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


Globular Protein - Structure

Introduction:

Globular proteins have no systematic structures. There may be single chains, two or more chains which interact in the usual ways or there may be portions of the chains with helical structures, pleated structures, or completely random structures. Globular proteins are relatively spherical in shape as the name implies. Common globular proteins include egg albumin, hemoglobin, myoglobin, insulin, serum globulins in blood, and many enzymes.

Myoglobin and hemoglobin are much alike in their action toward oxygen. All body cells require oxygen for metabolism, but unfortunately, oxygen is non-polar and not soluble in the aqueous blood. The problem then becomes, how do you get oxygen to the tissue cells?

Hemoglobin is used to transport oxygen in the blood in red blood cells to tissue cells where it is used directly. Myoglobin is present in skeletal muscles as an extra storage protein to enable muscles cells to have a readily available supply of oxygen.

Heme:

Hemoglobin and myoglobin both a have group called "heme", which is at the heart of the protein structure. The molecular structure is shown in the left graphic. It is made from a a series of nitrogen five member cyclic rings, that are further joined to each other by more rings.

At the center of the heme group is the iron +2 metal ion. The nitrogen atoms bind to the iron ion through what are called coordinate covalent bonds. This means that ,unlike normal covalent bonds where each atom contributes one electron for the bond, that the nitrogen contributes both electrons for the coordinate covalent bond. The oxygen molecule will ultimately bind to this iron ion also using a coordinate covalent bond.


Click for larger image 

Myoglobin:

Hemoglobin consists of four protein chains and four heme groups that carry oxygen from the lungs to the tissue cells. Myoglobin consists of a single protein chain with 153 amino acids and one heme group that stores oxygen in the muscle cells. Myoglobin has a stronger affinity for oxygen then hemoglobin, which enables the oxygen to shift from one to the other.

The presence of myoglobin gives meat its bright red color. The tertiary structure is such that it forms a "box-like" structure around heme. Heme is a series of flat, planar heterocyclic five-member nitrogen rings attached to iron as shown in the above graphic.

The heme is held in position by the bonding of a nitrogen on a histidine side chain from the protein to iron in heme. This histidine is shown in blue-gray on the left side of the heme. A second histidine is in the vicinity on the opposite side of the heme but is not bonded.

There is one bonding position on iron ion for the attachment of oxygen diatomic molecule (red). Note also that the oxygen is bonded at an angle to the plane of the heme.

Myoglobin - Chime in new window

Quiz:Which type of protein structure is represented by the green ribbons?
Which type of protein structure is represented by bends and folds of the green ribbons?  

 
Click for larger image 

 Hemoglobin:

The structure for hemoglobin is very similar to myoglobin except that it has a quaternary structure due to the presence of four protein chain subunits. Actually, there are two pairs of similar chains - two alpha types and two beta types. Each protein chain subunit contains a heme group with the iron attached. Each hemoglobin molecule can bind to a total of four oxygen molecules.

Individually, each of the chains is not that much different than that of myoglobin, so the same principles apply that were discussed previously. The diatomic oxygen molecules are shown bonded to the heme group.

The specific intrachain and interchain bonding interactions, are well known at this time. There are some changes in the structure of the deoxyhemoglobin when oxygen reacts to form oxyhemoglobin which is shown in the graphics on the left.

Hemoglobin - Chime in new window

Oxygenation of the hemoglobin occurs in the blood as it passes through the lungs. Oxygen in the air diffuses through the walls of the lungs into surrounding blood capillaries. As oxygen binds to the deoxyhemoglobin, a hydrogen ion is released. The hydrogen ion in turn causes the release of carbon dioxide from the blood into the lungs.

As the first oxygen is bond to one of the four heme groups, the protein begins to undergo small shifts in the structure, but sufficient to expel BPG (2,3-bisphosphoglycerate ion). This molecule resides in the "cleft" between the protein chains. See whether you can see this in the Chime model.

As soon as the BPG is expelled, a cascade of events causes the other three heme groups to bind to three more oxygen molecules in fast succession.

When the oxyhemoglobin reaches the vicinity of the tissue cells, carbon dioxide, in excess from metabolism which has used up the oxygen supplies, causes an increase in hydrogen ions. The increase in hydrogen ions then initiates the process for the hemoglobin to release the oxygen to the cells. Again BPG begins to work its way into the hemoglobin and causes faster loss of the oxygen molecules.

Quiz: What is the definition of quaternary structure?