Notes for 9/24/2014

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Chapter 5 - Function of Proteins

Function of Globular Proteins

Cellular structure

Transport

Storage

Recognition and/or defense

Muscle Contraction

Catalysis

Ligand Binding

Ligand - a molecule or ion that binds to a protein (Small, medium, or large)

Binds reversibly through attractive Intermoleculer Forces

Binding Site

Area on a protein to which the ligand binds

Has to be complementary to the ligand

Relatively specific

P = Protein

L = Ligand

PL = Protein Ligand Complex

P + L PL

Ka = equilibrium constant for association

Kd = equilibrium constant for dissociation

Ka = [PL]/[P][L]

Kd = [P][L]/[PL]

* Ka = 1/Kd

High Ka means ligand binds tightly

Low Ka means ligand binds weakly

* Usually Kd is reported - Small Kd means tight binding

Thermochemistry and Thermodynamics

Relationship between ΔG and K

ΔGº = -RTlnKa

ΔGº = RTlnKd

P + L PL

θ = Fraction of binding sites that are occupied by ligand

θ = [PL]/[P] + [L]

θ can be between 0 and 1

Kd = [L] when θ = 0.5

Ligand Binding

lock and key model

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