Chromoplast stroma

Chromoplasts from the daffodil flower (Narcissus pseudonarcissus) are well suited for in vitro investigations on the biosynthesis of tetraterpenoid carotenes. Soluble enzymes of the chromoplast stroma convert the hydrophilic substrate isopen-tenyl diphosphate (IPP) to the liphophilic and membrane associated phytoene, further carotenes are then formed by integrate membrane enzymes (Beyer et al. 1980). 

It turned out that the enzymes which are responsible for monoterpene biosynthesis are present as soluble proteins., in the chromoplast stroma. For qualitative and quantitative analysis of the volatile compounds incubations of chromoplast stroma in... 

The gaschromatographical analysis of the volatile compounds in comparison with the volatile pattern synthesized by whole daffodil flowers (Fig. 1) revealed that the compounds formed in vitro were monoterpene hydrocarbons plus linalool, mainproducts being myrcene (36.4 o), ocimene (18 o) and linalool (22 9 o). Moreover the bicyclic a-pinene (2.8 0 and the monocyclic limonene (l o) were formed. The low portion in cyclic monoterpenes is in good agreement with the natural pattern. Monoterpene ketones as well as sesquiterpenes which both occur in the natural pattern, were not obtained in chromoplast stroma incubations. On the other hand, linalool was not present in the in vivo pattern. 

Studies in the 1960s and 1970s on the cofactor requirements for phytoene synthesis by cell extracts of higher plants produced conflicting results (see Ref. 66). More definitive experiments over the past few years have clarified some of these anomalies. In particular, the enzyme has been purified to homogeneity from the chromoplast stroma of Capsicum annuum, to yield a single protein, 47,500, with an absolute requirement for Mn ", but no other cofactors. This enzyme catalyzes a two-step, kinetically... 

Figure 1. Time dependence of the isomerisation of isopentyl diphosphate and the formation of phytoene in the stroma of daffodil chromoplasts. A at 25 C. B at lO C.

Phytoene synthase is located in the stroma of Capsicum chromoplasts, chloroplasts, and amyloplasts, with the highest activity in the nonphotosynthetic plastids. It has also been found in the stroma of pea fruits and Triticum leaves." " In contrast, it is reported to be located on the chloroplast envelope of spinach" and to be a peripheral membrane protein of the inner membrane of Narcissus chromoplasts. ... 

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