FoundationalConceptsNeuroscience

of amino acids is called a polypeptide; if it is more than about forty
amino acids long, then it is called a protein. The threshold number
for defining when a polypeptide becomes a protein is somewhat arbi-
trary. Some might say thirty, some fifty.
Figure 3.4 is a diagram of a small protein called myoglobin. It is
a chain of 153 amino acids that binds, stores, and transports oxy-
gen molecules within animal muscles. In this drawing, myoglobin
is depicted as a ribbonlike structure representing the polypeptide
chain and overall shape of its folding. The chain of amino acids
spontaneously folds into a stable configuration characterized by an
energy minimum. The result is a unique three-dimensional structure
for the protein. In addition to its 153 covalently linked amino acids,
myoglobin contains a component called heme (not shown in this
diagram), a planar molecule embedded within the protein’s structure
that functions to bind a molecule of oxygen. The diameter of myo-
globin is only about two nanometers; even though proteins are con-
sidered relatively large molecules, they are still very tiny indeed.