Can somebody let me know what we need to know about the structures of proteins;
Primary Proteins, Secondary Proteins, Tertiary Proteins & Quaternary proteins
Do they have functions that we need to know?
& Can someone let me know what are the shapes of each required to know?
Thank you.
Protein Structure
Polypeptides are just a string of amino acids, but they fold up to form the complex and well-defined three-dimensional structure of working proteins. To help to understand protein structure, it is broken down into four levels:
1. Primary StructureThis is just the sequence of amino acids in the polypeptide chain, so is not really a structure at all. However, the primary structure does determine the rest of the protein structure. Finding the primary structure of a protein is called protein sequencing, and the first protein to be sequenced was the protein hormone insulin, by the Cambridge biochemist Fredrick Sanger, for which work he got the Nobel prize in 1958.
2. Secondary Structure
This is the most basic level of protein folding, and consists of a few basic motifs that are found in all proteins. The secondary structure is held together by hydrogen bonds between the carboxyl groups and the amino groups in the polypeptide backbone. The two most common secondary structure motifs are the a-helix and the b-sheet.
The a-helix. The polypeptide chain is wound round to form a helix. It is held together by hydrogen bonds running parallel with the long helical axis. There are so many hydrogen bonds that this is a very stable and strong structure. Do not confuse the a-helix of proteins with the famous double helix of DNA. Helices are common structures throughout biology.
The b-sheet. The polypeptide chain zig-zags back and forward forming a sheet of antiparallel strands. Once again it is held together by hydrogen bonds.
The a-helix and the b-sheet were discovered by Linus Pauling, for which work he got the Nobel prize in 1954. There are a number of other secondary structure motifs such as the b-bend, the triple helix (only found in collagen), and the random coil.
3. Tertiary StructureThis is the compact globular structure formed by the folding up of a whole polypeptide chain. Every protein has a unique tertiary structure, which is responsible for its properties and function. For example the shape of the active site in an enzyme is due to its tertiary structure. The tertiary structure is held together by bonds between the R groups of the amino acids in the protein, and so depends on what the sequence of amino acids is. There are three kinds of bonds involved:
1. hydrogen bonds, which are weak.
2. ionic bonds between R-groups with positive or negative charges, which are quite strong.
3. sulphur bridges - covalent S-S bonds between two cysteine amino acids, which are strong.
So the secondary structure is due to backbone interactions and is thus largely independent of primary sequence, while tertiary structure is due to side chain interactions and thus depends on the amino acid sequence.
4. Quaternary StructureThis structure is found in proteins containing more than one polypeptide chain, and simply means how the different polypeptide chains are arranged together. The individual polypeptide chains are usually globular, but can arrange themselves into a variety of quaternary shapes. e.g.:
Haemoglobin, the oxygen-carrying protein in red blood cells, consists of four globular subunits arranged in a tetrahedral (pyramid) structure. Each subunit contains one iron atom and can bind one molecule of oxygen.
Immunoglobulins, the proteins that make antibodies, comprise four polypeptide chains arranged in a Y-shape. The chains are held together by sulphur bridges. This shape allows antibodies to link antigens together, causing them to clump.
Actin, one of the proteins found in muscles, consists of many globular subunits arranged in a double helix to form long filaments.
Tubulin is a globular protein that polymerises to form hollow tubes called microtubules. These form part of the cytoskeleton, and make cilia and flagella move.
These four structures are not real stages in the formation of a protein, but are simply a convenient classification that scientists invented to help them to understand proteins. In fact proteins fold into all these structures at the same time, as they are synthesised.
The final three-dimensional shape of a protein can be classified as globular or fibrous.
globular structure
fibrous (or filamentous) structure
The vast majority of proteins are globular, including enzymes, membrane proteins, receptors, storage proteins, etc. Fibrous proteins look like ropes and tend to have structural roles such as collagen (bone), keratin (hair), tubulin (cytoskeleton) and actin (muscle). They are usually composed of many polypeptide chains. A few proteins have both structures: the muscle protein myosin has a long fibrous tail and a globular head, which acts as an enzyme.
This diagram below shows a molecule of the enzyme dihydrofolate reductase, which comprises a single polypeptide chain. It has been drawn to highlight the different secondary structures.
This diagram below shows part of a molecule of collagen, which is found in bone and cartilage. It has a unique, very strong triple-helix structure.
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I think that is all you need to know
