Medicines design Flashcards
1
Q
What is the sigmoidal behaviour of phosphofructokinase?
A
- PFK-1 is the primary control enzyme of glycolysis.
- It is composed of 4 subunits.
- ATP is both a substrate and an allosteric effector:
- High activity, Michaelis-Menten kinetics.
- Low activity, Sigmoidal kinetics.
2
Q
What are the 4 stages of the cori cycle?
A
- Recycles R-lactate to glucose.
- Lactate is transported from muscle to liver in blood.
- Lactate dehydrogenase in liver converts R-lactate to pyruvate.
- Pyruvate is converted to glucose by gluconeogenesis.
3
Q
How are glycolysis and gluconeogenesis regulated?
A
Both take place in the cytosol
Most enzymes are the same
Controlled by two processes
1. Energy levels in the cell (signalled by ATP)
2. Hormonal control (Signalled by fructose-2,6-bis-phosphate levels)
4
Q
What is the effect of glucagon on glycolysis and gluconeogenesis?
A
- Glucagon is secreted in response to low blood glucose levels
- Binding to a receptor initiates cAMP-mediated signalling.
- This activates protein kinase A leading to pyruvate kinase and so activity of the glycolysis pathway is reduced.
5
Q
What is the effect of insulin on glycolysis and gluconeogenesis?
A
- Insulin has the opposite effect to glucagon
- Secreted by pancreas in response to high blood glucose levels
- In the fed state it increases the concentration of GLUT4 enabling uptake of glucose into muscle and adipose cells.
- It counteracts the inhibitory effect of glucagon
6
Q
What is the effect of adrenaline on glucose metabolism?
A
- Adrenaline is a hormone produced in response to stress and exercise
- The stress response requires glucose to be mobilised to generate ATP
- Therefore, glycogen breakdown is stimulated, and glycogen synthesis is inhibited in a similar manner to glucagon (kinase-dependent phosphorylation of regulatory enzymes)
- Glycolysis is stimulated and gluconeogenesis is inhibited
- Increased activity of these pathways often occurs in ‘panic’ situations and therefore anaerobic respiration takes place.
7
Q
What is the TCA cycle?
A
- Pyruvate is converted to acetyl-CoA by pyruvate dehydrogenase complex
- Oxaloacetate and acetyl-coA are condensed
- Citrate is rearranged to isocitrate by cis-aconitase.
- Isocitrate is oxidised and decarboxylates.
- 2-Oxoglutarate is decarboxylated by the 2-OG dehydrogenase complex.
- Succinyl-CoA is converted to succinate.
- Desaturation and formation of FADH2.
- Hydration.
- Oxidation and formation of NADH.
8
Q
How is the TCA cycles regulated?
A
- The primary control enzyme is pyruvate dehydrogenase complex.
- High energy signals (ATP and NADH) and products of the TCA cycle are inhibitors.
- Succinyl-CoA inhibits the activity of the α-ketoglutarate dehydrogenase complex (negative feedback control).
- ADP (low energy signal) increases activity of isocitrate dehydrogenase.
9
Q
What is the anapleurotic reaction?
A
- Anaplerotic literally means ‘filling up’.
- TCA cycle is used to provide starting materials for biosynthesis.
- This results in depletion of oxaloacetate.
- Acetyl-CoA accumulates.
- High levels of acetyl-CoA decrease activity of pyruvate dehydrogenase complex.
- High levels of acetyl-CoA increase activity of pyruvate carboxylase
- Net effect is to rebalance oxaloacetate and acetyl-CoA levels.
- Production of oxaloacetate is also important for gluconeogenesis
10
Q
How is the TCA cycle regulated by the electron transport system?
A
- The terminal acceptor for the system is dioxygen.
- Under anaerobic conditions, the reduced electron acceptors in the transport system are not re-oxidized.
- This means that the TCA cycle products NADH and FADH2 are not reoxidized to NAD+ and FAD.
- This means that entry of pyruvate and flux through the TCA cycle is reduced.
- Pyruvate is converted to lactate instead
- Lactate is recycled via the Cori cycle
11
Q
How is glycogen metabolism regulated?
A
- Glucagon binds to its receptor, which produces cAMP.
- This activates protein kinase A which phosphorylates target enzymes:
- Phosphorylase kinase is activated.
- This activates glycogen phosphorylase.
- Glycogen synthase is inhibited.
- More glucose is produced.
- These effects are reversed by dephosphorylation by a phosphatase.
- Insulin inhibits phosphorylation of glycogen synthase and maintains activity.
12
Q
How is fatty acid β-oxidation and biosynthesis regulated?
A
- The key process is synthesis of malonyl-CoA.
- High levels of malonyl-CoA allow fatty acid biosynthesis.
- Malonyl-CoA inhibits import of fatty acids into mitochondria for degradation.
- Glucagon phosphorylates acetyl-CoA carboxylase and reduces malonyl-CoA levels.
- This inhibition can be overcome by high citrate levels.
- Insulin promotes removal of phosphate from acetyl-CoA carboxylase.
13
Q
What is the overall effect of glucagon?
A
- Glucagon increases blood glucose levels by cAMP-mediated activation of protein kinase A
14
Q
What is the overall effect of insulin?
A
- Insulin is secreted from the pancreas in response to high glucose blood levels. This causes:
- Increased GLUT transporters in muscle and adipose tissue allowing glucose uptake.
