In biology, a crystallin is a water-soluble structural protein in the lens of the eye, which accounts for the transparancy of the structure. There are four major families of crystallins, called the alpha, beta, gamma and delta crystallins. Of these, the 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 creatures.
Interesting, perhaps exciting from an evolutionary perspective, are the crystallins that 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.
