Pathways cytoplasmic

This figure is yet another example (see Chapter 5) of what might be considered Molecular Theology. On a more serious note we do not know what, if any, the physiological significance of a cytoplasmic aconitase might be, particularly since we do not know of any mammalian cytoplasmic pathways for isocitrate metabolism. 

The biosynthesis of all proteins starts on free ribosomes (top). However, the paths that the proteins follow soon diverge, depending on which target they are destined for. Proteins that carry a signal peptide for the i (1) follow the secretory pathway (right). Proteins that do not have this signal follow the cytoplasmic pathway (left). 

Cytoplasmic pathway. Proteins that do not have a signal peptide for the ER are synthesized in the cytoplasm on free ribosomes, and remain in that compartment. Special signals mediate further transport into the mitochondria (5 see p. 232), the nucleus (6 see p. 208) or peroxisomes (7). 

Figure 10.7 All terpenes are derived from allylic diphosphates which are polymers of repeating isopentyl units (IPP) put together by the action of prenyltransferases. In plants, IPP can be derived from the mevalonate biosynthetic pathway (a cytoplasmic pathway) or the methyl erythritol phosphate pathway (a plastidic pathway). Monoterpenes are then derived from the CIO precursor geranyl diphosphate (GPP), sesquiterpenes from the C15 precursor famesyl diphosphate (FPP), and diterpenes from the C20 precursor geranylgeranyl diphosphate (GGPP) by the action of terpene synthases or cyclases, which divert carbon into the specific branch pathways.

Figure 12.2 Plastidic and cytoplasmic pathways of active zoapatle diterpenes.

Newman JD, Chappell J (1999) Isoprenoid biosynthesis in plants carbon partitioning within the cytoplasmic pathway. Crit Rev Biochem Mol Biol 34 95-106... 

Brassica napus is a Ci6 3 plant possessing hexadecatiienoic acid (Ci6 3) MGDG [2]. It has been shown that in Ci6 3 plants MGDG is synthesized by both chloroplastic and cytoplasmic pathways, the former giving Cig/Ci6 molecular species and the latter C18/CI8 and C16/C18 molecular species [3]. We report here the effects of a heat shock on the molecular species composition of the main polar lipids in Brassica napus leaves. 

If a complete cytoplasmic pathway is not present in plant cells, the functional significance of the identifiable cytoplasmic isozymes remains to be established. In this context, it is of interest that glyceraldehyde-3-phosphate has been shown to be a better substrate of the cytosolic DAHP-Co " than erythrose-4-phos-phate (R. Jensen, personal communication), and that arabinose-5-phosphate is the preferred substrate of a third enzyme capable of facilitating DAHP synthesis (Morris et ai, 1989). Thus, the cytoplasmic forms of some enzymes apparently associated with aromatic amino acid biosynthesis may catalyze alternate reactions and/or participate in yet unidentified metabolic pathways. 

The enzymes of the glyoxylate cycle in plants are contained in glyoxysomes, organelles devoted to this cycle. Yeast and algae carry out the glyoxylate cycle in the cytoplasm. The enzymes common to both the TCA and glyoxylate pathways exist as isozymes, with spatially and functionally distinct enzymes operating independently in the two cycles. 

Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol (Figure 25.35). The enzymes responsible for this are all associated with the endoplasmic reticulum. The primary pathway involves 7-dehydroeholesterol as the penultimate intermediate. An alternative pathway, also composed of many steps, produces the intermediate desmosterol. Reduction of the double bond at C-24 yields cholesterol. Cholesterol esters—a principal form of circulating cholesterol—are synthesized by acyl-CoA cholesterol acyltransferases (ACAT) on the cytoplasmic face of the endoplasmic reticulum. 

A system of membrane enclosed cisternae in the cytoplasm. The ER is continuous with the outer membrane of the nuclear envelope. The part of the ER coated with ribosomes is called rough ER, the other part is called smooth-surfaced ER. The rough ER is the first compartment of the secretory pathway. Here, membrane proteins are integrated into and secretory proteins translocated across the ER membrane. Furthermore,... 

---