If any one class of molecules were to be considered
the fundamental building blocks of living organisms, it certainly
would be proteins. They are the most versatile of all molecules.
In some proteins the strands of polypeptide chains that we encountered
in Chapter 20 are twisted about one another into larger cables and
fibers, which are used for connections, support, and structure.
These are the fibrous proteins, found in hair, skin, claws,
muscle, tendons, and insect fibers. In a second class of proteins,
which have entirely different molecular architecture, the polypeptide
chains are coiled back and forth on themselves to make compact,
ellipsoidal molecules, 25Å to 200Å in diameter. These
are the globular proteins, which are chemical agents whose
job is to act with, or on, other molecules and macromolecules. The
catalytic enzymes are the most familiar of the globular proteins,
but others serve as oxygen carriers (hemoglobin), electron carriers
(cytochromes), and protective antibodies (gamma globulins).
Right: Polypeptide backbone and side groups
in a protein. With the help of Chapter 20, identify the non-carbon
atoms (H, N, O) and indicate one amino acid residue. (Only amide
hydrogens are drawn)