Low affinity sites

Figure 9. Proposed cyclic mechanism for ATP synthesis by complex V involving all three catalytic sites of F,. In this scheme only the a and p subunits of F, are shown these are connected by a short stalk to F, in the inner membrane. Proton translocation through Fq driven by the proton motive force (AP) causes sequential conformational changes in each of the p-subunits and ATP synthesis as described in the text hexagons, high-affinity sites semicircles, low affinity sites parallelepipeds, intermediate-affinity sites (with no movement of F,).

As the copper content of cell walls increases, the Cu " ions distribute between the two types of uronates most of them on high affinity sites first, at low copper contents more on low affinity sites afterwards. The percentage of the uronic acids that bind Cu with a high affinity is plotted on Fig. 5 as a function of the relative amount of copper in the walls. We can see that about 30% of the uronic acids consist of high affinity sites. 

The biochemical nature of these high and low affinity sites can be deduced from the literature. The high affinity sites (about 30% of all exchange sites, Fig. 5) have EPR spectra characteristic of homopolygalacturonic acid polymers [6]. The walls also comprise about 40% of RGI and hemicellulosic uronates [7, 8]. The remaining 30% thus represent low affinity homopolygalacturonic acids. 

The relative selectivity of drugs such as haloperidol for the higher affinity (+)SKF-10,047 labeled sites allowed the determination of the pharmacological profile of the low affinity sites, which was very similar to that of 3H-TCP labeled sites. PCP and... 

Nickel binds to multiple high- and low-affinity sites at the DNA molecule. 

Brodersen, R., Funding, L., Pedersen, A. O., and Roigaai-d-Petemen, H., Binding of bilirubin to low-affinity sites of human serum albumin in vitro followed by cocrystallization. Scand. J. Clin. Lab. Invest. 29, 343-346 (1972). 

Duff, H.J., Feng, Z.P. and Sheldon, R.S. (1995) High- and low-affinity sites for [3H] Dofetilide binding to guinea pig myocytes. Circulation Research, 77, 718-725. 

C.96 Based on the Leigh model95 a distance of 9.8 + 0.7 A between the Mn2+ and the spin label was calculated. This short distance indicates that the high affinity site is located on the extracellular side of the protein. Smaller changes in amplitude were observed for spin label at site 103C when Mn2+ was bound at the high affinity sites or at low affinity sites. It was concluded that at least part of the 3 or 4 low affinity sites are located on the cytoplasmic side of the protein. 

Li+ may bind at the Na+ site with activation42 while K+ binds with inhibition. For these internal Na+ sites, Km (Na+) = 1.23 mmol dm 3 and KT (K+) = 4.5 mmol dm-3. Both high and low affinity sites exist for K+ for the high affinity sites Km (K+) = 0.08 mmol dm 3 and (Na+) = 14.3 mmol dm 3, thus showing good discrimination between K+ and Na+. The low affinity sites have Km (K+) = 1.2 mmol dm-3 and appear to be involved in the phosphatase reaction Na+ binds with inhibition [X, (Na+) = 4.5 mmol dm-3]. Other Group IA cations activate at the K+ sites thus there is evidence for two sites for Rb+ per ouabain site. 

For the ATPase activity of the enzyme, Mg2+ is required. Kinetic studies confirm that MgATP is the substrate and that while free ATP binds to the enzyme it inhibits activity. It appears that MgATP binds at high and low affinity sites, with Km values about 3 p,mol dm-3 and 0.2 mmol dm-3 respectively. For phosphorylation of the enzyme, saturation of the high affinity site is sufficient for maximum activity. Binding at the low affinity sites inhibits ADP/ATP exchange and may represent a modulator role for these sites. This low affinity site may be the site for the substrate in the K+-phosphatase reaction. 

Troponin C from rabbit skeletal and bovine cardiac muscle has a molecular weight of about 18 000. Skeletal TN-C has four sites for Ca2+. Two of these (III and IV) are high affinity sites (K < 107 dm3 mol-1) which also bind Mg2+ competitively, with K = 103 dm3 mol-1. The remaining two (I and II) appear to be specific for Ca2+, although of lower affinity (K 105 dm3 mol-1).234 These two sites are the only sites in calcium-binding proteins that do not bind Mg2+ with constants in the range 102-103 dm3 mol-1. Cardiac TN-C contains two Ca2+-Mg2+ sites, one Ca2+-specific site and one low affinity site for Ca2+, in which the two aspartate residues in the skeletal TN-C protein are replaced by leucine and alanine residues.235... 

Osteocalcin has two high affinity and two or three low affinity sites for calcium. It limits growth of calcium phosphate crystals, and inhibits precipitation of hydroxyapatite. This cannot be achieved by the decarboxylated protein. Calcified tissue from various sources also contains gla proteins.459 Gla proteins are not found in invertebrates. 

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