PROTEINS Amino Acids

Amino Acid Molecular Structures
 Elmhurst College
Secondary Protein Structure Quaternary Protein Structure Denaturation  Chemistry Department
Tertiary Protein Structure Globular Proteins Minitopics  Virtual ChemBook


Quaternary Protein - Structure

Introduction:

The quaternary protein structure involves the clustering of several individual peptide or protein chains into a final specific shape. A variety of bonding interactions including hydrogen bonding, salt bridges, and disulfide bonds hold the various chains into a particular geometry. There are two major categories of proteins with quaternary structure - fibrous and globular.

Fibrous Proteins:

Actually, the final beta-pleated sheet structure of silk is the result of the interaction of many individual protein chains. Specifically, hydrogen bonding on amide groups on different chains is the basis of beta-pleated sheet in silk proteins.

See Secondary Protein structure for the discussion on silk.

Other fibrous proteins such as the keratins in wool and hair are composed of coiled alpha helical protein chains with other various coils analogous to those found in a rope. Other keratins are found in skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks, feathers, actin and mysin in muscle tissues and fibrinogen needed for blood clots.

Globular Proteins:

On the other hand, globular proteins may have a combination of the above types of structures and are mostly clumped into a shape of a ball. Major examples include insulin, hemoglobin, and most enzymes.


Click for larger image 

Collagen:

Collagens are found in tendons and other connective ligaments. Collagens have a triple helix as the major structure.

The keratin structure is described more fully in the next section. The main differences in various keratins arises from their sulfur content. If there are many cysteine disulfide cross-links, then there is very little flexibility as in horns, claws, hooves, or nails. In wool, skin, and muscle proteins, there are fewer disulfide cross-links which allows some stretching but returns to normal upon relaxation of tension.

The quaternary structure of collagen consists of three left-handed helices twisted into a right-handed coil. This structure is shown in the graphic on the left.

The basic properties of collagen are rigidity and resistance to stretching. The helices are grouped in a variety of more complex fiber type structures. A rigid pattern of interchain amide-carbonyl hydrogen bonding is best accommodated by a primary amino acid sequence of:

-gly-X-pro- or -gly-X-hypro

where X is any amino acid and hypro is hydroxyproline. In the graphic on the left X = arginine. Molecular model studies show that this sequence works the best for the triple helix structure. Glycine is needed because it is small and is the only amino acid which can fit in the interior of the triple helix.

Collagen - Chime in new window

Quiz:What are the atoms being represented around the outside of the helix in green?
What holds the protein in the alpha helical shape?  

 
Click for larger image 

 Structure of Insulin:

Human insulin contains two protein chains with a total of 51 amino acids. The chains are connected by two disulfide bonds. Insulin is classified as a hormone and is needed for the proper utilization of glucose. Diabetics must take insulin injections to maintain health. Since the available supplies of human insulin are so low, insulin from cows, hogs, and sheep must be used. The following table shows the differences in the primary structure of the insulin proteins.

 

 A Chain

 B Chain

 Position Nos.

 8 -- 9 -- 10

 30

 Human

 -thr-ser-ile-

 thr

 Cow

 -ala-ser-val-

 ala

 Hog

 -thr-ser-ile-

 ala

Sheep

 -ala-gly-val-

 ala

Fortunately, these differences are not significant in the final structure and action of insulin, and therefore humans can use all three types. However, none of the three animal types is quite as effective as the human insulin.

In recent years, human insulin is now manufactured by recombinant DNA techniques. It is used by some diabetics successfully, although some do not have as strong an awareness of a condition known as hypoglycemia, which must be corrected by eating some sugar.

The secondary structure of insulin is an example of the alpha helix (there are three segments). The intramolecular hydrogen bonding in the alpha-helix is between the amide groups. The hydrogen on the amide of one protein chain is hydrogen bonded to the amide oxygen of the neighboring protein chain.

See the graphic on the left.

A very interesting factor about insulin is the three disulfide bonds. Two of them hold the two chains together, and the third causes a loop within a single chain.

Insulin - Chime in new window

Insulin also illustrates an important property of many globular proteins. The interior of the protein contains mostly non-polar amino acid side chains (gray), while the exterior contains mostly polar amino acid side chains (red).

Quiz: List some of the amino acids which are non-polar.
List some of the amino acids which are polar.