- It reduces phosphorylation of glycogen synthase and glycogen phosphorylase, promoting glycogen synthesis.
15
Q
What is the effect of glucocorticoids on glucose and lipid metabolism?
A
- High levels of glucocorticoids promotes biosynthesis of gluconeogenic enzymes and suppresses production of glycolytic enzymes.
- There also appears to be a direct activation of gluconeogenic enzymatic activity in some circumstances.
- There is also a complex effect on fatty acid metabolism. These effects interact with that of insulin.
16
Q
How are ketone bodies formed?
A
- In diabetes or starvation glucose is scarce.
- Brain and erythrocytes are particularly dependent on glucose for energy.
- Fatty acids are kept in adipose tissue and are not easy to transport around the blood so are converted to acetyl-CoA.
- Three molecules of acetyl-CoA are used to acetoacetate and 3-hydroxybutyrate (a.k.a. ketone bodies) by the mevalonic acid pathway
- These are distributed by the blood and can be used as fuels instead of glucose.
- Acetoacetate is easily converted to acetone, hence the solvent smell on the breath of diabetics experiencing hyperglycaemia
17
Q
What are glycation reactions?
A
- Glycation is the non-enzymatic reaction of glucose with nucleophilic groups.
- Derivatives can further react to produce advanced glycation products (AGPs).
- Glycation of haemoglobin is often used to measure glucose control in diabetic and other patients.
- Glycation of haemoglobin A gives haemoglobin A1c, status reflects glucose control over the last 3 months:
Normal patients HbA1c = < 6% (<48 mmol/mol).
Uncontrolled diabetes = > 10% (>86 mmol/mol).
Target for type 1 diabetes < 7.5% (<58 mmol/mol)
Target for type 2 diabetes < 6.5% (<48 mmol/mol).
18
Q
What are monosaccharides?
A
- Carbohydrates are hydroxylated derivatives of aldehydes and ketones.
- Complex carbohydrates are built from monosaccharides ‘building blocks’.
- Aldohexose possess aldehyde groups, ketohexoses possess ketone groups.
19
Q
What are hexose monosaccharides?
A
- Most monosaccharides have D- configuration (the OH on carbon-5 is on the right hand side)
- The remaining three stereochemical centres can have variable configurations – hence 8 possible isomers.
- Glucose is the most abundant hexose.
- Galactose and mannose are epimers of glucose
20
Q
What is the ring closure reaction of glucose?
A
- Addition of alcohol group to aldehyde forms a hemiacetal
- Presence of OH group on carbon-5 means a 6 membered ring is formed (favourable).
- A new chiral centre is formed on hemiacetal formation, which can have one of two configurations (α- or β-) (down or out the side, respectively).
- These new isomers are called anomers.
- Ring-opened and ring closed forms are in dynamic equilibrium
21
Q
What are reducing sugars?
A
- Glycation can only occur with reducing sugars
- Reducing sugars reduce Cu2+ and Ag+ ions in basic solution and are themselves oxidised
- Ketoses undergo tautomerisation to aldehydes in alkali solution and so are also reducing.
- Reducing sugars (primarily glucose) are responsible for protein and lipid glycation.
22
Q
What is the glycation of proteins?
A
the spontaneous, non-enzymatic reaction of proteins with simple reducing sugars and related metabolites.
23
Q
What is the amadori reaction?
A
- Excess glucose can react with amino groups of intracellular and extracellular proteins in a nonenzymatic manner to form glycated residues termed as Amadori products
24
Q
What are other types of glycation?
A
- Glycation can occur with other proteins, including serum proteins such as albumin, and α-crystallins
- The most common residue to be glycated is lysine.
- Lysine residues in a particular protein are not glycated to the same extent.
- Other protein residues (e.g., Arg, His, Trp and Cys and the N-terminal amine) can be glycated. Nucleophilic residues will also react with the initial Amadori product to produce Advanced Glycation Products (AGPs).
- Cross-linking of protein strands and reaction with other nucleophiles can occur.
- Glycation also occurs with lipids containing amine groups, such as phosphotidylethanolamine (a phospholipid found in cell membranes) and low density lipoprotein (a blood particle used to transport cholesterol around the body).
- Glycation with other reducing sugars e.g., lactose, fructose also possible.
25
Q
What are the consequences of glycation?
A
- Glycation is recognised as a major cause of diabetic secondary complications, including nephropathy, retinopathy and diabetic cataracts and atherosclerosis.
- Glycated proteins are difficult to degrade, so there is a build-up of damaged proteins in the cell.
- Damage results in production of auto-antibodies, resulting in increased levels of inflammation.
26
Q
How do you measure glycation?
A
- Several analytical techniques can be used to measure early glycation products including:
- Ion-exchange chromatography.
- Affinity chromatography or gel electrophoresis
- Immunoassays such as ELISA which detect specific antigens.
27
Q
Which 3 three drugs have we got from nature?
A
Exenatide (gila monsters)
Aspirin (trees)
Digoxin (plants)
28
Q
What are the bioactive peptides from Gila monsters?
A
Exendins are of interest in the treatment of diabetes (especially Exendin-4)
Named as they were extracted from EXocrine glands and ENDocrine actions - EXENDIN
The Structure of Exendin-4 is similar to a human peptide – Glucagon-Like Peptide -1 or GLP-1
