One of the most important of these roles is that of hormones, and we will see many examples of hormonal control as we discuss biochemical reactions. Of course, other classes of compounds, particularly steroids, can be hormones.
The notoriety of steroid hormones in doping scandals in sports can obscure the fact that peptides also can be hormones. We can take a quick look at peptide hormones now to prepare the way for later discussions.
Some important peptide hormones have cyclic structures. Two well-known examples with many structural features in common are oxytocin and vasopressin (Figure). In each, there is an –S–S– bond responsible for the cyclic structure.
Each of these peptides contains nine amino acid residues, each has an amide group (rather than a free carboxyl group) at the C-terminal end, and each has a disulfide link between cysteine residues at positions 1 and 6.
The difference between these two peptides is that oxytocin has an isoleucine residue at position 3 and a leucine residue at position 8, and vasopressin has a phenylalanine residue at position 3 and an arginine residue at position 8. Both of these peptides have considerable physiological importance as hormones
● Small peptides, containing two to several dozen amino acid residues, can have marked physiological effects in organisms.
● Oxytocin and vasopressin each have nine amino acids and only differ in two positions.
● They both have marked biological effects in vertebrates.
● Oxytocin stimulates uterine contractions, lactation, mother–child bonding, pair bonding, feelings of trust, and altruistic behavior.
● Vasopressin is involved in control of blood pressure, as well as being implicated in male behavior of territoriality, aggression, pair bonding, and social group defense.