Protein biosynthetic system approaches

The o anization of peptide biosynthetic systems is primarily approached from the amino acid composition of the product. From the protein side, large multifunctional enzymes... 

Direct proteomic approaches can be used to identify protease enzyme proteins, as well as protein categories in the biological system, that are present in secretory vesicles for neuropeptide production and secretion. Knowledge of the secretory vesicle proteome can advance our understanding of neuropeptide biosynthetic mechanisms that operate within this organelle. 

The enzymes responsible for the conversion of lanosterol to cholesterol, as were those for the conversion of farnesyl pyrophosphate to squalene and lanosterol, are all integral membrane-bound proteins of the endoplasmic reticulum. Many have resisted solubiUzation, some have been partially purified, and several have been obtained as pure proteins. As a consequence, much of the enzymological and mechanistic studies have been done on impure systems and one would anticipate a more detailed and improved understanding of these events as more highly purified enzymes become available. Many approaches have been taken to establish the biosynthetic route that sterols follow to cholesterol. Some examples are synthesis of potential intermediates, the use of inhibitors, both of sterol transformations and of the electron transfer systems, and by isotope dilution experiments. There is good evidence that the enzymes involved in these transformations do not have strict substrate specificity. As a result, many compounds that have been found to be converted to intermediates or to cholesterol may not be true intermediates. In addition, there is structural similarity between many of the intermediates so that alternate pathways and metabolites are possible. For example, it has been shown that side-chain saturation can be either the first or the last reaction in the sequence. Fig. 21 shows a most probable series of intermediates for this biosynthetic pathway. 

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