Crystal structure, alkaline phosphatase

The X-ray crystal structure of the inorganic phosphate (an inhibitor) complex of alkaline phosphatase from E. coli (9) showed that the active center consists of a Zn2Mg(or Zn) assembly, where the two zinc(II) atoms are 3.94 A apart and bridged by the bidentate phosphate (which suggests a phosphomonoester substrate potentially interacting with two zinc(II), as depicted in Fig. 2), and the Mg (or the third Zn) is linked to one atom of the zinc pair by an aspartate residue at a distance... 

The most important aspect of the study of Co(II) metalloenzymes is the possibility of using the metal ion as a functional, built-in reporter of the dynamics of the active site. The spectral and magnetic properties of Co (II) carbonic anhydrase have given valuable clues to the catalytic function of this enzyme. The recent studies of Co(II) alkaline phosphatase and Co (II) carboxypeptidase A indicate the general applicability of this approach to enzymes where the probe properties of the constitutive metal ion are poor. The comparison of the absorption spectra of these enzymes and low-molecular weight models have shown that the proteins provide irregular, and in some cases nearly tetrahedral environments. It is obvious, however, that a knowledge of the crystal structures of the enzymes is necessary before the full potential of this method can be exploited. 

Alkaline phosphatases form a well-known class of proteins that perform quite interesting and complicated reactions. As previously reported, Zn enzymes, like carboxypeptidases, thermolysin, and carbonic anhydrases, consist of only one Zn atom per active center. Most of the alkaline phosphatases consist of two 96-kDa subunits, each containing two Zn and one Mg ion. The alkaline phosphatase from E. coli has been crystallized and described in full detail [4], and a mechanism has been proposed. Several enzymes in this category have been mentioned in recent years, some of them also containing different metal ions, such as iron and zinc, as in the purple acid phosphatase [5], It is likely that the detailed structure and mechanism of many more examples of enzymes that remove or add phosphate groups to proteins will become available in the next decade. 

Both arylsulfatases and phosphodiesterases are members of a protein superfamily that includes alkaline phosphatase, and the crystal structure of an arylsulfatase shows the presence of a metal binding site that is homologous to that of alkaline phosphatase, although it contains only a single... 

In the case of Escherichia coli alkaline phosphatase, a dimeric zinc metal-loenzyme where a total of six metal ions (three per monomer) are involved directly or indirectly in providing the enzyme with maximal catalytic activity and structural stability, Cd NMR, in conjunction with and NMR methods, were instrumental in separating out the function of each class of metal binding sites. Perhaps most importantly, these studies revealed the chemical basis for negative coopera-tivity that had been reported for this enzyme under metal deficient conditions. Also noteworthy was the fact that these NMR studies preceeded the availability of the X-ray crystal structure. 

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