Polypeptides, synthetic hydrolysis

The secondary structure of the polypeptide chain in hydrolytic enzymes ensures the spatial proximity of the necessary functional groups, which are responsible for the observed catalytic effect. In synthetic enzyme mimics, it is possible to bring the requisite functionalities into close juxtaposition only if there is a rigid framework to which these groups are attached. It was thus logical to examine cyclic peptides, especially cyclodipeptides, which bear the necessary functional groups for their catalytic activity in ester hydrolysis. 

There are a number of useful synthetic applications of these reactions of chelated amino acid esters (Fig. 3-12). For example, if the attacking nucleophile is not a simple amine, but is another amino acid ester or an O-protected amino acid, then peptide or polypeptide esters are formed in excellent yields. This may be developed into a general methodology for the metal-directed assembly (and, in the reverse reaction, the hydrolysis) of polypeptides. 

It can be concluded that the wide specificity of papain observed in studies with synthetic substrates is in agreement with the results of studies on small polypeptides. On the other hand, it will be difficult to predict which types of bonds will be hydrolyzed in a peptide of known composition, inasmuch as several structural features around a bond formed by an amino acid with a structurally favorable side chain must determine the rate of hydrolysis. 

The action of these two pancreatic exopeptidases on synthetic substrates, proteins, and peptides has been reviewed in detail by Neurath (1960). The specificity requirements which were deduced from studies with synthetic peptides have been confirmed by studies with polypeptides. The structural requirements of specific substrates for both types of carboxy-peptidase are analogous except for the nature of the amino acids which contain the free, ionized a-carboxyl group at the terminus of the substrate. Carboxypeptidase B hydrolyzes most rapidly those bonds formed by terminal lysyl and arginyl residues, whereas carboxypeptidase A hydrolyzes terminal bonds formed by a variety of aromatic, neutral, or acidic amino acids. Of the natural amino acids only carboxyl-terminal prolyl residues are resistant to the action of the enzyme. The rate of hydrolysis depends upon the nature of the side chains of the amino acids which form the susceptible bonds. Thus, differences in the rate of hydrolysis of different substrates may vary several thousandfold. The methods for application of these peptidases to hydrolysis of proteins have been discussed in detail by Canfield and Anfinsen (1963). 

The specificity of this enzyme from swine kidney has been established from detailed studies with synthetic substrates (reviewed by Smith and Hill, 1960). All peptide bonds formed by L-amino acids which are adjacent to a free a-amino group are susceptible to hydrolysis, although the rates of hydrolysis vary over a several thousandfold range. The best substrates are those which contain amino-terminal leucine and the poorest are those which contain the amino nitrogen of proline in peptide linkage, e.g., glycyl-proline (Hill and Schmidt, 1962). The action of leucine aminopeptidase on protein and polypeptide substrates (Hill and Smith, 1958, 1959) agrees with the specificity established with synthetic substrates. 

When optically active amino acids are released from peptides by aminopeptidase, they must be of the L-configuration. For this reason, complete hydrolysis of synthetic polypeptides with aminopeptidase provides a convenient means for evaluating the sterochemical homogeneity of synthetic peptides and peptide derivatives (Hofmann et al, 1962). 

Selection of Optical Antipode in the Hydrolysis of Chiral Ester Catalyzed by Synthetic Polypeptide... 

The synthetical polypeptides are completely hydrolysed by boiling with concentrated hydrochloric acid for five hours lO per cent, hydrochloric acid at 100° C. hydrolyses them very slowly, and normal alkali has only a very slight action. Their hydrolysis by enzymes, especially by trypsin, is of such importance that a special section is required for the description of these results. 

It is only by the knowledge of the properties of the synthetical compounds that Fischer has been able to invent methods for isolating them from the mixtures which result by the hydrolysis of the proteins and to identify these compounds. The extension of the study of the higher polypeptides, more especially of the mixed polypeptides, will lead without doubt to the isolation of greater complexes from the products of partial hydrolysis of the proteins, such as the separation of the proteoses and peptones, which from the results so far obtained appear to be more simple than was previously supposed, if salting out by ammonium sulphate of polypeptides containing four and six units, of which tyrosine is one, be taken as a typical example. 

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydrolyzed. Polyamides are normally much more resistant to hydrolysis than polyesters they may be cleaved by the methods usually employed for polypeptides and proteins. 

Several less common monomers have been polymerized anionically. Ree and Minoura" effected the conversion of aminoacetonitrile to NHCH2C(NH) n— by treatment with sodium s-butoxide in dioxan hydrolysis yielded the polypeptide—[NHCHaCO] —. Aminopropionitrile behaved similarly. High polymer has been formed from 8-nitrostyrene using alkoxide ion as initiator in protic solvents. Block copolymer is formed from polyisoprenylpotassium in benzene on adding 1-nitropropene. Vinyl ferrocene has been shown to undergo homo- and co-polymerization by butyl-lithium or lithium metal, but the utility of these synthetic procedures is severely limited by side reactions. The poly-... 

Proteinases catalyze the hydrolysis of peptide linkages in proteins. Those proteinases which have been purified have also been shown to have peptidase activity, that is, they catalyze the hydrolysis of synthetic polypeptides of known structure. Because the proteinases split proteins to yield lower molecular weight peptones, proteoses, and peptides they have been termed endopeptidases they are capable of hydrolyzing peptide bonds that are not terminal. Exopeptidases can hydrolyze only peptide bonds that involve terminal amino acids. The synthesis of peptide substrates has been authoritatively reviewed by Fruton. ... 

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