Nitrogen-Containing Compounds - Natural Products Chemistry: Sources, Separations, and Structures - page 69

7 Nitrogen- Containing

Compounds

7.1 PROTEINS, PEPTIDES, AND AMINO ACIDS
Proteins are present in and vital to all living cells. The protein holds together, pro-
tects, and provides structure to the body of multicelled organisms. Proteins cata-
lyze, regulate, and protect the body chemistry in the form of enzymes, hormones,
antibodies, and globulins. Furthermore, proteins affect the transport of oxygen and
other substances within a living organism. Examples include hemoglobin, myoglo-
bin, and various lipoproteins . Proteins possess positive attributes as antibiotics and
vaccines to help fight disease. Conversely, some proteins are recognized as toxins
and venoms (see Chapter 14), for example, toxins produced by tetanus and diphtheria
microorganisms; snake venoms; and the toxic protein known as ricin, found in castor
beans (see Figure 14.4).

7.1.1 C omponentS of p roteinS
First, it should be noted that proteins are fundamentally different from carbohydrates
and lipids. Lipids are made up of approximately 75% to 85% carbon, and carbohy-
drates contain about 50% oxygen and less than 5% nitrogen. However, the proteins
(and peptides) possess between 15% and 25% nitrogen and about an equal amount of
oxygen. Other elements include sulfur atoms and are specifically found in the amino
acids cysteine and methionine. Proteins and peptides differ in their size: peptides are
considered as small proteins, having molecular weights of less than 10,000 daltons.
But all proteins and peptides are constructed from amino acids.

7.1.2 p rotein -p eptide B uildinG B loCkS
To date, more than 20 essential amino acids (Figure 7.1) and more than 200 other
unusual amino acid compounds have been isolated from nature. The 20 key amino
acids are essential diet components. They are not synthesized by human metabolic
processes. When proteins are hydrolyzed in boiling aqueous acid or base, these small
amino acids are released by rupturing the peptide bonds.
Peptides are synthesized by the coupling of one of the carboxyl group or C-terminus
of one amino acid to the amino group or N-terminus of another (Figure 7.2). Peptides
can form linear chains of amino acids or can form cyclic peptides of six to eight
amino acids or more. Many of the latter are created by microbial and marine sources,
and they possess various biological activity.

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