Peptide sequences, modifications

Peptide sequence modification or addition of a tag region, e.g., a His tag, to reduce the flexibility and/or to increase stability. 

These two studies in particular are important. They demonstrate that not only can Pn peptide self-assembly be adjusted by sequence modification to suit a particular ionic strength and pH (e.g. conditions found in vitro or in vivo) but that it can be also made reversible and responsive. This raises the possibility that these peptides can be used in drug delivery, or as therapeutics that self-assemble post-injection. 

The pharmacokinetics of a radiopharmaceutical can be systematically altered by chemical modification of the targeting biomolecule or metal chelate, and the use of a pharmacokinetic modifying Hnker. The chemical modification of a biomolecule can be achieved by introducing various hydrophiUc or Hpophilic groups onto the side-chains of the targeting biomolecule. Sometimes a peptide sequence, such as polyaspartic acid, can be used to improve the hydrophiUcity. The chemical modification of the metal chelate can be achieved using BFCs with different charge and hydrophiUcity. 

Figure 4.6. Modulation of peptide conformational equilibrium can be achieved through systematic modifications of peptide sequences that direct receptor interactions toward therapeutic response, and away from untoward effects. A systematic peptide modification may lead to reduced concentrations of peptide conformations susceptible to metabolizing enzymes such as peptidases.

Any chemical or enzymatic process that modifies a therapeutic protein in the body is known as protein metabolism. Unfolded proteins are, in general, more susceptible to metabolism via proteolysis because of increased access to critical peptide sequences. Chemical modification of a protein can mark it for further degradation. This is thought to occur by altering the folding equilibrium in favor of the denatured conformation [3]. 

Proteins have a modular structure comprised of motifs and domains. Short peptide sequences bring specificity for certain modifications while in most cases... 

Table 1 Selected human opioid peptide sequences, (with modifications from Straftburger and Friderichs, 2002)...

The first and the second approaches have provided some positive results but, unfortunately, they depend upon the structure of the protein. Typical examples of sequence modifications to improve stability and pharmacokinetics are the preparation of humanized antibodies, where part of the mouse sequence is substituted by the human form, and the granulocyte colony-stimulating factor muteins, where up to seven amino acids are substituted. Examples of truncated sequence proteins with improved characteristic are the 7-36 analogues of glucagon-like peptides or the 1-29 sequence growth hormone-releasing factor [1, 2]. 

MALDI-TOF provides limited capabilities for mixture analysis, LC/MS methods are used to provide more detailed interrogation of protein expression and peptide sequence. The use of LC/MS approaches for protein identification in conjunction with 2-DGE offers distinct advantages such as the ability to handle low picomole (miniaturized) level samples, enhanced separation, detection, the amenability to N-terminally blocked proteins, and fast analysis. The LC/MS methods for protein characterization focus on four distinct goals (1) confirmation of putative sequence, (2) identification of amino acid modifications, (3) identification of known proteins, and (4) sequence determination of unknown proteins. 

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