Ch. 18 – Amino Acids and Proteins
The many various functions of proteins are:
Enzymes - used for biological catalysis of cellular reactions
Structural function – provide mechanical support to large animals and provide them with
their outer coverings (e.g., in collagen making up connective tissue or keratin in hair or
Nutrients – serve as sources of amino acids for embryos and infants, e.g., egg albumin and
casein in milk are examples
Protective (or Defense) – as antibodies against infectious agents (antigens)
Regulatory – control many aspects of cell function, like metabolism and reproduction, such
as hormones (insulin, glucagon, oxytocin, vasopressin)
Nerve impulse transmission – at the receptors of synapses and involved in vision
Motion – are necessary for all forms of movement, involved in contraction of muscle tissue,
through the interaction of actin and myosin
Transport – carry other compounds throughout body (e.g., the lipoproteins which carry
cholesterol in bloodstream to and from the liver, or hemoglobin and myoglobin which serve
to transport oxygen in bloodstream)
Proteins are in all living species, from bacteria to humans we all are constructed from the
same basic set of 20 amino acids, pp. 601.
As the name amino acid suggests, one has an
) on a C attached also to a
carboxyl group (namely the α C atom).
Amino acids are known exclusively by their common names as the IUPAC names are
difficult (you will not have to memorize them).
Also attached to the α C atom is an R group, which renders the amino acid its own unique
characteristics due to its size, shape, solubility, and ionization properties.
and biological activity of resulting proteins are seriously affected by it.
Chemists classify amino acids in 4 groups, which depend on the nature of the functional
group on the side chain (at neutral pH):
1) non-polar (or hydrophobic), 2) polar but neutral, 3) negatively charged, and 4) positively
charged (when these amino acids are at pH 7.0).