Steric blocking model

A series of striking observations, beginning in the early 1960s, led to the formulation of the so-called steric-blocking model, which has been the major paradigm for the mechanism of thin filament activation (reviewed by Cohen and Vibert, 1987). 

X-ray diffraction studies on intact muscle suggest that the binding of Ca + to troponin induces changes in the relationship of tropomyosin and actin on thin filaments (Haselgrove, 1972 Huxley, 1972 Parry and Squire, 1973 Kress et al., 1986). These studies are the basis for the well-known steric-blocking model of... 

Figure 7.5 Model of ferritin (and erythroid a-aminolaevulinate synthase) translation/ribosome binding regulation by IRP. In (a), with IRP not bound to the IRE (1) binding of the 43S preinitiation complex (consisting of the small ribosomal 40S subunit, GTP and Met-tRNAMet) to the mRNA is assisted by initiation factors associated with this complex, as well as additional eukaryotic initiation factors (elFs) that interact with the mRNA to facilitate 43S association. Subsequently (2), the 43S preinitiation complex moves along the 5 -UTR towards the AUG initiator codon, (3) GTP is hydrolysed, initiation factors are released and assembly of the 80S ribosome occurs. Protein synthesis from the open reading frame (ORF) can now proceed. In (b) With IRP bound to the IRE, access of the 43S preinitiation complex to the mRNA is sterically blocked. From Gray and Hentze, 1994, by permission of Oxford University Press.

One postulate which does resolve many of the inconsistencies in the present data base is that there is a strong tendency towards alternation in these polymers. Such alternation could result from a combination of an intrinsically greater reactivity of the ortho-relative to para-positions of coordinated phenoxide, coupled with an incibility of radicals of the type 5 to attack coordinated phenoxide at the ortho-position due to steric blocking. (An examination of models shows this to be a plausible assumption). ... 

Figure 2 Possible model for quadruplex formation inhibiting transcription by acting as a steric block, in or near the promoter region...

---