Protein fold class

Short description: Categories of protein tertiary structure

A summary of functional annotation of the most ancestral translation protein folds

In molecular biology, protein fold classes are broad categories of protein tertiary structure topology. They describe groups of proteins that share similar amino acid and secondary structure proportions. Each class contains multiple, independent protein superfamilies (i.e. are not necessarily evolutionarily related to one another).[1][2][3]

## Contents

* 1 Generally recognised classes
* 1.1 all-α
* 1.2 all-β
* 1.3 α+β
* 1.4 α/β
* 2 Additional classes
* 2.1 Membrane proteins
* 2.2 Intrinsically disordered proteins
* 2.3 Coiled coil proteins
* 2.4 Small proteins
* 2.5 Designed proteins
* 3 See also
* 4 References

## Generally recognised classes

Four large classes of protein that are generally agreed upon by the two main structure classification databases (SCOP and CATH).

### all-α

All-α proteins are a class of structural domains in which the secondary structure is composed entirely of α-helices, with the possible exception of a few isolated β-sheets on the periphery.

Common examples include the bromodomain, the globin fold and the homeobox.

### all-β

All-β proteins are a class of structural domains in which the secondary structure is composed entirely of β-sheets, with the possible exception of a few isolated α-helices on the periphery.

Common examples include the SH3 domain, the beta-propeller domain, the immunoglobulin fold and B3 DNA binding domain.

### α+β

α+β proteins are a class of structural domains in which the secondary structure is composed of α-helices and β-strands that occur separately along the backbone. The β-strands are therefore mostly antiparallel.[4]

Common examples include the ferredoxin fold, ribonuclease A, and the SH2 domain.

### α/β

α/β proteins are a class of structural domains in which the secondary structure is composed of alternating α-helices and β-strands along the backbone. The β-strands are therefore mostly parallel.[4]

Common examples include the flavodoxin fold, the TIM barrel and leucine-rich-repeat (LRR) proteins such as ribonuclease inhibitor.

## Additional classes

### Membrane proteins

Membrane proteins interact with biological membranes either by inserting into it, or being tethered via a covalently attached lipid. They are one of the common types of protein along with soluble globular proteins, fibrous proteins, and disordered proteins.[5] They are targets of over 50% of all modern medicinal drugs.[6] It is estimated that 20–30% of all genes in most genomes encode membrane proteins.[7]

### Intrinsically disordered proteins

Intrinsically disordered proteins lack a fixed or ordered three-dimensional structure.[8][9][10] IDPs cover a spectrum of states from fully unstructured to partially structured and include random coils, (pre-)molten globules, and large multi-domain proteins connected by flexible linkers. They constitute one of the main types of protein (alongside globular, fibrous and membrane proteins).[5]

### Coiled coil proteins

Coiled coil proteins form long, insoluble fibers involved in the extracellular matrix. There are many scleroprotein superfamilies including keratin, collagen, elastin, and fibroin. The roles of such proteins include protection and support, forming connective tissue, tendons, bone matrices, and muscle fiber.

### Small proteins

Small proteins typically have a tertiary structure that is maintained by disulphide bridges (cysteine-rich proteins), metal ligands (metal-binding proteins), and or cofactors such as heme.

### Designed proteins

Numerous protein structures are the result of rational design and do not exist in nature. Proteins can be designed from scratch (de novo design) or by making calculated variations on a known protein structure and its sequence (known as protein redesign). Rational protein design approaches make protein-sequence predictions that will fold to specific structures. These predicted sequences can then be validated experimentally through methods such as peptide synthesis, site-directed mutagenesis, or Artificial gene synthesis.

## See also

* Protein superfamily
* SCOP database
* CATH database
* FSSP database

* v
* t
* e

Protein tertiary structure

General|

* Structural domain
* Protein folding
* Structure determination methods

All-α folds:|

* Helix bundle
* Globin fold
* Homeodomain fold
* Alpha solenoid

All-β folds:|

* Immunoglobulin domain
* Beta barrel
* Beta-propeller
* Beta helix

α/β folds:|

* TIM barrel
* Leucine-rich repeat
* Flavodoxin fold
* Rossmann fold
* Thioredoxin fold
* Trefoil knot fold

α+β folds:|

* DNA clamp
* Ferredoxin fold
* Ribonuclease A
* SH2-like fold

Irregular folds:|

* Conotoxin

## References

1. ↑ Hubbard, Tim J. P.; Murzin, Alexey G.; Brenner, Steven E.; Chothia, Cyrus (1997-01-01). "SCOP: a Structural Classification of Proteins database". Nucleic Acids Research 25 (1): 236–239. doi:10.1093/nar/25.1.236. ISSN 0305-1048. PMID 9016544.
2. ↑ Greene, Lesley H.; Lewis, Tony E.; Addou, Sarah; Cuff, Alison; Dallman, Tim; Dibley, Mark; Redfern, Oliver; Pearl, Frances et al. (2007-01-01). "The CATH domain structure database: new protocols and classification levels give a more comprehensive resource for exploring evolution". Nucleic Acids Research 35 (suppl 1): D291–D297. doi:10.1093/nar/gkl959. ISSN 0305-1048. PMID 17135200.
3. ↑ Fox, Naomi K.; Brenner, Steven E.; Chandonia, John-Marc (2014-01-01). "SCOPe: Structural Classification of Proteins—extended, integrating SCOP and ASTRAL data and classification of new structures". Nucleic Acids Research 42 (D1): D304–D309. doi:10.1093/nar/gkt1240. ISSN 0305-1048. PMID 24304899.
4. ↑ 4.0 4.1 Efimov, Alexander V. (1995). "Structural Similarity between Two-layer α/β and β-Proteins". Journal of Molecular Biology 245 (4): 402–415. doi:10.1006/jmbi.1994.0033. PMID 7837272.
5. ↑ 5.0 5.1 Andreeva, A (2014). "SCOP2 prototype: a new approach to protein structure mining". Nucleic Acids Res 42 (Database issue): D310–4. doi:10.1093/nar/gkt1242. PMID 24293656.
6. ↑ "How many drug targets are there?". Nat Rev Drug Discov 5 (12): 993–6. December 2006. doi:10.1038/nrd2199. PMID 17139284.
7. ↑ Krogh, A.; Larsson, B. R.; Von Heijne, G.; Sonnhammer, E. L. L. (2001). "Predicting transmembrane protein topology with a hidden markov model: Application to complete genomes". Journal of Molecular Biology 305 (3): 567–580. doi:10.1006/jmbi.2000.4315. PMID 11152613.
8. ↑ Dunker, A. K.; Lawson, J. D.; Brown, C. J.; Williams, R. M.; Romero, P; Oh, J. S.; Oldfield, C. J.; Campen, A. M. et al. (2001). "Intrinsically disordered protein". Journal of Molecular Graphics & Modelling 19 (1): 26–59. doi:10.1016/s1093-3263(00)00138-8. PMID 11381529.
9. ↑ "Intrinsically unstructured proteins and their functions". Nat. Rev. Mol. Cell Biol. 6 (3): 197–208. March 2005. doi:10.1038/nrm1589. PMID 15738986.
10. ↑ "Function and structure of inherently disordered proteins". Curr. Opin. Struct. Biol. 18 (6): 756–64. December 2008. doi:10.1016/j.sbi.2008.10.002. PMID 18952168.

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