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Why are Proteins Important in Organisms?
Proteins are made up of monomers called amino acids. An amino acid has a central
carbon atom with four things attached: a hydrogen, an acid group (COOH), an amine
group (NH2), and a group which can vary, called the R group. The R group is the part of
the amino acid that is variable. All amino acids have the same acid group, amine group,
and hydrogen; they differ only in the R group. Twenty-two different amino acids are
found in proteins; each has its own characteristic R group. Amino acids are combined
by the reaction called:
As you read before, monomers are joined by synthesis.
Many amino acids, joined in a long unbranched chain, form a polypeptide. One or more
polypeptide molecules make up a protein. The most interesting thing about proteins is
the variety of shapes they can have. The shape of a protein is determined by the
sequence of amino acids in it, and the way the shape is produced involves three or four
levels of structure. The primary structure is just the linear sequence of amino acids; you
could start at one end of a protein and list the amino acids in it in order. Secondary
structure is the folding of the long chain into one of a few shapes, often a helix. This is
similar to a telephone cord (if you remember when telephone receivers used to be
connected to the base of the telephone by a cord, that is).
Two views of the protein hemoglobin; the pink parts show helical secondary structure.
Interactions between the amino acids stabilize this shape, so that, like the telephone
cord, it will automatically return to the shape after being stretched or bent. Pull your
telephone cord out straight and let it snap back. Not only does it return to its helixshaped secondary structure, the helix itself probably folds up into some sort of a glob
(this is one reason why everybody uses cordless phones instead). This additional
folding occurs in proteins, and is called tertiary structure. The tertiary structure is
determined also by interactions between the amino acids. More than one polypeptide
can be tangled up together; this interaction between whole separate polypeptides is
quaternary structure.
One important thing to note is that the complex shape, and all the levels of structure
determining it, depends on the primary structure or sequence of amino acids. A given
sequence will always automatically fold up into the same shape.
Why is the shape important? It may contribute to structure of things like hair, cartilage,
and muscle. More important, though, many proteins function as catalysts. Protein
catalysts are enzymes.
HOW DO ENZYMES ASSIST IN CHEMICAL REACTIONS?
Think about a reaction such as the synthesis shown above. What must happen before
the monomers can join together?
They can't join without breaking off an -OH and an -H. Breaking bonds requires energy.
How could you provide enough energy to get two glucose molecules to combine and
form maltose, a disaccharide?
One way would be by heating them in a liquid such as water. HEAT IS MOLECULAR
MOTION. If you heat the solution of glucose, the molecules will crash into each other
harder and harder, and more and more frequently. Occasionally two molecules will
crash in such a way as to break off the -OH and -H. How do the molecules have to be
aligned in order to do this?
The groups to be broken off must crash into each other. The problem with heating is
that the motion it gives to the molecules is random. An enzyme, though, has a dent on
its surface called the active site which fits the starting molecules (substrates). An
enzyme which would catalyze this reaction (call it sugarase) would have an active site
which fit two glucose molecules.
Once the two glucose molecules are in place in the active site, it requires only a small
amount of energy to get them to bump into each other and react, forming the product,
maltose. If you needed to get a certain number of molecules, say, 10000, to react with
each other, you could do it with or without a catalyst. Suppose you did this at body
temperature, 37 degrees centigrade. Here are two test tubes. One (A) contains 10000
molecules of glucose in water, the other (B) has 10000 molecules of glucose in water
with the enzyme, sugarase.
In which tube will all the reactant be converted to product more quickly? Explain how
you know, based on the discussion above
Please answer on the separate question sheet.
Without enzymes, none of the reactions that are essential to all your functions would
happen fast enough to notice. You couldn't move, grow, digest food, think (nerve
impulses are carried between cells by compounds made using enzymes) or even live,
since your heart wouldn't beat. All living things depend on a variety of enzymes, each
one with a very specific function.
What determines the function of an enzyme?
Please answer on the separate question sheet.
What determines the shape of the enzyme?
Please answer on the separate question sheet.
Since enzymes are so essential, and their function depends on their shape, and their
shape is determined by the sequence of amino acids, the big problem now is, what
determines the sequence of amino acids in a protein?
Aha – we will answer that one next time.