What About Coral Calcium? Mr. Barefoot?
Chemistry of the Coral Skeleton
A small, invertebrate animal with tentacles resides within a hard coral frame. It resembles an anemone, but unlike the anemone it (the polyp) can withdraw into the coral frame (the corallite) when disturbed. These tiny polyps secrete a limey substance that hardens into the coral structure. This substance is calcium carbonate (CaCO3). A small quantity of specialized proteins is also present in the coral skeleton.
There are two common forms of calcium carbonate, aragonite and calcite. They differ in their crystal shape, yet their chemical formula is the same. The denser of the two forms, aragonite, is found in the hard or scleractinian corals. Whether calcium carbonate becomes aragonite or calcite depends on the "seed crystals" growth pattern. Proteins secreted by the polyp undoubtedly play a role in the growth of the crystals of aragonite and probably determine whether aragonite or calcite forms.
Many mollusk shells are composed of the aragonite form of calcium carbonate. Abalone shells have a "mother-of -pearl" luster, as do many oysters. This nacre form of calcium carbonate is a special type of aragonite. Special proteins were found in mollusk shells that are probably responsible, in part, for producing the nacre. The proteins were separated, isolated and identified by polyacrylamide gel electrophoresis, electro-elution, and amino acid analysis, respectively. [Comparative Biochemistry and Physiology 115A (no. 4) 269, 1996]
Many proteins from shells are acidic;
however, a basic protein was also found. This basic protein may function to bind
the insoluble matrix to acidic proteins in such a manner that promotes the
formation of mother-of-pearl. The picture describes this complex of
matrix, basic protein H, and acidic proteins oriented in such a way that
mother-of-pearl is the result. Glycine (G), aspartic acid (D), valine (V), and
lysine (K) are present in relatively high amounts in protein H.
Since scleractinian coral is composed of the aragonite form of calcium carbonate, investigators chose to compare proteins from mollusks with corals to see if there were similarities. Two grams of ground coral were treated to conditions described (in reference 1). Acropora formosa and the shell proteins from the abalone, Haliotis rufescens, were separated on a 6% SDS-PAGE gel. Staining with Coomassie Brilliant Blue showed similar protein patterns with an H, M, and L zone. However, the amount of protein from the coral was much less than that from the abalone shell.
Three exposed corals, collected in the spray zone of a Fijian beach were also analyzed to see if any protein could be detected after a "long" period of exposure to the drying effects of the sun. Two species of Pocillopora and one Fungia were examined. Only a bleached specimen of the Pocillopora did not show proteins even when it was silver stained. So even old or sun bleached coral have detectable proteins.
Acropora formosa proteins were cut from a 6% PAGE gel and electro-eluted from four zones excised from the gel. The protein(s) from zone H in corals and mollusk shells has (have) a different amino acid content. On a relative mole % basis, the H from Acropora formosa is 11% Asp, 17% Glu, and 16% Ser. The basic amino acids are only 5% Lys, 6% Arg, and 0% His. Therefore, the dissimilarity in proteins between nacreous shells and corals would indicate a different role or function in the formation of the calcium carbonate skeleton (see Reference 6). It is worth noting that since protein H is found in nacreous aragonite but not in non-nacreous aragonite, this protein's presence may be the necessary factor in the formation of mother-of -pearl.
References:
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