Carotene reaction specificity

The Reaction Specificity of Carotene Dioxygenase Whereasthe principal site of carotene dioxygenase attack is the 15,15 -central bond of p-carotene, there is evidence that asymmetric cleavage also occurs, leading to formation of 8 -, 10 -, and 12 -apo-carotenals, as shown in Figure 2.4. These apo-carotenals are metabolized by oxidation to apo-carotenoic acids, which are substrates for /3-oxidation to retinoic acid and a number of other metabolites. 

Redmond, T.M. et al.. Identification, expression, and substrate specificity of a mammalian beta-carotene 15,15-dioxygenase, J. Biol. Chem., 276, 6560, 2001. Leuenberger, M.G., Engeloch-Jarret, C., and Woggon, W.D., The reaction mechanism of the enzyme-catalyzed central cleavage of beta-carotene to retinal, Ang. Chem. Int. Ed., 40, 2614, 2001. 

Table 3 summarizes some of the properties of PS I reaction center and specific functions of its individual subunits. The purified preparation contains about 100 Chi a molecules per P-700 [9,10]. However, this number can be decreased to about 40 while the order of the Chi a molecules increases [70]. Washing out more of the Chi molecules caused a decrease in the dichroic ratio, indicating that those 40 Chls are the highly oriented primary light-harvesting antenna of the reaction center. It was also shown that the S-carotene is in very close proximity to P-700 and it is highly oriented with respect to the latter [70]. P-700 as well as the primary electron acceptor (Aj) may be composed of specialized Chi a molecules [80]. There are at least three more electron acceptors which are part of the reaction center and their function is to slow down the rate of the reaction and thereby stabilize the redox potential difference [72]. It was not until Malkin and Bearden [81] discovered the bound ferredoxins that this part of PS I started to be understood. Today it appears that at least four different clusters are involved in the electron-accepting site of the... 

How are lipid molecules such as animal steroid molecules like estrogen and /5-carotene related to each other What biosynthetic reactions do these specific molecules have in common ... 

Bacterial p-carotene hydroxylases, CrtZ, were characterized with respect to substrate specificity and reaction mechanism after expression in E. coli. The reactions depend on oxygen and are stimulated by 2-oxoglutarate, ascorbate, and Fe L which is typical for a dioxygenase reaction. Similar mechanistic properties were also found for bacterial ketolases encoded by crtW. These enzymes catalyze the introduction of... 

A more refined test is the Schultz (1924) method for cholesterol, which involves the application of a mixture of concentrated sulfuric acid and glacial acetic acid to sections which have been oxidized with ferric ammonium sulfate (iron alum). A blue-green color results. The iron alum apparently oxidizes 3-hydroxy steroids and their esters to 7-oxy steroids, which give the Lifschlltz color reaction on the application of the acids. (Fieser and Fieser, 1949, p. 234). This test is considered to be more specific for a limited group of steroids than is the sulfuric acid method cited above, which reveals a large number of unsaturated polycyclic compounds. Recently, however, Kent (1952) has reported that a positive reaction also occurs with carotene. 

During the preparation of RC I and LHC I from PS 1-200 a considerable amount of chi (31 molecules) and B-carotene (7 molecules) is selectively lost as free pigments, as can be deduced from Tab. 1. This specific loss of the most lipophilic pigments of the thylakoid membranes leads to the suggestion, that these pigments are involved in the binding of the peripheral antenna to reaction centre polypeptides and allow an efficient energy transfer from the peripheral antenna to the core antenna. 

Normal-phase Spherisorb silica (3 pm) as well as ODS-1 and ODS-2 columns and Nucleosil C18 RP (3-pm) columns were used to resolve radiolabeled reaction products formed in specific carotenogenic enzymatic reactions, such as those catalyzed by P-carotene hydroxylase, lycopene cyclase, and phytoene synthase isocratic elution with a variety of mobile phases was used (179). 

PD Fraser, M Albrecht, G Sandmann. Development of high-performance liquid chromatographic systems for the separation of radiolabelled carotenes and precursors formed in specific enzymatic reactions. J Chromatogr 645 265-272, 1993. 

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