Internalization Step

It is a general property of internalized toxins and certain viruses that they form channels in membranes. The portion of the toxin molecule associated with channel formation is typically the same as that needed for internalization. This could mean that channel formation is the mechanism that underlies translocation, or that it is an epiphe-nomenon that occurs coincidentally with translocation. In either case, 

Regardless of the true role of channel formation in productive internalization, it is fascinating that microbial toxins, which presumably are ancient molecules, have this property. It is inevitable that investigators will compare the molecular biology and structure of toxin channels with corresponding properties of endogenous channels (e.g. sodium or potassium), and from this deduce something about the evolution of channels. 

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Dunn, R., and L. Hicke. Multiple roles for Rsp5p-dependent ubiquitination at the internalization step of endocytosis. J Biol Chem. 16 16.2001. 

Fig. 3.16 Internal motion of the inserted tube is blocked by the addition of an internal step.

Within steps 10-lla-12-9, the following internal steps are latently present (Eq. 48) ... 

Unlike diphtheria toxin, little is known about the structures required for the translocation of the enzymatic subunit of PT. In diphtheria toxin and Pseudomonas aeruginosa exotoxin A, the B moiety can be clearly subdivided into two distinct domains, one responsible for receptor binding, composed essentially of (3 sheets, and one responsible for translocation of the A subunits, essentially composed of a helices (Allured etal., 1986 Choe etal., 1992). There is no clear translocation domain in PT, and much less is known about the internalization step of PT, compared to diphtheria toxin and exotoxin A. 

It is crucial in performing TS analysis to know exactly which step of the reaction the experimental KIEs reflect. Using isotope-trapping experiments, it is possible to demonstrate whether formation of the Michaelis complex, E-S, is kinetically significant, and if necessary, to find conditions where it is not. However, internal steps can also complicate the interpretation of KIEs. These can include, but are not limited to (1) establishment of equilibria between different enzyme-bound intermediates, (2) isotopically insensitive steps, such as conformational changes in the enzyme or substrate, or (3) substrate channeling. 

An internal step is any step that occurs between formation of the Michael complex, E S, and dissociation of the enzyme product complex, E P. 

The most common instrument using the above principle is the vernier caliper (see Fig. 6.4). These instruments are capable of external, internal, step and depth measurements (Fig. 6.5) and are available in a range of measuring capacities from 150 mm to 1000 mm. 

Figure 6.5. External, internal, step and depth measurement...

Papageogiou, P, et al.. Preparation of Pt/g-AljOj pellets with internal step distribution of catalyst Experiments and theory. J. Catal, 158 pp. 439-451, 1996. 

Figure 8.25 Step fitments (a) external step fitment and (i>) internal step fitment. (Source Jordan I. Rotheiser, Joining of Plastics Handbook for Designers and Engineers, Hanser Publishers, Munich-Hanser Gardner Publications, Inc., Cincinnati, 1999.)...

ISC, International STEP centers. Available at http //www.pdesinc.org/... 

Although the vernier caliper is extremely versatile in its ability to carry out external, internal, step and depth measurements, calipers with special jaws are available for special purpose applications, a few of which are shown in Fig. 6.11. 

The actual internal step of solvent recovery involves more than simple condensation in a heat exchanger. The vacuum pump compresses the vapor (Chapter 2.2.3) removed from the work chamber, which results in condensation. This vapor is then condensed further in a heat exchanger. In the absence of air, heat transfer when condensing a pure solvent vapor can be very rapid. 

The characteristic of this process is that several processes are parallel or cross-carried out i.e., the reduction process for the each catalyst particle proceeds from surface to core step-by-step and the reduction process of whole catalyst in reactor (bed) proceeds from the top (outside) down (internal) step by step, and also reaction process of H2 with N2 forms ammonia on reduced catalysts. Therefore, temperature (t) of catalyst bed, the reduction degree (i ) of catalyst and water vapor concentration (y>) are changing. Trends of different types of reactors at different reduction stages are shown in Fig. 5.26. 

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