Essential Cell Biology 5th edition

60
HOW WE KNOW
THE DISCOVERY OF MACROMOLECULES
The idea that proteins, polysaccharides, and nucleic
acids are large molecules that are constructed from
smaller subunits, linked one after another into long
molecular chains, may seem fairly obvious today. But
this was not always the case. In the early part of the
twentieth century, few scientists believed in the existence
of such biological polymers built from repeating
units held together by covalent bonds. The notion that
such “frighteningly large” macromolecules could be
assembled from simple building blocks was considered
“downright shocking” by chemists of the day. Instead,
they thought that proteins and other seemingly large
organic molecules were simply heterogeneous aggregates
of small organic molecules held together by weak
“association forces” (Figure 2–32).
The first hint that proteins and other organic polymers
are large molecules came from observing their behavior
in solution. At the time, scientists were working
with various proteins and carbohydrates derived from
foodstuffs and other organic materials—albumin from
egg whites, casein from milk, collagen from gelatin,
and cellulose from wood. Their chemical compositions
seemed simple enough: like other organic molecules,
they contained carbon, hydrogen, oxygen, and, in the
case of proteins, nitrogen. But they behaved oddly in
solution, showing, for example, an inability to pass
through a fine filter.
Why these molecules misbehaved in solution was a
puzzle. Were they really giant molecules, composed
of an unusually large number of covalently linked
atoms? Or were they more like a colloidal suspension
of particles—a big, sticky hodgepodge of small organic
molecules that associate only loosely?
(A)
(B)
Figure 2–32 What might an organic macromolecule look
like? Chemists in the early part of the twentieth century debated
whether proteins, polysaccharides, and other apparently large
organic molecules were (A) discrete particles made of an
unusually large number of covalently linked atoms or (B) a loose
aggregation of heterogeneous ECB5 e2.30/2.32 small organic molecules held
together by weak forces.
One way to distinguish between the two possibilities
was to determine the actual size of one of these
molecules. If a protein such as albumin were made of
molecules all identical in size, that would support the
existence of true macromolecules. Conversely, if albumin
were instead a miscellaneous conglomeration of
small organic molecules, these should show a whole
range of molecular sizes in solution.
Unfortunately, the techniques available to scientists in
the early 1900s were not ideal for measuring the sizes of
such large molecules. Some chemists estimated a protein’s
size by determining how much it would lower a
solution’s freezing point; others measured the osmotic
pressure of protein solutions. These methods were susceptible
to experimental error and gave variable results.
Different techniques, for example, suggested that cellulose
was anywhere from 6000 to 103,000 daltons in
mass (where 1 dalton is approximately equal to the
mass of a hydrogen atom). Such results helped to fuel
the hypothesis that carbohydrates and proteins were
loose aggregates of small molecules rather than true
macromolecules.
Many scientists simply had trouble believing that
molecules heavier than about 4000 daltons—the largest
compound that had been synthesized by organic
chemists—could exist at all. Take hemoglobin, the oxygen-carrying
protein in red blood cells. Researchers tried
to estimate its size by breaking it down into its chemical
components. In addition to carbon, hydrogen, nitrogen,
and oxygen, hemoglobin contains a small amount of
iron. Working out the percentages, it appeared that
hemoglobin had one atom of iron for every 712 atoms
of carbon—and a minimum weight of 16,700 daltons.
Could a molecule with hundreds of carbon atoms in one
long chain remain intact in a cell and perform specific
functions? Emil Fischer, the organic chemist who determined
that the amino acids in proteins are linked by
peptide bonds, thought that a polypeptide chain could
grow no longer than about 30 or 40 amino acids. As
for hemoglobin, with its purported 700 carbon atoms,
the existence of molecular chains of such “truly fantastic
lengths” was deemed “very improbable” by leading
chemists.
Definitive resolution of the debate had to await the
development of new techniques. Convincing evidence
that proteins are macromolecules came from studies
using the ultracentrifuge—a device that uses centrifugal
force to separate molecules according to their size
(see Panel 4–3, pp. 164–165). Theodor Svedberg, who
designed the machine in 1925, performed the first studies.
If a protein were really an aggregate of smaller
molecules, he reasoned, it would appear as a smear
of molecules of different sizes when sedimented in an