Proteins
These are very large 3 dimensional macromolecules. They are very important as structural molecules in the cell, as energy sources, and most importantly as "enzymes", (protein catalysts which speed up chemical reactions in the cell without the need for high temperature or drastic pH changes). Proteins are often called "polypeptides" because they are made of long chains of building blocks called "amino acids".
Structure of some amino acids
- R groups can be any of 20 different forms giving 20 naturally occurring amino acids (in living things)
Structure of Proteins
Primary Structure (or primary level of organization)
Definition. "The sequence of amino acids in the polypeptide chain."
Amino acids are bound together with a "peptide" bond.
The "peptide" bond. To form a "dipeptide" molecule.
Therefore the primary structure is the sequence of amino acids.
Secondary Level of Organization of Polypeptides
There are two types of secondary structure in proteins, the
ahelix and the bpleated sheet. The attraction of the R groups within the same chain can cause the chain to twist into a "right handed" coil. This "ahelix" is held together by hydrogen bonds between the hydrogen and oxygen atoms of the amino acid backbone (amino groups and carboxyl groups). Such "Intrachain Hydrogen Bonding" often predominate in "globular proteins".
Keratin is a structural protein found in hair and nails, skin, and tortoise shells. The
aHelix nature of wool is what makes it shrink.
Another form of secondary structure the
bpleated sheet, is caused by hydrogen bonding between the hydrogen atoms (amino group) and the oxygen atoms (carboxyl group) of amino acids on two chains (or more) lying side-by-side.The
bpleated sheet structure is often found in many structural proteins, such as "Fibroin", the protein in spider webs.
spider web "silk" is a structural protein.
The Tertiary Structure of Proteins
When "proline", an oddly shaped amino acid occurs in the polypeptide chain a "kink" in the
ahelix develops. Kinks can also be caused by repulsive forces between adjacent charged R groups. These kinks create a 3 dimensional chain arrangement, ie. the "Tertiary" StructureThis 3 dimensional shape is also held together by weak hydrogen bonds but also by much stronger "disulfide" bonds between two amino acids of cystine ("covalent") disulfide "bridges" (linkages)
cystine -- s -- s -- cystine
Other strong linkages such as Ionic bonds can also form between regions of the same polypeptide creating the tertiary structure. And additional weaker interactions contribute to the overall three dimensional shape of the protein.
These strong covalent bonds hold the protein in its specific 3D shape. The 3D shape creates "pockets" or "holes' in the surface of the protein which are very important in enzyme function (as we shall see).
Quaternary Structure of Proteins
This last level of organization is simply taking 2 or more 3 dimensional (tertiary proteins) and sticking them together to form a larger protein. Many enzymes and transport proteins are made of two or more parts.
Hemoglobin: an oxygen carrying protein in red blood cells which is made of 4 parts.
Proteins when heated can unfold or "Denature". This loss of three dimensional shape will usually be accompanied by a loss of the proteins function. If the denatured protein is allowed to cool it will usually refold back into it’s original conformation.
