Proteins and Nucleic Acids: Information Carriers
Globular Proteins: Myoglobin and Hemoglobin
Enzymes and catalysts are the subject
of Chapter 24, so at this point we shall introduce globular proteins
by means of two molecules that perform other functions: hemoglobin,
which carries 02 in the bloodstream from the lungs to
the tissues; and myoglobin, which stores 02 in muscle
cells until it is needed. Both myoglobin and hemoglobin are hemoproteins,
with the protein chain enclosing a flat, planar iron-porphyrin ring
complex called a heme group, shown at the left. The iron atom and
the porphyrin ring together make up one large delocalized-electron
system similar to the magnesium-porphyrin system in chlorophyll.
Because of the delocalized electrons, both chlorophyll and the heme
group absorb light in the visible spectrum, and are brightly colored.
Chlorophyll is green because it absorbs strongly in the red end
of the spectrum; hemoglobin and myoglobin absorb in the yellow-green
and therefore have the red color familiar in blood and beefsteak.