Biosynthesis enzyme compartmentalization

The enzymes of flavonoid biosynthesis are compartmentalized by membranes that surround the organelles and those which separate cytoplasmic regions into different microcompartments. In addition, the association of biosynthetic enzymes as loose aggregates, as well as interaction between constituent enzymes, permit direct transfer of substrates from one enzyme to another and channeling of intermediates and final products to the different sites of accumulation (Hrazdina, 1992 Ibrahim, 1992). Flavonoid derivatives that... 

Figure 34-7 summarizes the roles of the intermediates and enzymes of pyrimidine nucleotide biosynthesis. The catalyst for the initial reaction is cytosolic carbamoyl phosphate synthase II, a different enzyme from the mitochondrial carbamoyl phosphate synthase I of urea synthesis (Figure 29-9). Compartmentation thus provides two independent pools of carbamoyl phosphate. PRPP, an early participant in purine nucleotide synthesis (Figure 34-2), is a much later participant in pyrimidine biosynthesis.

Chlorophyll b occurs as an accessory pigment of the light-harvesting systems in land plants and green algae, and comprises one-third (or less) of total chlorophyll. The biosynthesis of chlorophyll b has been an area of active research particularly regarding its compartmentalization in chloroplast membranes, identification of the gene for chlorophyllide a oxidase, and characterization of the enzymes involved. ... 

In spite of these limitations to our complete knowledge of starch biosynthesis, information about the pathway of starch biosynthesis gained from studies of maize endosperm mutants can probably be generalized to other plant species because related mutants have occurred in peas, sorghum, barley, rice and Chlamydomonas, and because the same enzymes are found in starch-synthesizing tissues in other plant species. Variation in the number of isozymes and their developmental expression, and variations in cellular compartmentation, however, could result in a range of pathways with significant differences. 

Unique subcellular compartmentation is also present in quinolizidine alkaloid biosynthesis, which occurs in the mesophyll chloroplasts of some legumes.158 One of the enzymes catalyzing the last two acylations of the pathway in Lupinus albus occurs in the cytoplasm, whereas the other resides in the mitochondria/59 Although the quinolizidine nucleus appears to be synthesized in the chloroplast, subsequent modifications can occur only after alkaloid intermediates are transported to the cytosol and mitochondia. Quinolizidine alkaloids appear to accumulate in vacuoles of epidermal cells where their defensive properties are most effective. 

AMANN, M., WANNER, G., ZENK, M.H., Intracellular compartmentation of two enzymes of berberine biosynthesis in plant cell cultures. Planta, 1986, 167, 310-320. 

RUEFFER, M AMANN, M, ZENK, M.H., S-Adenosyl-L-methionine columbamine-O-methyl transferase, a compartmentalized enzyme in protoberberine biosynthesis. Plant Cell Rep., 1986,3,182-185. 

Dihydroorotate dehydrogenase, the enzyme catalyzing the dehydrogenation of dihydroorotate to orotate (reaction 4 of the pathway Fig. 15-15), is located on the outer side of the inner mitochondrial membrane. This enzyme has FAD as a prosthetic group and in mammals electrons are passed to ubiquinone. The de novo pyrimidine pathway is thus compartmentalized dihydroorotate synthesized by trifunctional DHO synthetase in the cytosol must pass across the outer mitochondrial membrane to be oxidized to orotate, which in turn passes back to the cytosol to be a substrate for bifunctional UMP synthase. Mammalian cells contain two carbamoyl phosphate synthetases the glutamine-dependent enzyme (CPSase II) which is part of CAD, and an ammonia-dependent enzyme (CPSase /) which is found in the mitochondrial matrix, and which is used for urea and arginine biosynthesis. Under certain conditions (e.g., hyperammonemia), carbamoyl phosphate synthesized in the matrix by CPSase I may enter pyrimidine biosynthesis in the cytosol. 

Biosynthesis of IAA from tryptophan uses the L-form of the amino acid.75 Some of the enzymes that catalyze the conversion of specific intermediates have been identified, and some of the genes coding for the enzymes have been cloned. Such findings establish that plants are competent to carry out such metabolic conversions however, the specific involvement of these genes and intermediates requires confirmation, because biochemical studies carried out with applications to tissue segments or with extracts could disrupt tissue and cellular compartmentalization and because enzymes that catalyze the conversion of tryptophan to IAA in vitro may never come into contact with the intermediates in vivo. Thus, the physiological relevance of some of these pathways remains an open question.69 An additional concern is that many of the enzymes have wide substrate specificities, so it has been difficult to implicate them solely in IAA biosynthesis. Some of the intermediates and enzymes that have been described to have the competence to carry out these reactions are discussed below. 

The biosynthesis of SM exhibits a remarkable complexity. Enzymes are specific for each pathway and are highly regulated in terms of compartmentation, time and space. The same is true for fhe mechanisms of accumulation or the site and time of storage. In general, we find fhaf fissues and organs which are important for survival and multiplication, such as epidermal and bark tissues, flowers, fruits and seeds, have distinctive profiles of SM, and secondary compounds are stored in high amounts in them. As an example, the complex pattern of alkaloid synfhesis, transporf and sforage is illustrated in Fig. 1.7. 

Figure 2.12 A hypothetical view of compartmentation of indole alkaloid biosynthesis in Catharanthus roseus. Enzymes located with dashed arrows are hypothetical and circles indicate membrane associated enzymes (after Meijer et at, 1 993b). Cl OH, geraniol-1 0-hydroxylase NMT, 5-adenosyl-L-methionine 11 -methoxy 2,16-dihydro-16-hydroxytabersonine N-methyltransferase DAT, acetylcoenzyme A deacetylvindoline 1 7-0-acetyltransferase OHT, 2-oxyglutarate-dependent dioxygenase SSpC, strictosidine-((3)-glucosidase SSS, strictosidine synthase.

What is known about the biogenetic routes leading to the biosynthesis of the dimeric akaloids vinblastine and vincristine in C. roseus is represented in Fig. (4) to (6). Enzymes and genes that have been characterized are indicated, and the subcellular compartmentalization of the pathway is presented in Fig. (3). 

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