There are several types of secondary protein structures:
• alpha helix
• beta pleated sheet
• beta spiral / beta helix (IDK English version of this)
• omega loops* (I might be wrong about “loop” one)
That’s cool and all but why do we categorize them with alpha, beta and omega? Can’t we just say “this is a pleated sheet protein structure” or just a “helix structure”? Why exactly do we use those alpha, beta and omega terms? Is there alpha pleated sheet form? Or omega helix structure??
In: Chemistry
Alpha helices and beta pleated sheets date from the first x-ray structures of proteins conducted by a man named William Astbury in the 1930s. (X-ray diffraction is a method of determining the structure of molecules).
[His paper](https://www.jstor.org/stable/91227)
Astbury was being funded by textile companies to study the structure of wool, and while studying keratin, he discovered that keratin appeared to have a short sequence that repeated over and over, and also that it looked different when the fiber was stretched.
He referred to the structure of the unstretched fiber as the alpha form, and of the stretched fiber as the beta form.
>The second important result of the present investigation has been to show that the X-ray photograph of unstretched hair is quite different from that of stretched hair. On stretching the hair, the α-photograph, as we shall call it, fades away and is gradually replaced by the R-photograph, the interferences of which first become prominent at about 30 per cent. extension. The α-photograph is finally lost sight of at about 60 per cent. extension, at which point the β-photograph is almost as well defined as it is possible to obtain it by this method, since very soon after, in the neighbourhood of 70 per cent. extension, the fibres break. The occurrence and progress of this transformation from the α-form to the β-form accounts readily for the main features of the characteristic load/extension curve, and for the marked changes which are brought about in the physico-chemical properties of hair on stretching.
>
>W. T. Astbury and A. Street, *X-Ray Studies of the Structure of Hair, Wool, and Related Fibres. I. General*, *Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character*, Vol. 230 (1932), pp. 75-101
He further hypothesizes that the alpha structure is helical and the beta structure some sort of regular sheet.
When advances in x-ray crystallography in later decades confirmed the nature of the structures, it was decided to go with the original labels applied by Astbury.
A beta helix is call that because it’s a helical pattern such that one (or more) of the faces of the helix forms a beta sheet. That sounds weird, so think about it this way: in a beta sheet, you have several parallel (or antiparallel) sequences of amino acids that fit together to make that sheet, but you also have other stuff between the sequences of the sheet. In a beta helix, that other stuff is arranges so that the protein makes a helical shape.
The name of omega loops is incredibly boring: they look like a capital letter omega, Ω, with the loop being the loop, and the left and right “tails” on the bottom being the polypeptide chain going onwards to infinity in each direction.
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