bio4: protein structure and folding. (v1.0)

Why are proteins so important? Protein is found throughout our bodies. It is in muscle, bone, skin, hair, and virtually every other body tissue. It makes up the enzymes that power many chemical reactions and the hemoglobin that carries oxygen in your blood. This essay gives a brief introduction to proteins and protein structure.

In a previous essay I talked about peptide bonds joining amino acids to form an almost unlimited number of polypeptide and protein types. The order of amino acids matters but even more important are the shapes they form. For example, there are primary, secondary, tertiary, and quaternary protein structures. Silk is mostly a secondary structure. Hemoglobin is an intricately folded quaternary structure.

To revisit from the previous essay, part of the chemical structure of an amino acid is common to all. The varying part is called the radical or side chain. The common part consists of:

• ·         A carbon atom
• ·         A hydrogen atom
• ·         A carboxyl group (COOH)
• ·         An amino group (NH2)

The peptide covalent bond is between the carboxyl group of one amino acid and the amino group of the next amino acid and results in a water molecule removed (H2O). The chains so formed have an amino group on one end called the N-terminus and a carboxyl group at the other called the C-terminus. The amino acids have a single alphabet code which is their first letter except for KEVIN (where K is Lysine, E is Glutamic acid, V is Valine, I is isoleucine, and N is Asparagine). Every molecule of a given protein has the exact same sequence of letters.

But just the letter sequence is not enough. Interactions between amino acids in a protein chain must occur for a protein to function. The radical in the amino acid defines how it interacts with other radicals in other amino acids in the chain. These interactions give proteins its structure. For example, Lysine radical’s positive charge is attracted to the Aspartate radical’s negative charge. Conversely two radicals with the same charge are pushed apart. A hydrophobic or hydrophilic radical also results in interactions. Two Sulphur atoms in radicals may form a di-sulfide bond. There can also be hydrogen bonds formed. Iron, Zinc and Copper atoms in radicals can form ionic bonds. There are other interactions too. These interactions give the protein its 3-D shape. Without getting into details which can be very complex, there are primary, secondary, tertiary, and quaternary protein structures. These structures are called protein folding. These structures are essential for the functioning of a protein. Proteins losing structure is called Denaturation.

Collagen is an important protein with a triple helical secondary structure that is unique among proteins. It makes up 25 to 30% of the protein in the body and it is sometimes referred to as body glue. It holds cells together. If there is insufficient vitamin C, you can have unstable weakened Collagen that results in a disease called Scurvy. Scurvy was the cause of death of millions of sailors.

Hemoglobin is an important protein since it carries oxygen to cells. It has four distinct polypeptide chains interacting with each other. This is a complex quaternary structure that is elaborately folded.

There are two other interesting parts to proteins. I will take up protein synthesis in a cell in the metabolism essay. The other interesting part is how they function. I may take that up if I can locate good understandable references I can use.