TRAM protein

In biological pathways, dehydrations rarely occur with isolated alcohols but instead normally take place on substrates in which the -OH is positioned two carbons away from a carbonyl group. In the biosynthesis of fats, for instance, /3-hydroxybutyry) ACP is converted by dehydration to tram-crotonyl ACP, where ACP is an abbreviation for acyl carrier protein. We ll see the reason for this requirement in Section 11.10. 

TRAM is subject to control through phosphorylation by protein kinase C-e. It is phosphorylated on serine 16 which is located close to the myristoylation site which is TRAM cannot signal without this phosphorylation or if the myristoylation site has been mutated. 

Other proteins also participate in the integration process. One class is composed of molecular chaperones such as SecA in bacteria and Hsp70 or BiP in eukaryotes (Qi and Bernstein, 1999 Schekman, 1994 Mothes et al., 1997 Hamman et al., 1998 Pilon and Schekman, 1999). Another important player in the eukaryotic system is TRAM (translocating chain-associating membrane protein) (Walter, 1992). 

Biochemical studies have also suggested an asymmetric orientation of constituents in lipid globule membrane. By comparison of specific activities of enzymes in washed lipid globules and released membrane, Patton and Trams (1971) suggested that the active site of Mg2+-adeno-sine triphosphatase was accessible to substrates on both faces of the membrane and that of 5 -nucleotidase on the outer membrane face. Recent evidence from studies of Concanavalin A inhibition of globule membrane and plasma membrane 5 -nucleotidase support an outer surface localization for the active site of this enzyme (Carraway and Carra-way 1976 Snow et al. 1980). Kobylka and Carraway (1973) observed that exposure of lipid globules to proteolytic enzymes resulted in cleavage of all major membrane-associated proteins. They concluded that... 

The most stable elements of secondary structure of peptides and proteins are turns, helices, and extended conformations. Within each of these 3D-structures the most commonly found representatives are (3-turns,a-helices, and antiparallel (3-sheet conformations, respectively. y-TurnsJ5 310-helices, poly(Pro) helices, and (3-sheet conformations with a parallel strand arrangement have also been observed, although less frequently. Among the many types of (3-turns classified, type-I, type-II, and type-VI are the most usual, all being stabilized by an intramolecular i <— i+3 (backbone)C=0 -H—N(backbone) H-bond and characterized by either a tram (type-I and type-II) or a cis (type-VI) conformation about the internal peptide bond. In the type-I (3-turn a helical i+1 residue and a quasi-helical 1+2 residue are found, whereas in the type-II (3-turn the i+1 residue is semi-extended and the 1+2 residue is also quasi-helical but left-handed. This latter corner position may be easily occupied by the achiral Gly or a D-amino acid residue. 

To explore the changes in rhodopsin that precede release of all-tram-retinal from the protein, Torn Yoshizawa and Wald measured the optical absorbance changes that occurred when they illuminated rhodopsin at low temperatures. At 77 K, illumination caused the absorption band of the rhodopsin to shift from 500 to 543 nm. The product of this transformation is now called bathorhodopsin. Bathor-hodopsin is stable indefinitely in the dark at 77 K, but if it is warmed above about 130 K it decays spontaneously to a species that absorbs maximally at 497 nm. This is called lumirhodopsin. If the sample is warmed further to about... 

The cobalt center in MeCbl, one of the two important B12 coenzymes, is clearly involved in key steps in catalytic methyl transfer processes. Here, the Co center cycles between Co(I) and Co(III)CH3. In methionine synthase, the proposed mechanism involves direct nucleophilic attack on the C of the Co(III)CH3 group. In model reactions, the thiolate most frequently simply binds tram to the alkyl group to give a product recently established by an x-ray study of a model system. The protein may block access to the Co, thus preventing this reaction common in models. It is likely that the reactive form of the bound cofactor is five-coordinate in the key point in the catalytic cycle. This reactive form will lead to a four-coordinate Co(I) species. The axial coordination of the cofactor by a protein imidazole allows for a finer tuning of the Cbl chemistry and may permit control of the coordination number. Thus, recoordination of Co in the Co(I) state may facilitate attack on methyltetrahydrofolate and re-formation of Co(III)CH3. 

The activities of Fe— and Mn—SODs are decreased above pH 8.5, consistent with the involvement of an ionizable group of the protein with a pKa value between 9 and 10 in the catalytic cycle.77 The most likely residue appears to be Tyr-34, which is only 5 A apart from the Mn.51 A possible route of O2 to the Mn site in T. thermophilus Mn-SOD50 runs across the helix 1 between the Lys-29 and Tyr-34, and the O2 binds to the Mn3+ tram to Asp-163 with octahedral coordination (Figs. 10.7 and 10.8). The route to the metal is lined by aromatic and histidine residues including His-26, His-30, Tyr-34, His-81, Phe-84, Trp-165 and His-167. 

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