Biomimetics enzymes

Abstract Calix[n]arenes represent a well-known family of macrocyclic molecules with a broad range of potential applications in many branches of supramolecular chemistry. Because of their preorganisation, calix[n] arenes are frequently used as building blocks and molecular scaffolds in the construction of more elaborate systems, such as artificial enzyme biomimetics and receptors. This review is focused on the recent development of calixarene-based anion receptors. 

Many artificial systems have been designed recently to imitate the function and behaviour of native enzymes - biomimetic chemistry [27]. Among them, calixarene-based receptors bearing one, two or three Zn(II) complexes on the upper rim were prepared as a model for phosphoesterases [28-31]. Dinuclear receptor 25 was reported to enhance the rate of transesterification of the RNA model substrate 2-hydroxypropyl-p-nitrophenyl phosphate more than 20,000 times compared with the non-catalysed reaction. The complexation mode for the phosphate anion can be described as cascade complexation where the anion is coordinated within the cavity formed by two zinc cations. 

An interesting attempt to overcome this problem is the design of simplified systems which try to reproduce the activity of natural enzymes (biomimetic catalysts). This approach has produced, e.g., impressive advances in the chemistry of synthetic porphyrins and in understanding the activity of some enzymes e.g. cytochrome P-450) which catalyzes oxidation reactions by an iron-porphyrin centre. Furthermore, interesting similarities have been noticed between enzymes and completely different catalysts. For instance, selective adsorption in the channels of some zeolites provide a confined, relatively hydrophobic medium even in aqueous solvent (Annex 2). This strongly resembles the active sites of several enzymes (including cytochrome P-450) that are deeply buried in hydrophobic pockets where lipophilic substrates are readily oxidized. The more hydrophilic reaction products are promptly released into... 

In relation to copper-containing enzymes, biomimetic oxidation of catechol (200) was performed using dioxirane 553 in acetone-1,1,1-trifluoropropanone (TFP) (—20°C, 1 h) to produce selectively cw,cw-muconic acid (556) (88%) (Scheme 102). This result resembles the oxygenation assisted by metal complexes (see Scheme 40). 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

These were relatively low-resolution structures, and with refinement some errors in the initial structural assignments have been detected (4-7). Since the structures were first reported the subject has been extensively reviewed in this series (8) and elsewhere 9-15). This review will focus on the structure, biosynthesis, and function of the met-allosulfur clusters found in nitrogenases. This will require a broader overview of some functional aspects, particularly the involvement of MgATP in the enzymic reaction, and also some reference will be made to the extensive literature (9, 15) on biomimetic chemistry that has helped to illuminate possible modes of nitrogenase function, although a detailed review of this chemistry will not be attempted here. This review cannot be fully comprehensive in the space available, but concentrates on recent advances and attempts to describe the current level of our understanding. 

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

Biomimetic studies typically have one or more of the following objectives (i) to reproduce in a small synthetic molecule reactivity that was theretofore only observed in an enzyme (ii) to understand the mechanisms of an enzymatic reaction and the relationship between the stereoelectronic attributes of the catalytic site and its reactivity and (iii) to develop practical catalysts by exploiting and adopting solutions that evolved in Nature. Biomimetic studies of cytochrome c oxidase have been particularly impactfull in addressing aim (ii). On the other hand, this approach is... 

The mechanistic and structural chemistry of B12 may be separated into (i) investigations of cobalamin cofactors both apart from and in complex with their enzymes, and (ii) biomimetic model complexes, both structural and functional. 

Macrocyclic complexes of zinc have inspired interest in varied areas such as supramolecular and biomimetic chemistry including hydrolysis enzymes, such as phosphatases and esterases, and also for the fluorescent detection of zinc. The polyaza macrocycles and their A--functionalized derivatives are particularly well represented. An important aspect of macrocycle synthesis is the use of metal templates to form the ligand. Examples of zinc as a template ion will be discussed where relevant. 

The natural product panepophenanthrin (6/1-170), isolated in 2002 from the fermented broth of the mushroom strain Panus radus IFO 8994 [90], is the first example of an inhibitor of the ubiquitin-activating enzyme [91]. Retrosynthetic analysis based on a biomimetic analysis led to the conjugated diene 6/1-172 by a retro-Diels-Alder reaction via the hemiacetal 6/1-171. Further disconnections of 6/1-172 produces the vinyl stannane 6/1-173 and the vinyl bromide 6/1-174 [92]. 

Several current efforts are focusing on the portability of enzymatic heterocyclization. For example, novel chiral heterocyclic carboxylic acids were produced by using hybrid enzymes [62] (Figure 13.21). Stimulated by biosynthesis pathways, biomimetic heterocyclization methods have also been developed with high efficiency [63]. 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

However, it has to be realized that biological templates remain inserted in the final nanoparticles and this is not acceptable for many applications. Nevertheless, some recent examples indicate that such biomimetic materials may be suitable for the design of biotechnological and medical devices [32]. For instance, it was shown that silica gels formed in the presence of p-R5 were excellent host matrices for enzyme encapsulation [33]. In parallel, biopolymer/silica hybrid macro-, micro- and nanocapsules were recently obtained via biomimetic routes and these exhibit promising properties for the design of drug delivery materials (see Section 3.1.1) [34,35],... 

Capsules with high enzyme loading and activity prepared by templating BMS spheres can also be used as biomimetic reactors [89]. For example, PGA/ PLL capsules with pre-loaded urease (prepared via a BMS sacrificial template) are capable of catalyzing the hydrolysis of urea and have been shown to induce the exclusive formation of CaC03 particles inside the capsules [89]. 

Leonor, I.B., Azevedo, H.S., Alves, C.M. and Reis, R.L. (2003) Effects of the incorporation of proteins and active enzymes on biomimetic caldum-phosphate coatings. Key Engineering Materials, 240-242, 97—100. 

As we increasingly understand the chemistry performed by living systems, in particular that catalyzed by enzymes, we will continue to develop biomimetic methods to achieve some of the special selectivities that enzymes show. Enzymes can selectively bind a particular molecule out of the mixture of substances in the cell, then hold it in such a way that the geometry of the enzyme-substrate com-... 

Much work has been done to help understand how metal ions react or catalyze reactions in solution. Many enzymes also use bound metal ions to catalyze their reactions, and there is still need to understand how they work. When we do understand them in detail, we should be able to produce biomimetic catalysts for useful processes in manufacturing. 

---