Crystallin

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In biology, a crystallin is a water-soluble structural protein found in the lens of the eye, accounting for the transparency of the structure.

[edit] Classification

Crystallins from a vertebrate eye lens are classified into three types: alpha, beta and gamma crystallins. These distinctions are based on the order in which they elute from a gel filtration chromatography column. These are also called ubiquitous crystallins. Beta- and gamma-crystallins are similar in sequence, structure and domains topology, and thus have been grouped together as a protein superfamily called βγ-Crystallins. The α-crystallin superfamily and βγ-crystallins compose the major superfamily of proteins present in the eye lens.

In addition to these crystallins there are other taxon-specific crystallins which are only found in the lens of some organisms; these include delta, epsilon, tau, and iota-crystallins. For example, alpha, beta, and delta crystallins are found in avian and reptilian lenses, and the alpha, beta, and gamma families are found in the lenses of all other vertebrates.

[edit] Enzyme activity

Interestingly and perhaps excitingly from an evolutionary perspective, some crystallins are active enzymes. A number of crystallins are related to the serine and tyrosine proteases, and others to quinone oxidoreductases. Whether these crystallins are products of a happy accident of evolution, in that these particular enzymes happened to be transparent, or whether the enzymatic activity is a part of the protective machinery of the lens, is an active research topic.^{[citation needed]}