An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 48

blood serum and in certain cells of a vertebrate in response to foreign macromolecules. The
foreign macromolecule is called the antigen (Ag). The specific antibody molecules can
combine with the antigen to form an antigen e antibody complex. The complex formation
between Ag and Ab is called the immune response. In addition to their obvious clinical impor-
tance, antibodies are important industrial products for use in diagnostic kits and protein
separation schemes. Antibodies may also become a key element in the delivery of some anti-
cancer drugs. Antibodies have emerged as one of

the most important products of

biotechnology.
Antibody molecules have binding sites that are specific for and complementary to the
structural features of the antigen. Antibody molecules usually have two binding sites and
can form a three-dimensional lattice of alternating antigen and antibody molecules. This
complex precipitates from the serum and is called precipitin. Antibodies are highly specific
for the foreign proteins that induce their formation.
The five major classes of immunoglobins in human blood plasma are IgG, IgA, IgD,
IgM, and IgE, of which the IgG globulins are the most abundant and the best understood.
MWs of immunoglobulins are about 150 kDa except for IgM, which has a MW of 900 kDa.
A dalton is a unit of mass equivalent to a hydrogen atom. Immunoglobulins have four
polypeptide chains: two heavy (H) chains (about 430 amino acids) and two light (L) chains
(about 214 amino acids). These chains are linked together by disulfide bonds into a Y-
shaped, flexible structure ( Fig. 2.16 ). The heavy chains contain a covalently bonded oligo-
saccharide component. Each chain has a region of constant amino acid sequence and
a variable-sequence region. The Ab molecule has two binding sites for the antigen; the
variable portions of the L and H chains contribute to these binding sites. The variable
sections have hypervariable regions in which the frequency of amino acid replacement is
high. The study of how cells develop and produce antibodies is being actively pursued
worldwide. Recent developments have also led to insights on how to impart catalytic
activities to antibodies. These molecules have been called abzymes. Appling new develop-
ments in protein engineering to antibodies promises the development of extremely specific
catalytic agents.

2.3.2. Carbohydrates
Carbohydrates play key roles as structural and storage compounds in cells. They also
appear to play critical roles in modulating some aspects of chemical signaling in animals
and plants. Carbohydrates are represented by the general formula (CH 2 O) n , where n 3

and are synthesized through photosynthesis.

Light
Photosynthesis
Respiration

CO 2 + H 2 O CH 2 O + O 2

The gases CO 2 and H 2 O are converted through photosynthesis into sugars in the pres-
ence of sunlight and are then polymerized to yield polysaccharides, such as cellulose or
starch.