Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2010

Lecture Notes: 22 February

© R. Paselk 2006



Allosteric ("other site") enzyme or binding proteins are proteins with multiple interacting sites. Allosteric proteins can exhibit one or both of two types of allosterism:

Of course for any of these effects to exist, must see some degree of protein flexibility.


"Breathing" motions:

How do we know about the mobility of protein structures?


Look at cooperativity/regulation curves for enzymes/binding proteins. (Fig. 5.21) Get two families of regulators:

So how to explain the cooperative behavior of allosteric proteins? Need to explain both kinds of effects.

Two important models

Symmetry Model of Allosterism and Sequential Model of Allosterism.

Symmetry Model of Allosterism (Monod, Wyman & Changeau)

This may be diagramed in simplified form as in Figure 5.22a of your text, or in a more "classical picture":


Can also add binding of effectors to this model: positive effectors bind to R (circles) and shift equilibrium to right, negative effectors bind to T (squares) and shift equilibrium to left.

Sequential Model of Allosterism.

In this model the subunits are each influenced by binding to other subs, but change is step-wise rather than concerted. (Figure 5.22b, p 152)

Note that Hb seems to be a combination of both. Some enzymes appear to fit each model. (Figure 5.23, p 153)


Enzymes are the heart of Biochemistry

Enzymes generally have a cleft for active site, generally <5%of surface: look like pac man. Need large structure to maintain shape etc. with many weak bonds.

Six Classes of enzymes

  1. Oxidoreductases - redox reactions
  2. Transferases - group transfer reactions
  3. Hydrolases - hydrolysis reactions
  4. Lyases - lysis generating a double bonds (non-oxidative elimination reactions)
  5. Isomerases - isomerization reactions in a single moleculoe
  6. Ligases - join (ligate) two substrate molecules

Look at major aspects of enzyme study:

Pathway Diagrams

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Last modified 22 February 2010