Carboxy end

Figure 2.7 (a) Illustration of the twist of (3 sheefs. Befa sfrands are drawn as arrows from the amino end to the carboxy end of the p strand in this schematic drawing of fhe protein thioredoxin from E. coli, fhe sfrucfure of which was defermined in the laboratory of Carl Branden, Uppsala, Sweden, fo 2.8 A resolution. The mixed p sheet is viewed from one of ifs ends, (b) The hydrogen bonds between the P strands in the mixed p sheet of fhe same profein. [(a) Adapfed from B. Furugren.]... 

Figure 4.8 The active site in all a/p barrels is in a pocket formed by the loop regions that connect the carboxy ends of the p strands with the adjacent a helices, as shown schematically in (a), where only two such loops are shown, (b) A view from the top of the barrel of the active site of the enzyme RuBisCo (ribulose bisphosphate carboxylase), which is involved in CO2 fixation in plants. A substrate analog (red) binds across the barrel with the two phosphate groups, PI and P2, on opposite sides of the pocket. A number of charged side chains (blue) from different loops as welt as a Mg ion (yellow) form the substrate-binding site and provide catalytic groups. The structure of this 500 kD enzyme was determined to 2.4 A resolution in the laboratory of Carl Branden, in Uppsala, Sweden. (Adapted from an original drawing provided by Bo Furugren.)...

Figure 4.13 (a) The active site in open twisted a/p domains is in a crevice outside the carboxy ends of the P strands. This crevice is formed by two adjacent loop regions that connect the two strands with a helices on opposite sides of the P sheet. This is illustrated by the curled fingers of two hands (b), where the top halves of the fingers represent loop regions and the bottom halves represent the P strands. The rod represents a bound molecule in the binding... 

In almost every one of the more than 100 different known a/p structures 1 of this class the active site is at the carboxy edge of the p sheet. Functional residues are provided by the loop regions that connect the carboxy end of the strands with the amino end of the a helices. In this one respect a fun-I damental similarity therefore exists between the a/p-barrel structures and the I open a/p-sheet structures. 

Figure 4.15 Schematic diagram of the enzyme tyrosyl-tRNA synthetase, which couples tyrosine to its cognate transfer RNA. The central region of the catalytic domain (red and green) is an open twisted a/p stmcture with five parallel p strands. The active site is formed by the loops from the carboxy ends of P strands 2 and S. These two adjacent strands are connected to a helices on opposite sides of the P sheet.

Figure 4.19 Schematic and topological diagrams for the structure of the enzyme carboxypeptidase. The central region of the mixed p sheet contains four adjacent parallel p strands (numbers 8, 5, 3, and 4), where the strand order is reversed between strands 5 and 3. The active-site zinc atom (yellow circle) is bound to side chains in the loop regions outside the carboxy ends of these two p strands. The first part of the polypeptide chain is red, followed by green, blue, and brown. (Adapted from J. Richardson.)...

Figure 4.21 The polypeptide chain of the arabinose-binding protein in E. coli contains two open twisted a/P domains of similar structure. A schematic diagram of one of these domains is shown in (a). The two domains are oriented such that the carboxy ends of the parallel P strands face each other on opposite sides of a crevice in which the sugar molecule binds, as illustrated in the topology diagram (b). [(a) Adapted from J. Richardson.)...

The a/p-barrel structure is one of the largest and most regular of all domain structures, comprising about 250 amino acids. It has so far been found in more than 20 different proteins, with completely different amino acid sequences and different functions. They are all enzymes that are modeled on this common scaffold of eight parallel p strands surrounded by eight a helices. They all have their active sites in very similar positions, at the bottom of a funnel-shaped pocket created by the loops that connect the carboxy end of the p strands with the amino end of the a helices. The specific enzymatic activity is, in each case, determined by the lengths and amino acid sequences of these loop regions which do not contribute to the stability of the fold. 

Dimerization of pairs of Cro monomers depends primarily on interactions between p strand 3 from each subunit (Figure 8.4). These strands, which are at the carboxy end of the chains, are aligned in an antiparallel fashion and hydrogen bonded to each other so that the three-stranded p sheets of the monomers form a six-stranded antiparallel p sheet in the dimer (Figure 8.5). 

