An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 43

Each enzyme's molecule contains an active site to which its specific substrate is bound during
catalysis. Some enzymes are regulated and are called regulatory enzymes. Most enzymes are
globular proteins.
The building blocks of proteins are

-amino acids, and there are 20 common amino acids.
Amino acids are named on the basis of the side (R) group attached to the a -carbon. Amino
acids are optically active and occur in two chiral isomeric forms. Only L -amino acids are
found in proteins. D -amino acids are rare; only found in the cell walls of some microorgan-
isms and in some antibiotics.
Amino acids have acidic ( e COOH) and basic ( e NH 2 ) groups. The acidic group is neutral
at low pH ( e COOH) and negatively charged at high pH ( e COO ). At intermediate pH
values, an amino acid has positively and negatively charged groups, becoming a dipolar
molecule called a zwitterion.

a

H

H 3 N + COO - C

R
zwitterion

The pH value at which amino acids have no net charge is called the isoelectric point, which
varies depending on the R group of amino acids. At its isoelectric point, an amino acid does
not migrate under the influence of an electric field. Knowledge of the isoelectric point can be
used in developing processes for protein purification. A list of 20 amino acids and one
common type of derivatives that are commonly found in proteins is given in Table 2.5 .In
the structural drawings of R groups, every node has a carbon atom if no other atom is explic-
itly shown. All the carbon atoms have four bonds, with hydrogen H's as the default if no
other atom is explicitly shown. This is the convention to be used throughout this text. For
example, the e R group for Tyrosine (TYR),

OH

,

is e CH 2 e C 6 H 5 e OH.
Proteins are amino acid chains. The condensation reaction between two amino acids
results in the formation of a peptide bond as depicted below

H H O

H H

H 2 N CCOOH+HNCCOOH CCNCCOOH+H 2 O

H 2 N

H R 2 H

R 1 R 1 R 2

The peptide bond is planar. Peptides contain two or more amino acids linked by
peptide bonds. Polypeptides usually contain fewer than 50 amino acids. Larger amino
acid chains are called proteins. Many proteins contain organic and/or inorganic compo-
nents other than amino acids. These components are called prosthetic groups,andthe
proteins containing prosthetic groups are named conjugated proteins. Hemoglobin is