Rapid prosthetics

Iron-sulfur clusters constitute one of the most ancient, ubiquitous, and structurally and functionally diverse classes of biological prosthetic groups. For reviews see Cammack (1992), Johnson (1994, 1998), Beinert et al. (1997), Beinert and Kiley (1999), and Beinert (2000). Indeed there are now known to be in excess of 120 distinct types of Fe-S cluster-containing enzymes and proteins, distributed over all three kingdoms of life, and the list is growing rapidly. 

It is important to keep in mind that hydrophilic prosthetic groups are important for renal rather than hepatobiliary clearance. Wester et al. [62] have demonstrated this with their carbohydrated analogs. Since 18F possesses inherent imaging advantages, the development of a simple and rapid synthesis which will enable 18F-SSTr ligands to be clinically practical is also important. 

Xanthine Oxidase. This molybdoenzyme is readily available from cows milk in gram quantities (28) and is relatively stable, which accounts for the fact that it is by far the most intensively studied molybdoenzyme. Bray and Swann (5) have reviewed comprehensively the earlier literature, and recent papers by Olson et al. (20) summarize combined kinetic and thermodynamic approaches to the states of the prosthetic groups during catalysis. Two molybdenum, four iron-sulfur centers, and two FAD groups are present in each molecule. An important point raised by Edmondson, et al. (29) is that the rates of internal electron transfer among the prosthetic groups appear to be much more rapid than turnover. Olson et al., (20) deduced that the reduction potentials of the two processes Mo(VI) <— Mo(V) <— Mo(IV) were —60 and —31 mv, respectively, relative to the redox potential for one of the iron-sulfur centers (center II) in the molecule. Thus, at equilibrium one can never have more than a small fraction of molybdenum as... 

Thus, the presence and possible role of Zn and the nature of the flavin prosthetic group of D-2-hydroxyacid dehydrogenase have yet to be unambiguously demonstrated. However, it might be added that the enzyme can be inactivated by treatment with ammonium sulfate at acid pH, and reactivated by FAD, but not by FMN (339). Further, the flavin in EDTA-inactivated preparations is not reduced by D-lactate, but addition of Zn + results in rapid bleaching at 450 nm (5S0). These results have been considered as evidence that the flavin prosthetic group is FAD, and that the metal is necessary for the reduction of flavin by substrate. 

There is a rapid turnover of phosphopantetheine between AGP and CoA, in response to the metabolic state, and the need for fatty acid synthesis (and thus AGP in the fed state) or fatty acid oxidation (and thus GoAin the fasting state). Apo-AGP has a half-life of 6 to 7 days, whereas the prosthetic group turns over with a half-life of a few hours (Tweto and Larrabee, 1972 Volpe and Vagelos, 1973). 

A current goal is to establish long term biocompatibility of engineering and structural materials, especially for prosthetic devices, by promoting the rapid attachment and proliferation of living cells compatible with the site of ultimate implantation. Fine fabric meshes are of great utility in this application. The great sensitivity (and simplicity) of... 

The awareness of the many advantages of this modification reaction should be matched by the knowledge of the numerous side-reactions observed to result from the reaction of TNM with proteins. In addition to nitration of tyrosine, the following side-reactions have been reported in several (but not all) proteins studied (1) inter- and intramolecular cross-linking, (2) oxidation of sulfhydryl groups to a variety of products, (3) oxidation of methionine, (4) modification of tryptophan and histidine, (5) modification of prosthetic groups. It is apparent therefore that a successful application of TNM to the selective modification of tyrosine is achieved in those cases where an unusually rapid reaction... 

EPR signals for both the flavosemiquinone radical and the low-spin ferric heme have been reported (65, 78-82). The flavosemiquinone signal, which is easily observed at 123 K, shows a typical g value of 2.0039 0.002 (65). The bandwidth, which is around 15 G, is very like that of an anionic, or red, semiquinone (65). The EPR signal of the low-spin ferric heme can be observed at low temperatures ( 28 K) and shows g values of 2.99, 2.22, and 1,47 (65), which are similar to those found for cytochrome 65 (81). EPR rapid freezing studies have allowed the amounts of semiquinone and ferric heme to be monitored during reduction of the enzyme by L-lactate (66). This has proved to be extremely useful in the development of kinetic schemes to describe the flow of electrons in the enzyme. The distance between the prosthetic groups in H. anomala flavocytochrome 62 has been estimated from EPR experiments and spin-lattice relaxation measurements (82). Pyruvate was used to stablize the flavosemiquinone and the effect on the signal of this species from oxidized and reduced heme was measured. The results indicated a minimum intercenter distance of 18-20 A (82). 

In enzyme-substrate reactions, radicals may be formed either on the enzyme (on a prosthetic group or the polypeptide), the substrate, or both. Enzyme-derived radicals tend to be immobilized and, depending on the experimental conditions, can be long lived. In contrast, substrate-derived radicals generally are tumbling rapidly in solution and are transient. Therefore, the methods used to characterize and quantitate the two kinds of radical tend to be quite different enzyme radicals are harder to identify with certainty but quantitation may simply require measuring a radical concentration rather than a rate of radical formation. 

---