The structure and function of Proteins
The structure and function of Proteins
This essay clearly divides into two sections: structure, and function. Hence I will begin by talking about the structure of Proteins and will go on to talk about their functions.
The building blocks of proteins are "amino acids", which differ to other molecules due to their nitrogen content. The basic amino acid looks like this, with a line of N-C-C. The N has two H atoms bonded to it, while the end C atom has a double bond to an oxygen atom and has a hydroxyl group bonded to it. The middle carbon atom bonds with a H atom, and has a "variable side group" attached onto it.
Amino acids are very soluble, principally for the reason that the Oxygen and Nitrogen in them is very electronegative, meaning that the amino acids are readily carried around in aqueous state.
Amino acids are "amphoteric" meaning that in different circumstances they can act as both acids and alkalis. When in a specifically acidic or alkaline solution, the amino acid forms a "zwitterion":
When added to an acid, the zwitterion form of the amino acid acts as a buffer by taking up H+ ions to keep the concentration oh hydrogen constant, and hence to raise the pH.
However, when in an alkaline solution, the amino acid acts as a buffer in a different way. It gives away a H+ ion to the solution, to lower the pH and hence it controls the concentration of Hydrogen ions.
The way these amino acids formed proteins is through a process of "condensation of amino acids". In this example, two amino acids (Glycine and Alanine) condense together, forming an amide linkage (a peptide bond) and giving off a molecule of water. The end result is Glycanine, a dipeptide or Protein. Hence proteins are referred to as "polypeptides" due to the fact that they are many amino acids joined together by peptide bonds.
The structures of proteins themselves are put into several levels: primary, secondary, tertiary and quaternary.
Primary structure is the basic arrangement of amino acids that form the chain. E.g. Glycine + Alanine....
Secondary structure is determined by the interactions between amide linkages, like hydrogen bonding. Two structures can form in secondary structure of proteins:
. An Alpha Helix. Here, every amide linkage joins up with another, 4 atoms along the chain, causing a helix shape to form. This chain is flexible along it's axis, as the hydrogen bonds have no fixed length.
2. Beta ...
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The structures of proteins themselves are put into several levels: primary, secondary, tertiary and quaternary.
Primary structure is the basic arrangement of amino acids that form the chain. E.g. Glycine + Alanine....
Secondary structure is determined by the interactions between amide linkages, like hydrogen bonding. Two structures can form in secondary structure of proteins:
. An Alpha Helix. Here, every amide linkage joins up with another, 4 atoms along the chain, causing a helix shape to form. This chain is flexible along it's axis, as the hydrogen bonds have no fixed length.
2. Beta pleated sheet.
Tertiary structure is determined by interactions between amino acid side-chains, or between these side-chains and water:
. Covalent bonding: Takes place between 2 cysteine residues:
2. Hydrophobic / non-polar interactions: Non-polar side chains will tend to be found in the middle of a protein- far away from the exterior which is likely to be or is in immediate contact with water. These non-polar sidechains then interact with each other.
3. Polar interactions: Polar side chains tend to be found on the exterior of a protein, where they can interact with water. If however they are drawn to the interior because of other interactions (e.g. non-polar), they associate with other polar side-chains.
Quaternary structure involves the interaction of several polypeptide chains to form one functional protein. E.g. haemoglobin.
As discussed, there are many different types of protein, and all of these play different roles in the body. In this section, I will attempt to talk about these roles.
The first major role of proteins is that of enzymes. Enzymes are structurally made of globular proteins. They act as biological catalysts made by living cells, and they control metabolism and cell reactions. Enzymes never start reactions, but simply speed them up, by acting as biological catalysts, in reducing the activation energy for a reaction:
Shown above, most reactions require some kind of energy input to start off a reaction, as reactions don't just occur out of the blue.
Also, at the end of a reaction, enzymes remain unchanged or unaltered- hence they can be used again.
The formula for what goes on in an enzyme reactions is:
Enzyme + Substrate --> Enzyme-Substrate complex --> Enzyme + Product
There are two main theories about how enzymes react with molecules. The first is the Lock and Key Theory. It states that all enzymes' active sites are shaped specifically to fit certain molecules. If a molecule of that type is nearby, it will fit into the active site of the enzyme, and will be broken down. However, a different molecule will not fit into the active site of that particular enzyme:
The second theory is the Induced Fit Theory. This theory states that instead of enzymes having a set shape of active site, their active site is able to "grab" the molecule and fit around it:
It is thought that both types of reaction occur in the body.
There are many 1000s of enzymes in the body, all of which fulfil different roles in the body, in different reactions.
Proteins can also function as transport agents in the body, and are involved in transporting other substances around the body. A good example of this type of protein is "haemoglobin". This is an iron containing protein found in the blood that reacts with oxygen to form oxyhaemoglobin. This means that oxygen can be carried around the body in the blood- a very efficient way of supplying it to the whole body. Proteins also act as transport molecules in cell membranes, helping in facilitated diffusion. In this method of diffusion, instead of molecules simply passing through the membrane, some molecules may have to react with or be contained by proteins in the cell membrane, and transported inside the cell.
Structural proteins are vital for support in the body. Good examples are collagen and elastin, which provide a fibrous framework in animals' connective tissue, like tendons and ligaments. Keratin is another good example, as it is the protein of hair, horns, feathers and quills in animals.
Storage proteins play a vital role in their storage of amino acids around the body. Ovalbumin, for example, is the protein of egg white, which is used as an amino acid source for developing embryos. Casein, the protein of milk, clearly has a vital role in animal upbringing as the milk provided is used for baby mammals.
Proteins in the body also are found in the form of hormones. These coordinate and regulate bodily activities, and are essential in thermoregulation- the maintaining of constant bodily conditions. A clear example of this is the roles played by two hormones; insulin and glucagon. When the blood sugar levels rise above natural levels, the pancreas secretes insulin. This hormone breaks down sugary glucose into glycogen, thus lowering the blood sugar level. If however the blood sugar levels get too low, the pancreas secretes glucagon which converts glycogen into sugary glucose, thus raising the blood sugar levels. The action of these two hormones, secreted by the pancreas, helps regulate and control the blood sugar levels in the body.
Receptor proteins are built into the membranes of nerve cells in the nervous system of the body. They detect chemical signals released by other nerve cells, and are also involved in the cell's response to chemical stimuli. If these proteins where not present, the stimuli would stimulate the cell, but it would not be passed along nerves to the response centre, and no response would be made.
Contractile proteins are vital in movement of animals. Actin and Myosin, for example, are responsible for movement of muscles, creating body movement overall. Other contractile proteins control undulations of cilia and flagella, which propel many cells.
Finally, defensive proteins are vital in the body, as they fight off disease; like antibodies, which combat bacteria and viruses. Such proteins are called immunoglobulins.
Paul Wiltshire, L6DM
