Amino 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.]... 

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. 

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. 

The thioredoxin domain (see Figure 2.7) has a central (3 sheet surrounded by a helices. The active part of the molecule is a Pa(3 unit comprising p strands 2 and 3 joined by a helix 2. The redox-active disulfide bridge is at the amino end of this a helix and is formed by a Cys-X-X-Cys motif where X is any residue in DsbA, in thioredoxin, and in other members of this family of redox-active proteins. The a-helical domain of DsbA is positioned so that this disulfide bridge is at the center of a relatively extensive hydrophobic protein surface. Since disulfide bonds in proteins are usually buried in a hydrophobic environment, this hydrophobic surface in DsbA could provide an interaction area for exposed hydrophobic patches on partially folded protein substrates. 

Even though these enzymes have no absolute specificity, many of them show a preference for a particular side chain before the scissile bond as seen from the amino end of the polypeptide chain. The preference of chymotrypsin to cleave after large aromatic side chains and of trypsin to cleave after Lys or Arg side chains is exploited when these enzymes are used to produce peptides suitable for amino acid sequence determination and fingerprinting. In each case, the preferred side chain is oriented so as to fit into a pocket of the enzyme called the specificity pocket. 

Figure 14.7 Ribbon diagram of one subunit of the globular form of transthyretin. The p strands are labeled A to H from the amino end. Strands C and D are thought to be unfolded to produce the conformation that forms amyloid fibrils. (Adapted from C.C.F. Blake et al., /. Mol. Biol. 121 339-356, 1978.)...

Figure 16.17 The subunit structure of the bacteriophage MS2 coat protein is different from those of other sphericai viruses. The 129 amino acid polypeptide chain is folded into an up-and-down antiparallei P sheet of five strands, P3-P7, with a hairpin at the amino end and two C-terminai a helices. (Adapted from a diagram provided by L. Liijas.)...

BMI was also used as a crosslinking agent for poly(iminoethylene). The Michael addition takes place with the nucleophilic nitrogen of the imino group and the double bonds of the electrophilic BMI. The Michael addition of BMI is now adopted as a crosslinking reaction for polymers with amino end groups [2]. 

The end group of the polymers, photoinitiated with aromatic amine with or without the presence of carbonyl compound BP, has been detected with absorption spectrophotometry and fluororescence spectrophotometry [90]. The spectra showed the presence of tertiary amino end group in the polymers initiated with secondary amine such as NMA and the presence of secondary amino end group in the polymers initiated with primary amine such as aniline. These results show that the amino radicals, formed through the deprotonation of the aminium radical in the active state of the exciplex from the primary or secondary aromatic amine molecule, are responsible for the initiation of the polymerization. 

The polymers initiated by BP amines were found to contain about one amino end group per molecular chain. It is reasonable to consider that the combination of BP and such polymers will initiate further polymerization of vinyl monomers. We investigated the photopolymerization of MMA with BP-PMMA bearing an anilino end group as the initiation system and found an increase of the molecular weight from GPC and viscometrical measurement [91]. This system can also initiate the photopolymerization of AN to form a block copolymer, which was characterized by GPC, elemental analysis, and IR spectra. The mechanism proposed is as follows ... 

For a more complete treatment of the derivations and determination of experimental rate constants (to be discussed briefly below) refer to Ref. 46 for Gramicidin A and Ref. 47 for the malonyl dimer of Gramicidin A. (Malonyl Gramicidin A is formed by deformylation of Gramicidin A and then joining to amino ends together using the malonyl moiety, —CO—CH2—CO—, to form the covalent dimer.)... 

Next, add the four side chains in the positions marked R, R2, Rz, and Rj to give the final stmcture of the peptide. The leftmost amino acid, aspartic acid, is at the amino end. Aspartic acid is followed by methionine, valine, and tyrosine. Tyrosine is at the carboxyl end. Here are the individual amino acids, with their R groups highlighted ... 

The imidazolide group at the amino end of an amino acid is as reactive toward nucleophiles as the imidazolide group at the carboxylic end of an amino acid. If an N-protected amino acid is selected as nucleophile, this method can also be used for peptide synthesis. The amino-activated amino acids, for example N-( 1 -imidazolylcarbonyl)-amino acid esters, are prepared from a-isocyanatocarboxylic acids and imidazole. 

Meijer and co-workers also used a divergent dendrimer synthesis to prepare AB diblock structures (Figure 7.13B) in which the polystyrene linear block is used to initiate growth of the polypropylene imine) dendritic block [45], An amino end group had to be introduced in the polystyrene as a core for subsequent growth of the dendritic fragment via an iterative protocol of sequential... 

These dendritic boxes (Figure 13.7) were synthesized by the conjugation of a chiral shell of protected amino acids onto a flexible polypropylene imine) dendrimer with 64 amino end groups. In solution, the shell was highly hydrogen-bonded and dense-packed, displaying a solid-phase behavior, which was indicated by the low NMR relaxation time of the surface groups [11]. 

Aminopeptidasen spalten Polypeptiketten schrittweise vom freien Amino-ende her. Das meist verwendete Enzym ist hochgereinigte Leuzin-Amino-peptidase aus Schweinenieren (Fasold, Linharl und Turba 30)). Die... 

For most collagens, the folding of the triple helical domain proceeds from the carboxyl end toward the amino end of the trimeric molecule in a zipper-like fashion with a rate that is limited by cis—trans isomerization of peptidyl prolyl bonds." The fast propagation of the triple helix formation is followed by a slower folding... 

Stearoyl-terminated PIPAAms (PIPAAm-CigH35) was obtained by the reaction of the primary amino end group of PIPAAm-NH2 with a large excess of acyl chlorides [23]. A block copolymer of PIPAAm and poly(styrene) (PIPAAm-PSt) was obtained by a condensation reaction... 

These proteolytic enzymes are all endopeptidases, which hydrolyse links in the middle of polypeptide chains. The products of the action of these proteolytic enzymes are a series of peptides of various sizes. These are degraded further by the action of several peptidases (exopeptidases) that remove terminal amino acids. Carboxypeptidases hydrolyse amino acids sequentially from the carboxyl end of peptides. They are secreted by the pancreas in proenzyme form and are each activated by the hydrolysis of one peptide bond, catalysed by trypsin. Aminopeptidases, which are secreted by the absorptive cells of the small intestine, hydrolyse amino acids sequentially from the amino end of peptides. In addition, dipeptidases, which are structurally associated with the glycocalyx of the entero-cytes, hydrolyse dipeptides into their component amino acids. 

Myosin is quantitatively the most important protein in the myofibrils, representing 65% of the total. It is shaped like a golf club (bottom right). The molecule is a hexamer consisting of two identical heavy chains (2 X 223 kDa) and four light chains (each about 20 kDa). Each of the two heavy chains has a globular head at its amino end, which extends into a tail about 150 nm long in which the two chains are intertwined to form a superhelix. The small subunits are attached in the head area. Myosin is present as a bundle of several hundred stacked molecules in the form of a thick myosin filament. The head portion of the molecule acts as an ATPase, the activity of which is modulated by the small subunits. 

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