The active site of subtilisin is outside the carboxy ends of the central p strands analogous to the position of the binding sites in other a/p proteins as discussed in Chapter 4. Details of this active site are surprisingly similar to those of chymotrypsin, in spite of the completely different folds of the two enzymes (Figures 11.14 and 11.9). A catalytic triad is present that comprises residues Asp 32, His 64 and the reactive Ser 221. The negatively charged oxygen atom of the tetrahedral transition state binds in an oxyanion hole,... 

The L and the M subunits are firmly anchored in the membrane, each by five hydrophobic transmembrane a helices (yellow and red, respectively, in Figure 12.14). The structures of the L and M subunits are quite similar as expected from their sequence similarity they differ only in some of the loop regions. These loops, which connect the membrane-spanning helices, form rather flat hydrophilic regions on either side of the membrane to provide interaction areas with the H subunit (green in Figure 12.14) on the cytoplasmic side and with the cytochrome (blue in Figure 12.14) on the periplasmic side. The H subunit, in addition, has one transmembrane a helix at the car-boxy terminus of its polypeptide chain. The carboxy end of this chain is therefore on the same side of the membrane as the cytochrome. In total, eleven transmembrane a helices attach the L, M, and H subunits to the membrane. 

Five of the six loop regions (G1-G5 in Figure 13.4) that are present at the carboxy end of the p sheet in the Ras structure participate in the GTP binding site. Three of these loops, G1 (residues 10-17), G3 (57-60), and G4 (116-119), contain regions of amino acid sequence conserved among small GTP-binding proteins and the Ga subunits of trimerlc G proteins. 

An implication of the kinetic analysis presented in Sec. IV.A is that the rate of chain scission of polyesters can be retarded by endcapping to reduce the initial carboxylic acid end-group concentration. Alternatively, the rate may be increased by acidic additives that supplement the effect of the carboxy end groups. The first expectation was confirmed by partial ethanolysis of high molecular weight... 

FIGURE 25 Reduction in the rate of hydrolytic chain scission of PCL achieved by ethoxylation of the carboxy end groups of the polymer. The experimental result is compared with calculated predictions of the effect of varying degrees of ethoxylation. (From RGf. 49.)... 

In these processes hydroxy and carboxy end groups and salts capable to reinitiate copolymerization are generated. New end groups can again be formed in termination reactions. In fact, Eqs. (77) and (78) describe a chain transfer. This is also supported by the fact that during copolymerization about 10 chains are formed per one molecule of the tertiary amine at equimolar epoxide — to — anhydride ratio 39-44). 

The system may be regarded as involving a Na+/Mg2+ co-catalysed phosphorylation step and a K+ catalysed dephosphorylation. Each phosphorylation/dephosphorylation step involves a pseudorotation of an Mg2+-stabilised 5-coordinate intermediate, resulting in transport of the alkali metal cations. The cation transport ability of the enzyme is a direct result of the enzymatic reactivity of the protein. There are three binding sites with high Na+ affinity and two with K+ affinity (occupied by Rb+ in the crystal structure determination). The structure (which is of the E2K state of the system) reveals that carboxy end of the a-subunit is held in a pocket in between transmembrane helices and acts as an unusual regulating element that controls sodium affinity and may be influenced by the membrane potential. 

Deffieux et al. have already prepared a-styrenyl-cw-acetal heterodifunctional vinylic polymers using living anionic polymerizations [26-30]. The heterotelechelic polystyrene chains containing a-hydroxy-cw-carboxy end groups using free radical polymerization were also prepared, and the intramolecular cyclization (unimolecular process) was examined [37]. The... 

Copolymers are prepared by the reaction of a carboxy end-blocked diorganopolysiloxane, and organodicarboxy-termi-corona esp.onwire nated imide and an and cable organodiisocyanate to... 

Styryl carbanions readily react with carbon dioxide 31) to yield carboxy end groups. These terminal groups are also introduced by reaction with anhydrides 19) whereas the use of oxirane 23) leads to the formation of hydroxy end groups esters and nitriles are used to introduce carbonyl functions at the chain end. These reactions can be carried out at low temperature in THF solution and proceed quantitatively if no deactivation by protons occurs. 

In addition, telomers which exhibit carboxy end-groups have been esterified with telechelic polyether diols and a,co-difunctional PDMS have been mainly used as the central block, as in the three following examples [132-134] ... 

Such polyesters are prepared by condensation in xylene of isophtalic acid with a slight excess of 1,5-pentanediol. The molecular weights are adjusted by the ratio diol/diacid. The carboxy end groups are... 

Size, Surface Occupied by Carboxy End Groups and Conformation... 

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