Biosynthetic subunits

Fig. 6.4 Biosynthetic subunits to anatoxin-a from feeding various labeled substrates [13],...

Fig. 6.9 Biosynthetic subunits of saxitoxin as determined by various precursor-feeding studies [104].

Dissection of the chemical structure of jamaicamides A-C led to the speculation that these metabolites derive from a mixture of polyketides (nine acetate units), amino acids (t-Ala and p-Ala), and the S-methyl group of methionine. To map out the biosynthetic subunits of these molecules, isotopically labeled precursors were supplied to I. majuscula JHB, and the labeling patterns discerned by NMR spectroscopy (Figure 6.12). From these experiments, insights were gained into the biochemical transformations that produce the jamaicamides, especially the mechanism of formation of the vinyl chloride group [157]. 

Chaperones. Figure 2 The multiple roles of BiP in the biogenesis of the secretory proteins. BiP, immunoglobulin heavy chain binding protein ER, endoplasmic reticulum ERAD, ER-associated degradation ERj, resident ER protein with J-domain Sec61, core subunit of the protein translocase UPR, unfolded protein response that involves several signal transduction pathways that are activated in order to increase the biosynthetic capacity and decrease the biosynthetic burden of the ER... 

As we have noted, the outcome of a virus infection is the synthesis of viral nucleic acid and viral protein coats. In effect, the virus takes over the biosynthetic machinery of the host and uses it for its own synthesis. A few enzymes needed for virus replication may be present in the virus particle and may be introduced into the cell during the infection process, but the host supplies everything else energy-generating system, ribosomes, amino-acid activating enzymes, transfer RNA (with a few exceptions), and all soluble factors. The virus genome codes for all new proteins. Such proteins would include the coat protein subunits (of which there are generally more than one kind) plus any new virus-specific enzymes. 

Not much has been added to the biochemistry of STRs in the recent years. Therefore, we will only briefly treat the biosynthetic pathway here. Biosynthesis of the aminocyclitol (streptidine) and 6-deoxyhexose (streptose) moieties are well understood, with some remaining gaps in the enzymology. Much less is known about the residual biosynthetic phases, the formation of A-methyl-L-glucosamine, the condensation of subunits, and the late modification steps potentially coupled with export. ... 

The PAs, or condensed tannins, are polymers synthesized from flavan-3-ol monomer units. The phlobaphenes are 3-deoxy-PAs formed from flavan-4-ol monomers. The biosynthesis of both types of PAs follows the biosynthetic route of anthocyanins from chalcones through to the branch points to flavan-3-ol and flavan-4-ol formation. In this section, the specific enzymes forming the monomers are discussed, along with a discussion on the polymerization process. Although the chemistry of tannins is described in detail elsewhere in this book, it is useful to briefly mention the nature of the monomer subunit types and the polymer forms. 

Irreversible degradation of the porphyrin nucleus can be accomplished with oxidants such as chromium trioxide or potassium permanganate. As shown in Scheme 3, the products from the Cr03 reaction are maleimides, and this procedure has been used extensively for structure elucidation, and for tracking down the fate of isotopic labels in biosynthetic studies (B-75MI30702). In the absence of side chains such as formyl and vinyl, which are labile towards Cr03, it is possible to identify the various substituents on porphyrin subunits. 

There is evidence that lignin can be synthesized de novo. This lignin is synthesized locally, and specifically in response to pathogen attack. There is new evidence that this lignin requires different biosynthetic enzymes, which results in a different subunit composition than the lignin of the vascular tissue. 

Type II PKSs such as the bacterial aromatic PKSs are comprised of several mono-or di-domain proteins. Although little is known about the interactions among these proteins, and the relevance of these interactions to enzyme function and selectivity, such interactions are presumably important, given the extremely high lability of the inferred biosynthetic intermediates in these pathways. More recently, protein chemical studies and kinetic analysis have provided evidence for interactions between the ACP and the rest of the minimal PKS, as well as between the auxiliary subunits and the minimal PKS [23,24],... 

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