Aspartic acid residue

Mammals, fungi, and higher plants produce a family of proteolytic enzymes known as aspartic proteases. These enzymes are active at acidic (or sometimes neutral) pH, and each possesses two aspartic acid residues at the active site. Aspartic proteases carry out a variety of functions (Table 16.3), including digestion pepsin and ehymosin), lysosomal protein degradation eathepsin D and E), and regulation of blood pressure renin is an aspartic protease involved in the production of an otensin, a hormone that stimulates smooth muscle contraction and reduces excretion of salts and fluid). The aspartic proteases display a variety of substrate specificities, but normally they are most active in the cleavage of peptide bonds between two hydrophobic amino acid residues. The preferred substrates of pepsin, for example, contain aromatic residues on both sides of the peptide bond to be cleaved. 

Steps 1-2 of Figure 27.14 Epoxide Opening and Initial Cyclizations Cyclization is initiated in step 1 by protonation of the epoxide ring by an aspartic acid residue in the enzyme. Nucleophilic opening of the protonated epoxide by the nearby 5,10 double bond (steroid numbering Section 27.6) then yields a tertiary carbo-cation at CIO. Further addition of CIO to the 8,9 double bond in step 2 next gives a bicyclic tertiary cation at C8. 

Aspartic peptidases bind and activate water via two aspartic acid residues. 

Another competing cyclisation during peptide synthesis is the formation of aspartimides from aspartic acid residues [15]. This problem is common with the aspartic acid-glycine sequence in the peptide backbone and can take place under both acidic and basic conditions (Fig. 9). In the acid-catalysed aspartimide formation, subsequent hydrolysis of the imide-containing peptide leads to a mixture of the desired peptide and a (3-peptide. The side-chain carboxyl group of this (3-peptide will become a part of the new peptide backbone. In the base-catalysed aspartimide formation, the presence of piperidine used during Fmoc group deprotection results in the formation of peptide piperidines. 

Hydrophobic interaction chromatography (HIC) can be considered to be a variant of reversed phase chromatography, in which the polarity of the mobile phase is modulated by adjusting the concentration of a salt such as ammonium sulfate. The analyte, which is initially adsorbed to a hydrophobic phase, desorbs as the ionic strength is decreased. One application demonstrating extraordinary selectivity was the separation of isoforms of a monoclonal antibody differing only in the inclusion of a particular aspartic acid residue in the normal, cyclic, or iso forms.27 The uses and limitations of hydrophobic interaction chromatography in process-scale purifications are discussed in Chapter 3. 

Cacia, J., Keck, R., Presta L. G., and Frenz, J., Isomerization of an aspartic acid residue in the complementarity-determining regions of a recombinant antibody to human IgE identification and effect on binding affinity, Biochemistry, 35, 1897, 1996. 

DW Urry, SQ Peng, TM Parker, DC Gowda, RD Harris. Relative significance of electrostatic-induced and hydrophobic-induced pKa shifts in a model protein—The aspartic acid residue. Angew Chem Int Ed 32 1440-1442, 1993. 

Reactivity of Aspartic Acid Residues 6.3.3.1. Reaction Mechanisms... 

It is a common observation that a peptide bond formed by an aspartic acid residue is cleaved in dilute acids at a rate at least 100-fold faster than other peptide bonds. Three reaction mechanisms are now known to account for this specific reactivity [9] [90—95]. 

Fig. 6.27. Simplified representation of the reaction mechanisms by which aspartic acid residues can facilitate the hydrolytic cleavage of Asp-Xaa or Xaa-Asp peptide bonds. Pathway a begins as a nucleophilic attack of the C-flanking N-atom (i.e., the amide N-atom of the n+1 residue) at the carbonyl C-atom of the Asp side chain. Pathway b, which cleaves the Asp-Xaa bond, begins as a nucleophilic attack internal to the Asp residue. Pathway c, which cleaves the Xaa-Asp bond, is analogous to Pathway b, except that the attack is at the carbonyl C-atom of the N-flanking residue (i.e., the carbonyl of the n-1 residue).

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