When the primary chain spirals upon itself, into a helical form it reaches a secondary structure. “This is a result of hydrogen bonding to form either an alpha helix, which is the most common form.” (© UK-Learning 2002) Others that exist are beta-pleated sheets. Proteins that remain at the secondary level of structure are known as fibrous proteins. Fibrous proteins are incredibly strong and resist stretching. The most well know fibrous proteins are collagen and keratin. Without it we would not have skin, therefore we would not have a protective layer surrounding us. Cartilage and tendons are also produced from collagen without them we would not be able to move our bones and they would grind against one another. Keratin makes up the outmost layer of skin protecting our bodies from our surrounding environment. It is insoluble making skin waterproof. The number of disulphide bonds present is great, making it an extremely stable protein. Fibrinogen is a protein responsible for clotting. The structure is important here because if we did not have it then if we cut ourselves we could die.
The helix from the secondary protein coils further into a complex, three-dimensional, globular structure. The structure is described as tertiary. “Unlike fibrous proteins, globular proteins have a compact, roughly spherical shape.” (Biochemistry for Advanced Biology, p.39, 1994) They have metabolic roles in the body for example haemoglobin, enzymes and hormones etc. The structure of the protein haemoglobin is important because it carries oxygen to body cells for respiration. If the shape was uncomplimentary it would be unable to carry oxygen and consequently we would die.
The most proteins in the human body exist as enzymes. Enzymes act as biological catalysts in living organisms. They have individual, specific shapes so that they are able to react at the active site, with complementary substrates to form an enzyme-substrate complex. Hormones are globular proteins their structure is important because they initiate activities within the body, like metabolism and the production of digestive enzymes. If their shape were disrupted then they would not function properly, resulting in necessary chemical reactions not taking place.
There is a further structure known as quaternary. This happens when two or more polypeptide chains interconnect to form biologically active proteins for example haemoglobin.
Three bonds are present in all these structures - order to hold the structure together and maintain its function. Hydrogen bonds are used between adjacent amino acids, which collectively are very strong. Ionic bonds that occur between oppositely charged amino acids. Disulphide bridge is a covalent bond between sulphur atoms in adjacent amino acids. Structure is very important to proteins. If the previous bonds mentioned are broken then the protein is no longer stable and loses its vital structure. Hence, parts of the chain separate – structure and function are lost, i.e. the protein denatures. Factors that cause denaturation are:
- Temperature because as it increases the atoms within the amino acids gain kinetic energy and vibrate. Eventually the hydrogen and ionic bonds break because they are not strong enough.
- pH – if this changes then the charges on amino acids may change causing ionic bonds to break.
“Most enzymes have an optimum temperature of 37°C and an optimum pH of 7.” (© Oxford Encyclopaedia, 2001)
Proteins are overall extremely important to living organisms. Their different levels and varied structures give them their unique and vital functions - which are involved in many processes such as conveying information, transport, movement, chemical reactions, immune protection etc. If there were any deformation to the structure of a protein then it loses its function therefore the functions previously mentioned would not take place. Proteins without their exclusive structure and function are useless – making them a necessity to all living organisms.
