What do Proteins do?

There are several roles filled by proteins in the human body. Below are some examples. If you press on the buttons below, you will see an example of each type of protein, as described in the text.

As you bring these structures into the screen at the left you should use the "Chime" pull down menu to change the representation of the molecules. Often turning on "rotation" will help in viualising the shape of the molecules. You should experiment as much as you want. To return to an original image just click on the button that you used to bring it up originally. If you really get in a mess then use the Netscape "Reload" command which is in the "View" menu in the MenuBar.

Proteins can be enzymes, that is, they can catalyse a chemical reaction. This image is a protein called catalase, which is an enzyme which catalyses the reaction H₂O₂ ---> H₂0 in the peroxisome in the cell. The protein is coloured in order from from one end of the chain to the other. (Blue through the rainbow to red.) However the protein is a single polypeptide chain of amino acids.

The catalase molecule is shown as a spacefilling model in this view. This gives a view where the atoms are represented as spheres with a volume approximating the electron shell. Toget an idea of the different representations of the protein molecule turn on "Rotation" in the pop-up menu in the image frame and then in the "Display" submenu select the different representations in turn. This will give you an idea of the various ways to represent the same molecule.

This is another view of catalase. This time the protein part of the enzyme is shown as a stick representation. Notice the grey and red molecule in the midst of the protein? That is a haem group, which we will get to later. This is not an amino acid but a molecule attached to the catalase protein which is essential for its activity. But keep an eye out for the same heam group which occurs in several other proteins.

This protein is an oxygen carrier called haemoglobin. It is responsible for carrying oxygen in the red cells of the blood.

Notice that this protein too, has a haem group, for which it is named. It also has a very globular shape. Haemoglobin has four protein strands, and in this image each strand has been coloured differently. Because of this haemoglobin is called a tetramer. Each monomer (individual protein chain of a tetramer) has it's own haem group. You may find it easier to see if you rotate the image.

Antibodies are very important proteins. Without antibodies in the blood, animals would have no resistance to foreign bodies, such as bacteria or viruses. Antibodies enable the body to fight off an infection, and are involved in what is termed the "immune response". Although antibodies are usually seen as a complex with their antigen (the molecule which they bind to and inactivate as antibodies), this image is of an antibody on it's own.

Antibodies are made up of several protein chains. In this image each chain is coloured differently. To get a better idea of the arrangement of the chains use the pop up menu in the structure frame to change the display to "Backbone" Turn rotation on to see the relationship between thechains.

Insulin is a hormone It is made in the pancreas and secreted into the blood when blood sugar levels are high. Insulin tells the cells of the muscles and fat tissues to take up glucose from the blood and store it as glycogen or fat. Insulin is a messenger which is involved in co-ordinating the energy metabolism of the body. A deficiency of insulin is one cause of diabetes, a condition where the blood sugar rises because the muscle and fat cells cannot take up glucose unless told to by insulin.

There are two protein subunits in each insulin molecule. Each subunit has two chains and is bound to a single zinc atom. Atoms and molecules which bind to proteins, such as zinc to insulin are called ligands. Ligands are bound to the proteins by non-covalent interactions and are usually more easily removed from the protein than are prosthetic groups. Proteins can have many different types of ligands and these are often important for the function of the protein.

You should experiment with the insulin display using the popup menu in the structure frame. Because insulin is a small molecule it loads into the program quickly and also responds quickly when the display parameters are changed.

This image is of part of the complex of human growth hormone with its receptor. Cells in multi-cellular organisms need to be told what to do and to respond to signals from other cells and tissues in the body. The body does this through messenger or signaling molecules (like insulin).

One class of signals are the hormones which are made in a tissue in one part of the body and are released into the blood and carried around the body. The hormone reaches all cells but only causes a response in cells which have a specific receptor .

Insulin and growth hormone are examples of hormones which interact with receptors in the cell membrane of target cells. This image shows part of the complex of growth hormone with its receptor. Growth hormone binds to the extracellular parts of two receptor molecules (shown in the image) and draws them close together in the cell membrane. The receptors pass through the cell membrane and extend into the cytoplasm. (In this image the membrane spanning and intracellular parts are not shown.)

When the hormone binds to the extracellular part of the receptor the intracellular part is activated and produces a signal inside the cell which causes the cell to respond.

Different messages require different receptors, so this is an example of a large family of proteins, and not all look like this.

A message to a cell may tell it to start making some proteins, or to stop making other proteins. There is a large group of proteins which are involved in transmitting these messages to the genes in the DNA So once the receptor has recieved a message, this is transmitted to these proteins which regulate gene expression by interacting with the DNA and turning on the expression of some genes and turning off others.

This protein is the lac repressor which is involved in signalling to E. coli that there is lactose in the medium and helps control the expression of the genes involved in metabolising lactose.

The protein interacts with the DNA through the two helices at the left hand side of the molecule as it is shown in the structure frame when first loaded (red and light green helices).

Collagen is a structural protein. It is a major component of skin and connective tissue. Collagen needs to be elastic as well as strong. The structure of the protein is very important in producing these characteristics. What you see in the image is only a small part of the collagen molecule. The display shows only the peptide backbone in a smoothed format. Use the "Chime" pop up menu to change the display to "Sticks" or "Wireframe" to get a better idea of the complexity of even this small part of the protein.

Viruses are housed in protein capsids. Here you can see, in the image frame, two molecules of the capsid protein of the HIV (AIDS) virus. In the picture below the overall structure of the HIV virus is shown. The red oval structures on the outside of the virus are the capsid protein.

The capsid attaches to receptors on the outside of the cell membrane of the T-cell and the viral RNA is injected into the cell. If the capsid protein does not interact with the receptors on the T-cell then the viral DNA cannot be injected and there is no infection of the cell.

Now you've come to the end of the first part of the protein structure tutorial. You may have taken notes, and should know enough something about the diversity of protein structures and the roles played by proteins in the body.

If you want to go over anything before before moving to the next tutorial scroll the right hand (text) frame back to the top.

If you are ready to move on to the next stage of the tutorial You can do this by pressing the NEXT button in the lower (small) frame to the left, or by selecting the HOME button in this frame and then selecting the Amino Acids tutorial.

If you need help at any time press HELP in the small frame. This will take you to the short section on the use of the Chime pop-up menus etc.

NOTE: You may need to scroll the small window to see all of the buttons.