Insulin is a hormone produced by the pancreas, and responsible for keeping the sugar level in the bloodstream from dropping too low or rising to dangerous levels. Synthetic insulin is used for the treatment of diabetes.
From the biochemical point of view, insulin consists of two small protein chains, adding up to around 280 carbon atoms (grey), 70 nitrogen atoms (blue), 140 oxygen atoms (red) and 6 sulphur atoms (yellow). A ball-and-stick model shows all these atoms in detail (as for sucrose), but quickly becomes very cluttered and difficult to interpret.
Fortunately, we can make things clearer by taking the chemical nature of the molecules into account. A protein is a polymer chain formed by connecting building blocks, called amino acids, into a linear chain. After all amino acids are connected, the chain can fold up into a specific spatial structure. By drawing only the connections between the consecutive amino acids, we can clearly see the global shape of the protein chain, without getting lost in the details of all the individual atoms:
We can give the two protein chains distinct colors to make them easier to tell apart:
Each amino acid has two parts. One the on hand, there is a common substructure for all amino acids, which enables the polymerisation reaction. On the other hand, each of the roughly 20 types of naturally occuring amino acid has a variable part, which gives each type a distinct chemical identity. After being linked together, the constant parts form the regular main chain of backbone of the protein, while the variable parts extend from the main chain as a series of side chains:
Some parts of the main chain adopt an irregular shape, while others follow a more regular and recognizable pattern. In the insulin structure we find two spiral-like elements, which are known as alpha helices. There are specific, idealised ways to represent these elements:Another common pattern is for the main chain to be almost linear. The resulting extended regions are called beta strands.