An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 45

TABLE 2.5 Chemical Structure of 20 Amino Acids of the General Structure dCont'd

eR Group or whole Name Abbreviation Symbol Class

Proline PRO P

O

N

OH

H

d CH 2 SSCH 2 d Cystine Disulfide, a derivative of cysteine

a conjugated protein and has four heme groups, which are iron-containing organometallic
complexes.
The three-dimensional structure of proteins can be described at four different levels.

(1) Primary structure: The primary structure of a protein is its linear sequence of amino acids.
Each protein has not only a definite amino acid composition, but also a unique sequence.
The one-dimensional structure of proteins (the amino acid sequence) has a profound
effect on the resulting three-dimensional structure and, therefore, on the function of
proteins.
(2) Secondary structure: This is the way the polypeptide chain is extended and is a result of
hydrogen bonding between residues not widely separated. Two major types of
secondary structure are helixes and sheets. Helical structure can be either a -helical
or triple helix. In an a -helical structure, hydrogen bonding can occur between the
a -carboxyl group of one residue and the -NH group of its neighbor four units down
the chain, as shown in Fig. 2.13 . The triple-helix structure present in collagen consists
of three a -helixes intertwined in a superhelix. The triple-helix structure is rigid and
stretch-resistant. The a -helical structure can be easily disturbed, since H bonds are
not highly stable. However, the sheet structure ( b -pleated sheet) is more stable.
The hydrogen bonds between parallel chains stabilize the sheet structure and provide
resistance to stretching ( Fig. 2.14 ).
(3) Tertiary structure: This is a result of interactions between R groups widely separated along
the chain. The folding or bending of an amino acid chain induced by interaction between
R groups determines the tertiary structure of proteins. R groups may interact by covalent,
disulfide, or hydrogen bonds. Hydrophobic and hydrophilic interactions may also be
present among R groups. The disulfide bond can cross-link two polypeptide chains
(for example, insulin). Disulfide bonds are also critical in proper chain folding,
as shown in Fig. 2.16 . The tertiary structure of a protein has a profound effect on its
function.
(4) Quaternary structure: Only proteins with more than one polypeptide chain have quaternary
structure. Interactions among polypeptide chains determine the quaternary structure
( Fig. 2.15 ). Hemoglobin has four subunits (oligomeric), and interaction among these