1: The Three Dimensional Structures of Proteins
Cut 55 inches of wire. Wrap it several turns around a pencil.
Compare the results with right-to-left turns and with left-to-right
turns, as viewed from the back end of the pencil. The
left-to-right coiling simulates how proteins can form a coil
that is called an alpha helix. Linus Pauling (our "Story Time" subject)
discovered that proteins coil naturally in one direction only
(clockwise, as viewed from the rear of the pencil) and the helix
is stabilized by hydrogen bonds.
out the wire that you just coiled. Start at one end, leave 3 inches free and then begin wrapping the
wire left-to-right around a pencil for 4 turns, stop wrapping
for about a half inch, then wrap another section for 4 turns.
Repeat until you have 7 sections of coils, each separated by a
gap. Leave about 3 inches free at the other end.
Fold the coils and cluster them so that you have the seven coils
adjacent to each other. Looking down on it you should see that
the coils surround an open space. The free, uncoiled ends
protrude from each end.
4. Use the
Sharpie color marker to put a few "hot spots" on portions of
each free end.
This simulates how
receptor proteins are made. Receptors are very specific. For
example, one kind binds one kind of hormone, another kind binds
another hormone. Receptor proteins differ in the number of coils
in the membrane, the number of "wraps" in each coil, and in the
nature of the amino acids in the free ends outside and inside
The coils clump inside membranes
because the amino acids in the coils have no electrical charge
and they are attracted to lipid and repelled by water. Water is
on the inside and outside of the membrane, but not in it. The
free ends contain amino acids with electrical charge, and they
are therefore attracted to water and repelled by lipid in the
The hot spots represent
the amino acids that are electrically charged. They provide
binding sites for drugs, hormones, neurotransmitters and other
chemicals. When binding occurs, either on the outside face of
the membrane or the inside face, it puts a new mechanical stress
on the coils, which may alter the size of the hole in the
center. This acts as a gate mechanism to open or close the pore,
which in turn affects whether or not molecules can pass in or
out through the cell membrane.