Introduction to Nanotechnology

324 BIOLOGICAL MATERIALS
so each word or codon on the mRNA is a composed of three letters from the set A,
C, G, U. Each codon corresponds to an amino acid, and the sequence of codons of
the mRNA provides the sequence in which the corresponding amino acids are
incorporated into the protein being formed. Since DNA inventories the codewords
for thousands of proteins, and mRNA contains the codewords for one protein, the
mRNA molecule is very short compared to DNA; it is a short nanowire. The mRNA
brings the message of the amino acid sequence from the nucleus of the cell where the
transcription from DNA takes place to nanoparticles called ribosomes in the
cytoplasm region outside the nucleus where the proteins are synthesized. A
number of additional protein nanoparticles called enzymes function as catalysts for
the processes of protein synthesis.
Two of the twenty amino acids have a single word or codon allocated to them,
and this word on RNA is UGG for tryptophan, as noted in Fig. 12.5. Other
amino acids have two to six words assigned to them, and some examples are given in
Fig. 12.5. In the terminology of linguistics, all the words assigned to a particular
amino acid are synonyms, and in the language of science the code is called
degenerate because there are synonyms. Three of the words, namely, UAA, UAG,
and UGA, are stop codons used to indicate the end of a specification of amino
acids in a part of a polypeptide. For example, the sequence of codons or
words ACU GCA GGC UAG corresponds to the triple-amino-acid polypeptide
Thr-AZa-Gb, where the symbols for these amino acids are given in Fig. 12.5, and
the final codon UAG indicates the end of the tripeptide.
12.4. BIOLOGICAL NANOSTRUCTURES
12.4.1. Examples of Proteins
There are many varieties of proteins, and they play a large number of roles in
animals and plants. For example, biological catalysts, called enzymes, are ordinarily
proteins. In this section we describe several representative proteins.
The protein hemoglobin, with molecular weight 68,000 Da, consists of four
polypeptides, each of which has a sequence of about 300 amino acids. Each of these
polypeptides contains a heme molecule (C34H3204N4Fe) with an iron (Fe) atom that
serves as a site for the attachment of oxygen molecules O2 to the hemoglobin for
transport to the tissues of the body. Every red blood cell or erythrocyte contains
about 250 million hemoglobin molecules, so each cell is capable of carrying
approximately one billion oxygen molecules.
Collagen constitutes 25-50% of all the proteins in mammals. It is the main
component of connective tissue, and hence the major load-bearing constituent of soft
tissue. It is found in cartilages, bones, tendons, ligaments, skin, and the cornea of the
eye. The fibers of collagen are formed by triple helices of proteins containing
repetitions of tripeptide sequences -GlyProXrs-, where the amino acid denoted by
Xrx is usually proline (Pro) or hydroxyproline (Hpro). Collagen differs from other
proteins in its high percentage of the amino acids proline (12%) and hydroxyproline