Messenger Proteins

Another major class of proteins is the messenger proteins. They are one of the means by which cells in one part of the body communicate with cells in another part of the body. They are generally quite small as proteins go. Many are hormones. But not all hormones are proteins.

Oxytocin and Vasopressin

Two examples are oxytocin, which occurs in females and stimulates uterine contractions during childbirth, and vasopressin, whose major function is as an antidiuretic. A commercially prepared oxytocin, I am told, is routinely given intravenously to women in labor to help stimulate their uterine contractions. Vasopressin is also produced commercially under a variety of names. It prevents fluid loss by stimulating the kidney tubules to reabsorb water. Because it also causes constriction of the blood vessels, it produces a sharp rise in blood pressure.

Each of these has nine amino acid residues with an amide group on the carboxylic acid end. They have the same primary structures with the exception of two amino acid residues. That difference of two out of nine results in their different functions. (This diagram is also shown in Example 10 in your workbook.)

68065.jpg (16159 bytes)

Insulin

A more familiar polypeptide hormone is insulin, which regulates glucose metabolism in the body.

This messenger protein consists of two polypeptide chains, one 21 amino acids long, called the "A" chain, and a second, the "B" chain, which is 30 amino acids long. The two chains are bound together in their quaternary structure by two disulfide bridges. In addition, a third disulfide bridge between two amino acids in the "A" chain help stabilize its tertiary structure. The secondary structure of both chains is mostly alpha helical.

Amino acid sequence in insulin. [68066.jpg]

Insulin is created as a single peptide chain which is looped around, bonded to itself by covalent disulfide bonds between cysteine residues and then cleaved by an enzyme to make the smaller insulin molecule.

Variations on the primary structure of insulin occur in different species. Yet each of those variations serves the same function. So sometimes changes in the primary structure of a protein do not cause a change in the function of the protein. With insulin those variations occur in the eighth, ninth and tenth positions of the smaller chain (the A chain).

Another view of the amino acid sequence in insulin. [68069.jpg]

This table shows which amino acids are present at the 8^th, 9^th and 10^th amino acid positions of insulin in several different species. Which do you think is most similar to human insulin?

Animal	AA#8	AA#9	AA#10
Sheep	Ala	Gly	Val
Cow	Ala	Ser	Val
Pig	Thr	Ser	Ile
Horse	Thr	Gly	Ile

Insulin regulates the use of glucose by the body by somehow controlling the rate at which cells can absorb glucose from the blood. In diabetics, whose insulin is defective, the blood can contain high levels of glucose, yet the cells, especially of the muscle and liver, cannot absorb it and starve to death while literally bathed in nutrients.

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