2.1) Metabolic Pathways

Question 1

• What is ‘metabolsim’?

Answer 1

• The combination of all enzyme catalysed reactions within a cell.

Question 2

• What are ‘catabolic’ pathways?

Answer 2

1. Breakdown of molecules into sub-units 💥
2. Releases energy ⚡
3. Provides building blocks 🧱

Question 3

• What are ‘anabolic’ pathways?

Answer 3

1. Synthesis of molecules from sub-units ✨
2. Requires energy ✅
3. (e.g. synthesis of proteins from amino acids)

Question 4

• What is a summary of the control of metabolic pathways?

Answer 4

• Metabolic pathways are controlled by the presence (or absence) of particular enzymes in the pathway,
  ➞ and through the regulation of the rate of reaction of key enzymes within the pathway.

Question 5

• What are the 3 properties of a catalyst?

Answer 5

1. Lowers the activation energy ⚡📉
2. Speeds up the rate of a chemical reaction 🧪
3. Remains unchanged at the end of the reaction 👍

Question 6

• What is ‘activation energy’ and how can it be lowered?

Answer 6

• ‘Activation energy’ is the input of energy to start the reaction off.
• ‘Activation energy’ can be lowered with the use of enzymes (biological catalysts)

Question 7

• What is ‘enzyme action’?

Answer 7

• Enzymes are made of proteins folded into a specifc shape
  ➞ exposing the active site (region where substrate fits)
• Enzymes are specific to one type of substrate.
  ➞ The molecules of a substrate are complementary to the active site, showing a high affinity (chemcial attraction) ⭐
• Products have a low affinity for the active site
  ➞ so are released after the reaction.

Question 8

• Describe ‘induced fit’

Answer 8

• Induced fit
  ➞ is when the shape of the enzyme changes slightly to make the active site fit very closely round the substrate molecule,
  ↳ as the active site is dynamic and flexible.

Question 9

• Explain the effect of substrate concentration

Answer 9

1. At low concentrations of substrate 🫘
   ➞ the reaction rate is low 📉
   ↳ due to there being too few substrate molecules present to make maximum use of all the active sites.
2. An increase in substrate concentration 🫘
   ➞ means higher reaction rates 📈
   ↳ since more active sites are being used.
3. A further increase in substrate 🫘 concentration
   ➞ fails to increase the reaction rate ❌
   ↳ because all the active sites are occupied (enzyme concentration + limiting factor)
4. The graph levels off 〰️
   ➞ because there are more substrate molecules than active sites.

Question 10

• Explain ‘reversibility’

Answer 10

• Most metabolic pathways are reversible. 🔄
  ➞ Often an enzyme can catalyse a reaction in both directions.
  ↳ The actual direction depends on the relative concentration of the reactants and products ✨
• If the concentration of metabolite B **was to *increase to an unusally high level and A were to *decrease, 📈📉
  ➞ enzyme 2 could go into reverse and convert some of B back into A until a balanced state (equilibrium) was restored.

Question 11

• Explain ‘regulation of metabolic pathways’

Answer 11

• Some metabolic pathways
  ➞ (e.g. glycolysis in respiration)
  ↳ are needed to operatre continuosuly.
• The genes that code for their enzymes must always be switched on so that the enzymes are always present in the cell

Question 12

• REVIEW QUESTIONS:
  1. State the term which describes a metabolic patwhay in which simple molecules are built up into complex molecules.

2. i) Describe how the genetic code for glycogen synthase might be altered in an individual with the disease.
   ii.) Explain why this altered genetic code fails to produce glycogen synthase.

Answer 12

1. Anabolic pathway
2. i.) The genetic code will contain a different nucleotide/base
   ii.) The protein contains different amino acids.

Question 13

• What is an ‘inhibtor’ and can you name the three types?

Answer 13

• An ‘inhibitor’ is a substance which decreases the rate of an enzyme-controlled reaction.

1. COMPETITIVE
2. NON-COMPETITIVE
3. FEEDBACK INHIBITION (BY AN END PRODUCT

Question 14

• Describe and explain ‘competitive inhibitors’

Answer 14

• A competitive inhibitor
  ➞ is a molecule which has a similar shape as the enzyme’s substrate.
• It joins with the active site
  ➞ and prevents the substrate from joining to the enzyme.
• As the inhibitor and substrate are competing for space on the active site,
  ➞ the reaction rate is reduced.

Question 15

• Describe and explain ‘non-competitive inhibitors’

Answer 15

• A non-competive inhibitor
  ➞ binds to a location on the enzyme away from the active site (allosteric site)
• It alters the enzyme’s shape
  ➞ and therefore the active site.
• The substrate is prevented from binding to the enzyme’s active site
  ➞ and the reaction rate is severely reduced.
• Additionally, increasing the substrate concentration has no effect on the reaction with a non-competitive inhibitor.

Question 16

• Describe and explain ‘feedback inhibition by an end product’

Answer 16

• ‘Feedback inhibition by an end product
  ➞ occurs when an end product in a pathway reaches a critical concentration.
• An end product
  ➞ can bind to an enzyme that catalyses a reaction early on in the pathway.
• This blocks the pathway,
  ➞ preventing further synthesis of the end product.
• EXAMPLE:
  ➞ As the concentration of the end product (metabolite D) builds up.
  ↳ some of it binds to enzyme 1 in the pathway.
  ➜ This slows down the conversion of metabolite A to B and in turn regulates the whole pathway.
• As the concentration of D drops,
  ➞ fewer molecules of enzyme 1 are affected and more of A is converted into B and so on.
• The pathway is kept under control by this (negative feedback)
  ➞ and wasteful conversion and accumulation of intermediates and final products are avoided.