Translocation hypotheses

If proton translocation were involved, it would be important to know where the proton is removed. The 9- and 13-methyl groups of retinal cannot be involved, as 9-and 13-desmethylrhodopsins did form batho intermediates [145]. Synthetic retinals 3 [145], 4 [146,147] and 5 [148] missing the crucial hydrogens assumed to intervene in the proton translocation hypothesis, were prepared and were found to form rhodop-sin analogs. Rhodopsin analogs from 3, 4 and 5 all formed red-shifted, batho... 

No final comprehensive hypothesis of the chain of events in biological calcification processes can be constructed at the present time. All current major theories on the sequence of events — leading to translocation of mineral ions into initial mineral phase — involve overcoming the energy of activation required for forming the first primitive stable mineral clusters. Then calcium and phosphate are present at levels... 

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

Blobel discovered the mechanism for protein translocation across the endoplasmic reticulum membrane—the signal hypothesis. 

Figure 3.3 The action mechanism of mitochondrial H+-ATP-synthase the hypothesis of substrate translocation, (a) 2H+ transferred through F0 to catalytic site / j are consumed in ATP synthesis (in ADPOPOf scheme) and H20 from ADP and phosphite (ADPO3 and OPO], respectively). The following two H+ ions may enter the reaction only after ATP release from the catalytic site. They rest in X groups up to the moment, when [Mg2+ ADPOP03 ]2+complex is removed from this site by electric repulsion force, which is stipulated by the presence of +XH group, (b) H+ ions participating in H20 production (not shown in the scheme) differ from those symported with synthesized ATP as it is removed from the catalytic site [22].

Several laboratories have studied the assimilation of specific lysosomal enzymes using as model systems skin fibroblasts deficient in the enzyme under study. The underlying mechanism for the translocation of lysosomal enzymes was hypothesized to involve binding of carbohydrate-containing recognition markers to specific cell surface receptors (1 5). In support of this hypothesis Hickman, Shapiro, and Neufeld (16J found that treatment of N-acetyl-B-hexosaminidase with periodate under conditions that dTd not affect enzymatic activity prevented the efficient assimilation of this enzyme by Sandhoff fibroblasts. Additionally, Kresse and von Figura (1 7) found that treatment of f -acetyl-a-hexosaminidase with B-galactosidase reduced the assimilation of this enzyme by San-filippo B fibroblasts. 

Fig. 7. Hypothesis for the action of ACTH in increasing the level of steroidogenic enzymes, in this case 17a-hydroxylase. In this model, cyclic AMP activates cyclic AMP-dependent protein kinase which by a series of unknown steps results in the accumulation of mRNA coding for a regulatory protein (17a-RP). After translation of this mRNA, the 17a-RP is hypothesized to translocate to the nucleus, where it activates the transcription of the cytochrome /M50,7 gene. This is one of several hypotheses which account for the sensitivity of cytochrome P-450 gene expression to inhibition of protein synthesis. From Ref. 27.

An alternate pathway for starch synthesis has been proposed, which is based on the finding of a putative ADPGlc translocator in the envelope of both amyloplasts and chloroplasts. Akazawa et al. (1991) proposed that ADPglucose is synthesized in the cytosol by the sucrose synthase (rather than in the plastid by the action of the ADPGlc PPase, as is widely accepted), and is then transported into the plastid where it is converted into starch by the starch synthase. A critique of this hypothesis is presented in the chapter, The Site of Starch Synthesis in Nonphotosynthetic Plant Tissues The Amyloplast, where metabolite transport into the plastids is discussed, but it is worth mentioning here that this pathway does not fulfill the criteria mentioned in the preceding—that is, the experimental evidence does not support this alternative pathway. 

To test the hypothesis that phosphate supply from the can limit the rate of photosynthesis (Sivak and Walker, 1986), antisense experiments were performed by Schultz et al. (1993). A cDNA for the potato triose phosphate translocator was identified and a fragment of this cDNA in reverse orientation was expressed in trangenic potato plants under the control of the constitutive cauliflower mosaic virus 35S promoter (Rismeier et al., 1993). This experiment confirmed that Pi supply can limit photosynthesis since a... 

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