Inhibition of the electron transport chain stops the electron transfer from NADH/FADH2. Carbon monoxide (CO) and cyanide (CN-) inhibit the transfer of electrons from complex 4 to O2. ATP production will stop and cell death will occur. No ATP will be produced in the electron transfer chain as no proton motor force is generated.
The function of uncoupling proteins in thermogenesis
Uncoupling proteins create a proton leak on the inner mitochondiral membrane and uncouple the proton gradient. The protons will move through the UCP1 or thermogenin to generate heat. They are transmembrane proteins that cross the mitochondrial membrane. The protons bypass Complex 5. The uncoupling proteins are known as UCP1/thermogenin, you find these in Brown adipocytes which are around the thymus, you have more of them in neonates. This is a sympathetic response to the cold
Gluconeogenesis
A metabolic process in which we take non carbohydrate material and turn them into Glucose, helping to maintain blood Glucose in prolonged starvation
The three substrates of Gluconeogenesis
- Lactate- from anaerobic glycolysis, it is taken up by the Liver and converted to Pyruvate (Cori cycle)
- Glycerol- produced by hydrolysis of triglycerides in adipose tissue. It is taken up by the liver and converted to dihydroxyacetone phosphate (DHAP). DHAP is an intermediate of Glycolysis.
- Alpha-Ketoacids (carbon skeletons of amino acids)- glucogenic amino acids can be converted to oxaloacetate which can be converted to Phosphenol pyruvate- intermediate of Glycolysis.
Where does Gluconeogenesis happen
In the cytosol and mitochondria of the Liver, requires energy which comes from FA oxidation
Why is Gluconeogenesis not the reverse of Glycolysis?
Glycolysis has 7 reversible reactions and 3 irreversible reactions. Gluconeogenesis can therefore not be the reverse of Glycolysis as there are 3 irreversible reactions which can not be overcome. We have to bypass these steps
The three steps in Gluconeogenesis which are different to Glycolysis
- Pyruvate is converted into Oxalocetate by Pyruvate carboxylase and is then converted into 3-Phosphoenolpyruvate by PEP carboxykinase.
- Fructose-1.6-bisphosphate can be directly converted to Fructose-6-Phosphate by Fructose 1,6-bisphosphatase.
- Glucose 6-Phosphate can be directly converted into Glucose by Glucose 6-Phosphatase.
The enzymes which are bypassed in Gluconeogenesis
- Hexokinase /Glucokinase bypassed by Glucose 6-phosphatase
- Phosphofructokinase glycolysis bypassed by Fructose 1,6-bisphosphatase
- Pyruvate kinase bypassed by Pyruvate carboxylase (Mitochondria) and PEP carboxykinase (Cytosol)
Energy needed to produce one Glucose molecule
Produced from two molecules of oxaloacetate, you need 4 x ATP, 2 x GTP, 2 x NADH. In the fasting state ATP and NADH are provided by the beta oxidation of fatty acids
Regulating Gluconeogenesis
- Changes in enzyme levels- increased Glucagon promotes expression of PEP carboxykinase (increased gluconeogenesis).
- Changes in substrate availability- decreased insulin, increased amino acid catabolism and increases Gluconeogenesis (problem of diabetes)
- Acetyl CoA- fatty acids from TAG breakdown which saturates the liver. This results in increased Acetyl CoA, increased Pyruvate carboxylase activity and increased Gluconeogenesis.
