Biology Reference
In-Depth Information
of fundamental particles in the entire observable universe (~10
80
). So many more
different kinds of protein are possible in principle than have actually appeared in
evolution. We now have the genetic engineering technology to make any of these
novel proteins, and some of them may well turn out to be useful.
How does the unique sequence of amino acids in the chains of a given protein
determine the specific properties of that protein? The answer is that this sequence
determines how each chain folds into a specific three-dimensional shape, called
its conformation. Figure 4.4 shows a model of the specific conformation of one
molecule of the enzyme hexokinase.
Each sphere in this model represents the size and position of an atom of either
carbon, oxygen or nitrogen; hydrogen atoms, of which there are many, are omit-
ted for clarity. The conformation of hexokinase is unique to that enzyme, and each
molecule of hexokinase is identical. The structure contains an active site which
binds a glucose molecule by interactions between the chemical groups of the glu-
cose molecule and the chemical groups of the amino acids that line the active site
of the enzyme. The way in which a chemical fits into an active site is sometimes
described as analogous to the way a key fits into a lock, in the sense that there is a
complementarity of surface features.
The conformation unique to hexokinase is formed by specific interactions
between the side chains of the different amino acids along the polypeptide chain.
What interactions are possible, and hence the shape of the folded molecule, is deter-
mined solely by the sequence of amino acids. Once that sequence is specified and
synthesized, the chain folds spontaneously into its functional conformation. This
fact is sometimes called the “principle of self-assembly” because all the information
for the conformation is contained within the sequence. Modern techniques enable us
to determine the sequence of amino acids in any polypeptide chain, but, as yet, it is
not possible to predict how a given chain will fold. A major goal of protein chemists
is to determine the rules of folding so that we can make novel proteins with useful
properties.
Principle 3:
The amino acid sequence in each chain of a protein is determined by
the sequence of bases in the gene encoding that protein.
The amino acid sequences of proteins are determined by other sequences writ-
ten in a different chemical language - the sequences of bases in nucleic acids.
Nucleic acids are linear polymers of four different kinds of base and two differ-
ent kinds of sugar phosphate. The two types of sugar phosphate define the two types
of nucleic acids; ribose defines ribonucleic acid or RNA, while deoxyribose defines
deoxyribonucleic acid or DNA. Although both proteins and nucleic acids are linear
polymers, they differ greatly in the sizes of the individual molecules. In the case of
proteins, the functional units are the individual molecules, such as hexokinase, but
in the case of DNA, the functional units, or
genes
, are joined together to form giant
molecules consisting of millions of bases and sugar phosphates. RNA is more like
protein however, consisting of hundreds to thousands of bases and sugar phosphates
in each molecule. This difference in size between molecules of DNA and molecules
of RNA reflects their different functions - DNA encodes the genetic information
