Glycine
If you like minimalism you choose glycine. It couldn’t get any simple, right? Just two carbon atoms, no chirality… Yet, when people want to minimize influence of a residue, they mutate it to alanine, not glycine, why? This has a reason. Likewise proline, glycine offers a different backbone structure, therefore these three: 1) glycine, 2) proline, 3) all others have each a different map of the allowed conformational states (i.e. Ramachandran plot).
Glycine is special, despite all the minimalism. Due to the absence of chirality and substituents, this can be a helix breaker, and in general, the conformation of this residue are often tricky to decipher.
Overall, there are some regularities in the use of glycine in peptide structures. This residue is very often found on the interfaces between secondary structures. For example, in membrane proteins, the assembly of helical bundles is made through the help of glycine residues, which allow maximally intimate proximity of the helices to one another. The same principle is used in collagen triple helices, where individual stretches are brought into close proximity through the help of glycine residues. No other amino acid can substitute glycine in these cases, and mutations may lead to diseases.
In metabolism, glycine is one of the first components in the purine biosynthesis, the central C4-C5 carbon atoms in purines are inherited from glycine.
Interesting readings:
- Bowie, J. U. Solving the membrane protein folding problem. Nature, 438, 2005, 581-589, doi: 10.1038/nature04395
General remarks to membrane protein architecture. These are having glycine on interfaces quite often.
- Pace, R. A. et al. Collagen VI glycine mutations: Perturbed assembly and a spectrum of clinical severity. Ann. Neurol., 64, 2008, 294-303, doi: 10.1002/ana.21439
Glycine mutations impair assembly of collagen in the development of muscular dystrophies.
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