anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

, a crystallin is a water-soluble structural

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

found in the

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

and the

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast cancer tumors. Since it has been shown that lens injury may promote nerve regeneration, crystallin has been an area of neural research. So far, it has been demonstrated that crystallin β b2 (crybb2) may be a

neurite A neurite or neuronal process refers to any projection from the cell body of a neuron. This projection can be either an axon or a dendrite. The term is frequently used when speaking of immature or developing neurons, especially of cells in cultur ...

-promoting factor.

Function

The main function of crystallins at least in the

of the eye is probably to increase the

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

while not obstructing light. However, this is not their only function. It has become clear that crystallins may have several metabolic and regulatory functions, both within the lens and in other parts of the body. More proteins containing βγ-crystallin domains have now been characterized as calcium binding proteins with Greek key motif as a novel calcium-binding motif.

Enzyme activity

Some crystallins are active enzymes, while others lack activity but show

homology Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor * Sequence homology, biological homology between DNA, RNA, or protein sequences *Homologous chrom ...

to other enzymes. The crystallins of different groups of organisms are related to a large number of different proteins, with those from birds and reptiles related to

lactate dehydrogenase Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of lactate to pyruvate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one ...

and

argininosuccinate lyase The enzyme argininosuccinate lyase (EC 4.3.2.1, ASL, argininosuccinase; systematic name 2-(''N'' ω-L-arginino)succinate arginine-lyase (fumarate-forming)) catalyzes the reversible breakdown of argininosuccinate: :2-(''N'' ω-L-arginino)succina ...

, those of mammals to

alcohol dehydrogenase Alcohol dehydrogenases (ADH) () are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to NA ...

and quinone reductase, and those of cephalopods to

glutathione S-transferase Glutathione ''S''-transferases (GSTs), previously known as ligandins, are a family of eukaryotic and prokaryotic phase II metabolic isozymes best known for their ability to catalyze the conjugation of the reduced form of glutathione (GSH) ...

and

aldehyde dehydrogenase Aldehyde dehydrogenases () are a group of enzymes that catalyse the oxidation of aldehydes. They convert aldehydes (R–C(=O)) to carboxylic acids (R–C(=O)). The oxygen comes from a water molecule. To date, nineteen ALDH genes have b ...

. Whether these crystallins are products of a fortuitous accident of evolution, in that these particular enzymes happened to be transparent and highly soluble, or whether these diverse enzymatic activities are part of the protective machinery of the lens, is an active research topic. The recruitment of protein that originally evolved with one function to serve a second, unrelated function is an example of an exaptation. AlignmentHumanCrystallinABG

Classification

Crystallins from a vertebrate eye lens are classified into three main types: alpha, beta and gamma crystallins. These distinctions are based on the order in which they elute from a

gel filtration chromatography Size-exclusion chromatography (SEC), also known as molecular sieve chromatography, is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight. It is usually applied to large molecules ...

column. These are also called ubiquitous crystallins. Beta- and gamma-crystallins (such as

CRYGC Crystallin, gamma C, also known as CRYGC, is a protein which in humans is encoded by the ''CRYGC'' gene. Function Crystallins are separated into two classes: taxon-specific, or enzyme, and ubiquitous. The latter class constitutes the major prot ...

) are similar in sequence, structure and domains topology, and thus have been grouped together as a protein superfamily called βγ-Crystallins. The α-crystallin family and βγ-crystallins compose the major family of proteins present in the crystalline lens. They occur in all vertebrate classes (though gamma-crystallins are low or absent in avian lenses); and delta-crystallin is found exclusively in reptiles and birds. In addition to these crystallins there are other

taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

-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.

Alpha-crystallin

Alpha-crystallin occurs as large aggregates, comprising two types of related subunits (A and B) that are highly similar to the small (15-30kDa) heat shock proteins (

sHsp The heat shock protein Hsp20 family, also known as small heat shock proteins (sHSPs), is a family of heat shock proteins. Prokaryotic and eukaryotic organisms respond to heat shock or other environmental stress by inducing the synthesis of protei ...

s), particularly in their C-terminal halves. The relationship between these families is one of classic gene duplication and divergence, from the small HSP family, allowing adaptation to novel functions. Divergence probably occurred prior to evolution of the eye lens, alpha-crystallin being found in small amounts in tissues outside the lens. Alpha-crystallin has chaperone-like properties including the ability to prevent the precipitation of denatured proteins and to increase cellular tolerance to stress. It has been suggested that these functions are important for the maintenance of lens transparency and the prevention of

cataract A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble ...

s. This is supported by the observation that alpha-crystallin mutations show an association with cataract formation. The N-terminal domain of alpha-crystallin is not necessary for dimerisation or chaperone activity, but appears to be required for the formation of higher order aggregates.

Beta and gamma crystallin

Beta- and gamma- crystallin form a separate family. Structurally, beta and gamma crystallins are composed of two similar domains which, in turn, are each composed of two similar motifs with the two domains connected by a short connecting

peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

. Each motif, which is about forty amino acid residues long, is folded in a distinctive

Greek key pattern __NOTOC__ A meander or meandros ( el, Μαίανδρος) is a decorative border constructed from a continuous line, shaped into a repeated motif. Among some Italians, these patterns are known as "Greek Lines". Such a design also may be called ...

. However, beta crystallin is an oligomer, composed of a complex group of molecules, whereas gamma crystallin is a simpler

monomer In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

References

External links

* *
Lens Crystallin Crystal Structures
by Christine Slingsby, Birkbeck College
Crystallins: Molecule of the Month
, by David Goodsell, RCSB Protein Data Bank {{Eye proteins Structural proteins Moonlighting proteins Human eye anatomy