Proteins generally are pretty small. This makes sense, because the cells in which they function are also pretty puny. When the cells in which these proteins function are taken together as a tissue, it can be seen how small proteins can have functions that impact on entire tissues. Sometimes, though, proteins break the mould.

The mass of proteins is generally measured in kilodaltons (kDa), a measure of molecular weight equal to 1.67×10−24 kg (so very, very small). The average mass of all human proteins is around 53 kDa, though significant variety exits depending on the location and function of a protein. No proteins, however, can measure up the adequately named Titin, a juggernaut of a protein that weighs in at nearly 4000 kDa and is seen as the single largest protein in the human body. The single Titin gene contains nearly 180,000 base pairs and to put this in context, that is larger than the entire genomes of most common viruses.

Image from Wikipedia Commons (http://upload.wikimedia.org/wikipedia/commons/2/27/1BPV.png)
The structure of Titin, in comparison with the average size of a human protein (indicated by the arrow). Image from Wikipedia Commons (http://upload.wikimedia.org/wikipedia/commons/2/27/1BPV.png)

A good representation of Titin’s enormous size can be found in how long it takes to say its proper chemical name, its IUPAC name, for example Hydrogen Chloride for common table salt. Titin’s IUPAC name contains nearly 200,000 letters and has been estimated to take around 3 hours to say.

So what does this massive protein do? Titin is one of the major constituents of muscle, along with myosin and actin. It is thought to mediate the passive elasticity of the muscle, allowing the function of the muscle without direct contraction. Mutations within the Titin gene pose an interesting question – since it is so large, mutations are quite likely. However its size may also be protective, providing insulation so that a single mutation may not result a noticeable change in protein function. Nonetheless, Titin has been implicated in numerous diseases including a wide range of muscular dystrophy disorders.

So, with Titin size clearly does count. With muscle contributing around 40% of the human body mass, the proteins that play major roles within this tissue have to size up!

‘Til next time…


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