Cation binding site

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

The estimated distances between lanthanides bound at various cation binding sites on the Ca -ATPase, and FITC, TNP-ATP, Cr-ATP or eosin bound at the nucleotide binding site range between 10 and 47 A [389,390,400,401,407-409,413] (Table II). 

X-ray diffraction studies on gramicidin commenced as early as 1949 218-219> and this early work pointed to a helical structure 220). Recent work by Koeppe et al. 221) on gramicidin A crystallised from methanol (/%) and ethanol (.P212121) has shown that the helical channel has a diameter of about 5 A and a length of about 32 A in both cases. The inclusion complexes of gramicidin A with CsSCN and KSCN (P212121) have channels that are wider (6-8 A) and shorter (26 A) than the uncomplexed dimer 221 222). Furthermore there are two cation binding sites per channel situated either 2.5 A from either end of the channel or 2.5 A on each side of its centre 222) Unfortunately these data do not permit a choice to be made from the helical models (i)—(iv) and it is not certain if the helical canals studied are the same as those involved in membrane ion transport. 

I. 9 A resolution, revealing a cation-binding site near the catalytic site. Acta Crystallogr D Biol Crystallogr 2001, 57, 1950—1954. 

Hl-loop cation-binding sites can stabilize the flexible [14] a 1 helix designated as the thumb epitope, which is spatially close to the wrist epitope (Fig. 6d). This segment is found only among the TGF-b isoforms and the activins but not in other members of the TGF-b superfamily and may also participate in the recognition of type I receptor or other TGF-b activity regulation proteins, such as betaglycan. 

Binding of Mn2+ to the high affinity cation binding site of bacteriorhodopsin caused a 50% decrease in signal amplitude for spin label attached at residue... 

There are two cation-binding sites per subunit, classified nL and n2. The n, site is a structural site, which may involve the reorientation of a glutamate carboxyl group. Metals bind first at this site. The n2 site is the catalytic site and may bind metal or metal-nucleotide. Co111 and Cr111 can be incorporated into the nj metal-binding sites in un-adenylylated glutamine synthetase from E. co/i.318 Both derivatives were inactive, but were able to bind Mn2+ at the n2 site. Comparison of the quaternary enzyme-Crin-Mnn-ADP (which shows spin-spin interaction between the two metal centres) with enzyme-Com-Mnn-ADP leads to an estimate of the distance between n, and n2 sites of 7 2 A. 

The interface between the polar phospholipid headgroups and the aqueous electrolyte solution provides a membrane surface which contains weakly selective cationic binding sites. (10) This generates a reservoir of cations available for conduction and is apparently much more important than bulk solution Ion content in the determination of permion and ion current density. (11)... 

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