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232 Biopolymers

are found in cartilage, for example, in a matrix contain-
ing chondroitin and keratan sulfates attached to a protein
core. The protein polysaccharide linkage usually involves
the OH group on carbon 1 of the reducing end of the
polysaccharide and hydroxyproline, serine, threonine or
asparagine residues of proteins. A variety of proteogly-
cans are known, differing in the nature of the core protein
and the number and nature of polysaccharide chains at-
tached to it. In cartilage, several hundred chains of chon-
droitin sulfate and fewer of keratan sulfate are linked to
a core protein called aggrecan. Several of these proteo-
glycan molecules are, in turn, noncovalently bonded to a
single long hyaluronic acid chain. The whole complex is
stablilized by the presence of another protein, a link pro-
tein. On cell surfaces, heparan sulfate is the main polysac-
charide constituent of proteoglycans, and these molecules
appear to be involved in cell adhesion and in modulating
signals at the cell surface, and are implicated in inflam-
mation and wound repair.
Glycoproteins consist of molecules that are mainly pro-
tein with some side branches of mono- or oligosaccharide.
Theycancontainaslittleasonesugarresiduepermolecule
or several complex chains each of about ten monosaccha-
ride rings. The functions of glycoproteins are as diverse as FIGURE 16 Structure of peptidoglycan: (a) repeat disaccharide
those of the proteins themselves. Some are enzymes; anti- of polysaccharide, and (b) oligopeptide cross-link.
bodies are glycoproteins, as are many structural proteins.
Indeed collagen contains some galactose and glucose units nucleic acids, the ribonucleic acids (RNA), are involved
bonded to hydroxylysine residues of the protein. It is be- in protein biosynthesis (i.e., the translation of the code on
lieved that glucose can be involved in cross-links bind- DNA into protein structure in a cell). Ribonucleic acids
ing collagen molecules together, and that an increase in can be divided into three major types: messenger (m–)
such cross-links can bring about some of the deleterious RNA which carries the coded message specifying pro-
effects of aging seen in collagenous tissues. Plant and ani- tein structure from the DNA to the protein-synthesizing
mal structural tissues often contain glycoproteins in addi- “machinery,” the ribosomes; ribosomal (r–) RNA, which
tion to proteoglycans and fibrous polymers. Glycoproteins forms an integral part of those ribosomes, and transfer (t–)
are common on the outer surface of cell membranes; the RNA, which brings to the ribosomes the amino acids to
oligosaccharides may function as probes with which the be incorporated into proteins. Some RNA is now known
cell interacts with its environment. In fact, cancer cells are to have important catalytic properties.
known to carry altered oligosaccharides on the glycopro-
teins of their surfaces.
1. Structure
The monomers from which nucleic acids are made are
C. Nucleic Acids
more complex than those of either proteins or polysac-
Nucleic acids are the basis of heredity—they transmit ge- charides. These are nucleoside triphosphates and they
netic information from one generation of living organisms can be considered to consist of three parts (Fig. 17)—a
to the next. The genetic information specifies, in a code triphosphate group, a five-carbon sugar in the furanose
form, the structure of all the proteins in a cell; the proteins, ring form, and a cyclic base—that is a ring system with
particularly the enzymes, then determine the characteris- basic properties (which can associate with H in water).
+
tics of each cell and hence of the complete organism. Five different bases are commonly found in nucleic acids.
For all life forms, except a few viruses, protein structure InDNAthesugaris2-deoxy-D-ribose.InRNAthesugar
is encoded in the deoxyribonucleic acids (DNA). These, is D-ribose. From this difference came the names of the
in complex organisms, are found in cell nuclei, associated two polymers. The common bases are adenine, guanine,
with protein in structures known as chromosomes. Other cytosine, thymine, and uracil; adenine and guanine are