Biomineralization - Some Typical Hydrophilic Polymers

The functional biopolymer matrix in biominerals is a multicomponent mixture with the possibility of all kinds of interactions and this makes the analysis of the individual polymer components difficult. Also, the biomineralization proteins are often polydisperse, have a non-globular shape and multiple charges and post translational modifications, which further hampers the protein separation. It is even more difficult to reveal their function as they are not only present in a polymer mixture with the associated possibUity of polymer interactions, but are furthermore often only active for a certain time. For example, in calcified parts of crustacean cuticles, 33 different proteins have already been identified [ 153] so that it is difficult to reveal the fimctions of individual biomineralization polymers. 

Nevertheless, whereas the active proteins for sihca biomineraUzation are cationic to catalyze the silica polycondensation [154], those found to interact with Ca-biominerals are anionic to maintain an interaction with Ca .  

Important molecules of this class are dentin phosphoryns or sialoproteins containing high aspartic acid (Asp) and extremely high serine (Ser) amoimts, which can be phosphorylated by the enzymes casein kinases I and II. Examples include rat dentin phosphoprotein with 43% Ser and 31% Asp [155], rat phosphoryn with 56% Ser and 32% Asp [ 156], hiunan dentin phosphophoryn with 58% Ser and 26% Asp [157] or a moUuscan shell glycoprotein with 32% Ser and 20% Asp [158]. It seems that phosphoserin (P-Ser) is of especial importance for polymer-crystal interactions, very Ukely in blocked sequences. In addition, the so-called Asprich proteins were recently identified in nacre and can contain up to 53 mol % Asp and 11% Glu [ 159,160]. 

polysaccharides are usually of importance when they covalently bind to proteins (glycoproteins for example in eggshell mineralization). They only rarely occm unboimd as a soluble matrix in biomineralization and often have an alternating structure, usually of the AB copolymer type. They are therefore closer to the structme of polyelectrolytes than to that of proteins/peptides with multiple domain structures and fimctionahties. 

Summarizing, most biomineralization proteins have a multidomain structure and can often be glycosylated by hydrophilic and often acidic polysaccharides, which also can form the domains interacting with the mineral. 

---