Cell Bio: Chapter 7- Regulating Enzymes

Lineweaver-Burk Plot

• Double reciprocal plot: 1/V vs 1/[S]

• slope = K_m / V_max

• Only need two points to graph straight line (x int, y int)

Enzyme acticity can be regulated through

Inhibition (mostly allosteric but can be steric/competitive)

Activation

Steric/competitive inhibition

Inhibitor competes with substrate for binding to the enzyme’s active site

• Binding of substrate to enzyme is reversible (connects through weak bonds)

• Bindings of inhibitor to enzyme is reversible (connects through weak bonds, such as ionic & hydrogen bonds but inhibitor has a greater affinity for active site and will bind tighter)

• Ex: aspirin inhibits inflammation through steric inhibition

Competitive inhibition on Lineweaker-Burk Plot

• Competitive inhibitor has an increase in slope compared to no inhibtor

• 1/Vmax has not changed so the substrate can outcompete inhibitor

• K_m increases because substrate has been blocked by inhibitor, so more substrate is needed to reach 50% velocity

Allosteric Inhibition

• Inhibitor binds to enzyme at another site and alters the enzyme’s conformation which decreases the ability of the active site to bind the substrate

  • When the active aite changes the amino acids in the enzymes will not be able to form the bond to the substrate

Allosteric Activator

• Binds to enzyme at site other than active site and changes enzyme’s conformation to bind the substrate better to the active site

Rate limiting step

Only one key enzyme needs to be regulated in a pathway which controls the production of the product

End-product/Feedback Inhibition

• When the end product of a pathway is used to inhibit the enzyme starting the reaction

• Ex. enzyme 1 binds to substrate threonine to produce isoleucine

  • When enough isoleucine has been made, it can serve as an allosteirc inhibitor and block the binding of threonine and enzyme 1

Covalent allosteric modification

• Can be inhibiting or activating

• Most common type: phosphorylation

  • reversible

• Ex: glycogen phosphorylation

  • glycogen phosphorylase kinase adds phosphate groups to produce glycogen phosphorylase alpha which removes glucose from glycogen

• Ex: proteolytic activation

  • removing amino acids from an enzyme to activate it

Irreversible inhibitors

• Organophosphates covalently bind and inhibit the enzyme acetylcholinesterase (neurotransmitter)

• Usually acetylcholinesterase breaks down acetylecholine

• Serine reacts with acetylcholine and breaks the ester bond between acetyl group and choline

• Serine forms a temporary covalent bond with the acetyl group until the acetate is hydrolyzed away

• Active site participates in reaction

Organophosphates (OP)

• Not natural products

• When OP binds with serine, it is not hydrolyzed away

• Many OPs are used as insecticides to kill insects

• G-series, V-series, and Novichok agents are OPs that are higlhy poisnous to mammals