Polymers a large molecules made up of a chain of smaller molecules, known as monomers. The monomers that a polymer is made up of decide its structure and therefore it’s function. These monomers are linked and coiled in a very specific manor giving the polymer a specific tertiary structure (an extensively coiled and linked polymer chain caused as a result of the formation of more bonds forming). This tertiary structure is crucial to the function of a polymer as it denotes the necessary shape of the complementary molecule. This is easier explained through an example. An example of a polymer, is a protein.
The monomer used to make up is an amino acid. When two amino acids join together a dipeptide is formed, this is then made into a polypeptide chain by the joining of additional amino acids, it finally becomes a protein when more than one of these polypeptide chains link to one another in a condensation reaction. Proteins have a variety of functions, these functions are determined by their shapes. For example, haemoglobin, this is a substance in the blood that transports oxygen from the respiratory organs to the rest of the body and returns the waste product carbon dioxide to be exhaled.
In order to do so it must therefore have a specific shape reflecting its function; it must be folded in a particular way that allows oxygen and carbon dioxide to bind to it, and as much as possible. One haemoglobin molecule can carry four oxygen molecules each bound to a specific site. Haemoglobin has a quaternary structure. Most of the amino acids in haemoglobin form alpha helix eps, connected by short non-helical segments. Hydrogen bonds stabilise the helical sections inside this protein, causing attractions within the molecule, folding each polypeptide chain into a specific shape.
Even though carbon dioxide is carried by haemoglobin, it does not compete with oxygen for the iron-binding positions, but is actually bound to the protein chains themselves, this relies on the chain being the correct shape for the CO2 molecules to bind to. Haemoglobin is just one of the many possible forms and functions of a protein polymer. They all have different structures, which makes them specialised to carry out a particular function. For example enzymes. They are roughly spherical in shape due to the tight folding of the polypeptide chains.
Enzymes play important roles in most biological processes, in particular metabolism and synthesis. The tertiary structure of an enzyme is of particular importance to its function and a slight change in the chain sequence and therefore it’s shape can result in it becoming inactive. Enzymes work by combining with a substrate to form an enzyme-substrate complex, the substrate is then broken down and released. Each enzyme has a particular substrate that is can breakdown, this will be the substrate with the complimentary structure to that of the active site of the enzyme.
For example starch is only broken down by amylase. As the shape of the active site depends on the shape of the polymer shape so does the over all function of the enzyme. Antibodies are involved in the immune response. They’re made up of two light polypeptide chains and two heavy polypeptide chains bonded together. Antibodies have a variable region which acts in a similar way to the active site of an enzyme. Each antibody has a variable region that is the complementary shape for a particular antigen and can therefore destroy it.
Any change in the polymer chain means the variable region is no longer effective. And finally, DNA. DNA contains the genetic information for every living thing and therefore it is vital that the sequence and therefore shape are correct. DNA is made up of bases (nucleotides) every three bases codes for an amino acid this is a codon and is known as the triplet code. A change in just one of these bases can change the shape of the DNA polymer by coding for a different amino acid. It is not only the function of proteins that is affected by the shape of the polymer chain.
Polysaccharides make up carbohydrates. They are formed when more than two monosaccharides are linked together by a glycosidic bond in a condensation reaction. Starch is an example of a polysaccharide, because it is broken down by an enzyme (amylase) it relies on a constant structure in order for this breakdown to occur otherwise it would no be able to fit into the active site of an amylase enzyme. In conclusion, the shape of a polymer is of vital importance to the role it plays and even a slight alteration in its shape can result in the polymer becoming inactive and completely useless.
