Proteomics WORLD OF MICROBIOLOGY AND IMMUNOLOGY456•
tion of the molecular structure. For instance, the majority of
soluble proteins of an organism, such as blood proteins, have
globular structures, like small eggs. Some proteins are fiber-
like and are associated in bundles, forming fibrils such as those
of wool and hair. Myosin, the protein that makes muscles con-
tract, has both globular and fibrous elements in its structure;
whereas collagen, the protein of connective tissues, is consti-
tuted by three triple helices of fibrils that form super structures
in the shape of a fibrous rope. Collagen represents one third of
all proteins of the human body and together with elastin is
responsible for both cohesion and elasticity of tissues.
Every enzyme is also a protein. Enzymes are proteins
that function as catalysts of biochemical reactions. Most phys-
iological activities in organisms are mediated by enzymes,
from unicellular life forms to mammals. Enzymes speed up
chemical reactions, allowing organic systems to reach equilib-
rium in a faster pace. For instance, every phase of the cell
cycleis controlled by enzymes that alternately inhibit or stim-ulate specific cellular activities as well as geneexpression or
repression, hence affecting the time of specific physiological
activities within each phase of the cell cycle. Enzymes are
highly selective in their activities, with each enzyme acting
over a specific substrate or group of substrates. Substrate is a
term designating any molecule that suffers enzymatic action,
whether being activated or inhibited.
The main property of catalyst molecules is that they are
not altered by the chemical reactions they induce, although
some rare exceptions are known where some enzymes are
inactivated by the reactions they catalyze. Enzymatic catalysis
involves the formation of protein complexes between sub-
strate and enzyme, where the amount of enzymes is generally
much greater than the amount of substrate.
Some families of enzymes play an important role during
the process of DNAreplication. For example, when DNA syn-
thesis activates, helicases break hydrogen bridges and some
topoisomerases separate the two DNA strands. DNA-poly-
merases synthesize the fragments of the new DNA strand, whileComputer representation of the three-dimensional structure of a protein.womi_P 5/7/03 11:11 AM Page 456