Amino acids: characteristic chemical structure

The DQFCOSY spectrum of RpII in D O is shown in Figure 2. Each cross peak in this spectrum identifies a pair of coupled spins of the amino acid side chains. Since couplings are not propagated efficiently across amide bonds, all groups of coupled spins occur within individual amino acids. The chemical structure of an amino acid side chain is reflected in the characteristic coupling network and chemical shifts (13). Valine spin system (CH-CH-(CH3)2) explicitly shown in Figure 2 as an example. 

Of the twenty amino acids that are normally found in proteins, only two contain sulfur, cysteine and methionine. Cysteine has long been recognized as being easily oxidized and this oxidation is associated with the loss of biological activity of many proteins. In recent years, it has been shown that methionine also shares these characteristics. Methionine was first isolated by Mueller19 and was one of the last amino acids discovered. Its structure was later proven to be y-methylthio-a-aminobutyric acid by Barger and Coyne20 who named the amino acid methionine as a contraction for its chemical name. 

The specificities of the various digestive exo- and endopep-tidases suggest that they act synergistically to fulfill a major nutritional function. The concerted action of trypsin, chy-motrypsin, pepsin, and carboxypeptidases A and B facilitate and ensure formation of essential amino acids. The chemical characteristics and metalloenzyme nature of two bovine exopeptidases, lens aminopeptidase and pancreatic carboxy-peptidase A, indicate similarities in their mechanisms of action. However, the aminopeptidase exhibits an unusual type of metal ion activation not observed unth carboxy-peptidase. Chemical and physicochemical studies reveal that the latter enzyme has different structural conformations in its crystal and solution states. Moreover, various kinetic data indicate that its mode of action toward ester substrates differs from that toward peptide substrates. The active site metal atom of carboxypeptidase figures prominently in these differences. 

Solid state 13C CPMAS NMR spectra of Wheat High Molecular Weight (W.HMW) subunits show well resolved resonances identical with spectra of dry protein and peptide samples [24], Most of the amino acids side-chain resonances are found in the 0-35 ppm region followed by the alpha resonances of the most abundant amino acids glycine, glutamine and proline at chemical shifts of 42, 52 and 60 ppm, respectively, and the carbonyl carbons show a broad peak in 172-177 ppm region. The CPMAS spectra of hydrated whole HMW provides important information on the structural characteristics. 

Beyond their practical value, extremophile enzymes present scientists with a fundamental puzzle. Fike all molecular characteristics, their exceptional stability must originate in their chemical structures. However, it is not yet certain what structural features determine these properties. What is known is that in their active folded form, cold-resistant enzymes appear to have relatively fewer structure-stabilizing interactions between different parts of the amino acid chain. As a result, they remain more flexible at a lower temperature than ordinary enzymes but unfold and lose their activity more quickly as the temperature is raised. Conversely, heat-resistant enzymes seem to have a larger number of... 

Stabilization of a P-hairpin structure can be achieved in two ways, promoting a stable (or restricted) turn structure (as done with mimetics) or linking the two arms either chemically, or, more naturally, by hydrophobic interactions. In an approach to utilizing both methods, a D-Pro-Gly linkage was used to stabilize a left-handed turn (type I or II ) and various charged and hydrophobic residues were used to stabilize the molecule and enhance the interaction between arms. I252"254 Examples of these peptides studied in nonaqueous solution by IR, VCD and NMR spectroscopy exhibit characteristics of well-formed hairpins. 255 Alternatively, in aqueous solution, IR, VCD, and ECD results for related peptides agree with the NMR interpretation of conformations characterized as hairpins stabilized at the turn and frayed at the ends. 256 These latter results also have a qualitative match with theoretical simulations. Recently, examples of hydrophobically stabilized hairpins studied by NMR spectroscopy have avoided use of a nonnatural amino acid. 257,258 ... 

---