Page 175 - The Manga Guide to Biochemistry

Quaternary structure of a protein and subunits
Many proteins are able to operate as a protein or as an enzyme at the tertiary structure
stage. However, some proteins create an aggregation in which multiple polypeptide chains
that have taken tertiary structures are assembled together into a large functional unit.
For example, our red blood cells contain many iron-binding proteins called hemoglobin,
which are used to transport oxygen. Hemoglobin is formed by assembling four polypep-
tide chains called globin (two each of two types, α and β, which are indicated below as α1,
α2, β1, and β2). An enzyme such as RNA polymerase II, which creates RNA in our cells, is
formed by assembling 12 polypeptide chains.
This state is called the quaternary structure, and each of the polypeptide chains used
to create the quaternary structure is called a subunit.

α2 β1

β2 α1

α1 , α2 , β1 ,
and β2

are each
"subunits."
Since the subunits of hemoglobin are very similar in
structure to those in myoglobin, hemoglobin in this figure
has been drawn with the same structure as on page 160.
However, the structures are actually somewhat different.

Quaternary Structure

Enzymes Are the Keys to Chemical Reactions 161