Isomerizations and bioconversions

The retinal pool of 11 -cis-retinal is clearly critical for photon detection and visual signal transduction. There are two key aspects regarding this that are important to consider the precursor(s) from which 11 -cis-retinal derived, and, bioconversion of isomerized trans-retinal back to 11-cis-retinal that can recombine with opsin to re-form rhodopsin. The latter aspect is obligatory for restoration of the dark state, regenerating a photosensitive receptor capable of undergoing another cycle of photon detection and signal transduction. Considered together, these events represent the retinoid cycle in the visual process (22). The entire process of re-isomerization and formation of a new rhodopsin molecule occurs in two different retinal tissues and involves several enzymatic steps that are described below (see Fig. 7.14). 

Reversed micelles have also shown to be useful not only in bioconversions, but also in organic synthesis. Shield et al. (1986) have reviewed this subject and brought out its advantages in peptide synthesis, oxidation or reduction of steroids, selective oxidation of isomeric mixtures of aromatics, etc. In the oxidation of aromatic aldehydes to carboxylic acids with enzymes hosted in reverse micelles, the ortho substituted substrates react much more slowly than other isomers. 

Extremely thermostable amylases and pullulanases, active above 100 °C and stable/active at acidic pH values wiU significantly improve the industrial starch bioconversion processes of liquefaction, saccharification, and isomerization. The lack of enzymes with these characteristics means that the bioconversion of starch to glucose and fructose must be carried out in a multistage process (step 1 pH 6.0 to 6.5, 95 to 105 °C step 2 pH 4.5, 60 to 62 °C step 3 pH 7.0 to 8.5, 55 to 60 °C),... 

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