ENZYME REGULATION

METABOLIC PATHWAYS

RATE LIMITING STEP

  • The slowest step in a pathway

  • Highest activation energy

  • Limited by (S)

COMMITTED STEP

  • First irreversible enzymatic reaction in a pathway to a specific product.

1) Allosteric

2) multiple forms of enzymes

3) reversible covalent modification

4) Proteolytic activation

5) Expression control

6) Inhibition

Inhibitors - Type of inhibition

  1. competitive

    • methotrexate, dihydrofolate analouge for DHFR inhibitor in Cancer

  2. Uncompetitive

    • lithium inhibition of inositol monophosphatase as anticonvulsant

    • inhibitor binds to the enzyme when the substrate binds to that enzyme

  3. Non-competitive

    • it changes the shape of the enzyme to stop the substrate from entering the active site

    • phosphonoformic acid (PFA) Inhibition of DNA polymerases of Herpesviruses and Hepatitis B

  4. Irreversible OR suicide inhibitors

    • inhibitor covalently binds to the enzyme and the inhibitor becomes denatured

    • reacts with the active site

    • E.g. penicillin (binds to the cell wall), aspirin (modifies cox1 to stop activity)

  5. Product inhibition

    • product negative feedback

1) Competitive Inhibitor - the substrate needs to displace the competitive inhibitor to get to the enzyme-substrate complex that can form the product

  • Vmax remains the same it just needs more substrate to get to 100%

  • Km increases more (s) is needed

  • the slope is steeper (Km/Vmax)

2) Uncompetitive inhibitor - inhibitor only binds to enzyme when it is in substrate-bound form. It takes away the enzyme and makes it nonproductive

  • the Vmax decreases because you’re taking away a portion of the enzyme

  • Km is lower because of the increased Affinity

  • No change in slope

3) Non- Non-competitive inhibitor

  • combines with the enzyme with or without the substrate-bound. But when it is bound it is a non-productive form of that enzyme

  • Km Remains the same

  • slope is steeper

ALLOSTERIC REGULATION

the activity of one functional site within a multi-domain protein complex like hemoglobin ( 4 active sites) affects the activity of the others

  1. distinct regulatory sites

  2. multiple functional sites

T STATE = LOW OXYGEN PRESENT

R STATE = High oxygen present

S SHAPE = Sigmoidal curve

allosteric regulations rapidly change when you reach a threshold across a short span of substrate regulation.