Lecture 1: Introduction to Metabolism Flashcards
Define metabolism.
Metabolism occurs through a series of enzyme catalysed reactions, each involving a small, specific chemical change. Although the entire pathway must be thermodynamically favourable, it is not necessary that every reaction be favourable.
Give 3 methods of regulating metabolism.
1) Change the amount of enzyme
2) Change the activity of the enzyme
3) Change the amount/availability of the substrate
Which is the slowest method of metabolic regulation. Explain why this method is the slowest.
Changing the amount of enzyme. This requires a change in the rate of transcription of the enzyme or a change in the rate of enzyme degradation (turnover). This method can take an hour or two to produce a change in the rate of a metabolic pathway.
Give four examples of ways of changing the activity of the enzyme to regulate metabolism and identify the fastest method.
1) Associate the enzyme with a regulatory protein
2) Allosteric activation/inhibition (fastest)
3) Covalent modification
4) Sequestration (keeping enzyme away from the substrate)
Describe the process of allosteric activation.
The allosteric enzyme undergoes a conformational (shape) change, as a result of binding of a regulatory molecule (binds to a site distinct to the active site - not competitive). The conformational change changes the shape/structure of the active site, so the enzyme’s affinity for the substrate increases. The enzyme is said to be activated.
During allosteric inhibition, the changed active site has a lower affinity for the substrate and the enzyme is deactivated.
Explain why phosphofructokinase is a key regulatory enzyme in glycolysis.
Phosphofructokinase is the enzyme responsible for catalysing the reaction in which fructose-6-phosphate is converted to fructose-1,6-bisphosphate.
Phosphofructokinase is allosterically inhibited by ATP and citrate and is allosterically activated by AMP.
What is catabolism?
Catabolism is the removal of energy and reducing power from energy-containing nutrients, producing energy-depleted ‘waste’ products.
What is anabolism?
Anabolism is the synthesis of cell macromolecules, e.g. proteins, from precursor molecules e.g. amino acids, using energy and reducing power.
Give 3 examples of energy-depleted ‘waste’ products produced by catabolism.
NH3, CO2, H2O
Can a catabolic reaction be reversed into an anabolic reaction using the same enzymes?
No. There are distinct enzymes for the catabolic and anabolic pathways comprised of the same reactions.
Are catabolic and anabolic reactions usually localised to the same or different compartments in the cell?
Different compartments.
Give an example of a biologically important dehydration/condensation reaction.
The conversion of two amino acids into a dipeptide.
Does reduction involve the loss or gain of electrons?
Gain. Remember OIL RIG: Oxidation is Losing electrons, Reduction is Gaining electrons.
What is the product if an alcohol is a) oxidised, b) reduced?
a) aldehyde (primary alcohol) or a ketone (secondary ketone)
b) alkene
What is the product is a carboxylic acid is oxidised?
Carbon dioxide
Give an example of an isomerisation reaction in glycolysis.
1) Glucose-6-phosphate is isomerised to Fructose-6-phosphate.
2) 3-phosphoglycerate is isomerised to 2-phosphoglycerate
Give 5 examples of common types of reaction in metabolic pathways.
1) Hydrolysis/dehydration
2) Oxidation/Reduction
3) Isomerisation
4) C-C cleavage
5) Group transfer
Is glycolysis common to all organisms?
Yes
When is glycolysis most important?
In anaerobic conditions, when the TCA cycle and the ETC stop, glycolysis is the only source of ATP.
Why is glycolysis particularly important in red blood cells?
Red blood cells have no mitochondria, so cannot perform the TCA cycle or the ETC and so rely on glycolysis for ATP production.
Is the TCA cycle catabolic or anabolic?
Catabolic. It is the breakdown of complex molecules into simpler ones, with the removal of energy and reducing power.
Where do carbohydrates enter the TCA cycle?
Via pyruvate, via glucose and glycolysis. Starch, sucrose and glycogen are all converted to glucose and undergo glycolysis to enter the TCA via the pyruvate route?
How do lipids enter the TCA cycle?
As Acetyl-CoA. Fatty acids undergo beta oxidation to become Acetyl-CoA.
How do amino acids enter the TCA cycle?
As pyruvate, alpha-ketoglutarate, fumarate and oxaloacetate.
From which TCA cycle intermediate are fatty acids and sterols synthesised through anabolism?
Citrate.
Which type of molecules can be synthesised from TCA cycle intermediate Succinyl-CoA?
Porphyrins (contain haem group)
From which TCA cycle intermediates can amino acids be synthesised through anabolism?
Alpha-ketoglutarate and oxaloacetate.
Fill the gap. Regulation only occurs at ______ reactions.
Irreversible
Give the enzymes involved in regulating glycolysis and the regulatory molecules which they are activated or inhibited by.
Hexokinase (inhibited by glucose-6-phosphate, the product of its reaction)
Phosphofructosephosphate (allosterically inhibited by ATP and citrate and activated by AMP)
Pyruvate kinase (allosterically inhibited by ATP and Acetyl CoA and activated by Fructose-1,6-bisphosphate (a substrate earlier in glycolysis) in a feed forward stimulation)
