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Although the term beta helix is much more descriptive for this fold, the term solenoid is used by CATH. The obvious feature of these folds are the coiled shape formed by repeating parallel beta sheets. There are two main types, 3-solenoids and 2-solenoids - with the corresponding number of strands in each turn of the coil. As well as the number of sheets, there is the question of handedness of the coil. There doesn't seem to be a preference for either left or right handed coils.

This makes 3-solenoids a triangular shape, with broad flat surfaces. The 2-solenoid is more squashed, resembling an extended sandwich fold. Although it would seem plausible to extend the series to four or five sheets (4-solenoid etc), the core of the coil has to be reasonably tightly packed to ensure the protein folds correctly. It is concievable to have a porin-like hole down the middle, but this has not (yet?) been observed.

As a structural protein, you would expect it to be quite stable. Indeed the tailspike proteins of some viruses adopt such folds, which have the added advantage of easy extensibility. Since each turn of the coil is formed from the same basic structure (strand-turn-strand-turn-strand), this can presumably be easily duplicated at the level of the gene. The structure becomes more impressive in the P22 tailspike adhesin - three subunits associate together (face to face, like piled logs) and also form an interdigitated helix that is to say, each coil provides a turn in a 'superhelix'.

There are also enzymes that adopt this fold, notably pectate lyase C which breaks down carbohydrate chains. Since the chains can bind along one of the flat surfaces of the helical prism the binding is presumably more stable. Metal ions bound to this surface at the active site break the chain in the middle and the halves are released. Another application, which exploits the regularity of the fold, is as an ice binding protein. Polar groups spaced at just the right distance apart - one on each turn - bind to the crystal and prevent it growing.

In fact, regular 'ladders' of residues running up the coil aid the stability of the fold. Stacked phenylalanines provide additional interactions to supplement the beta-sheet hydrogen-bonding. This shows again the importance of repetion of the pattern, both of sequence (the ladders) and the secondary structure (the strands). All in all, I suspect that this is the kind of structure people thought proteins would have - not the relatively messy globin fold. :)

Example : 1air00 NeHehhEeEeEHEeEhEeEEeEEeEEeEEehEeHhC
1:3Z3:7P6:8P7:9P8:12P9:13P10:14Z12:16P 13:17P14:18Z16:19P17:20P18:21Z19:22P 20:23P21:24Z22:25P23:26P24:27Z25:28P 26:29P27:31Z28:32A29:32R

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