Structural requirements of position Aα-157 in fibrinogen for the fibrin-induced rate enhancement of the activation of plasminogen by tissue-type plasminogen activator

The sequence fibrinogen-Aα-(148-160) can mimic part of the fibrin-induced rate enhancement of the activation of plasminogen by tissue-type plasminogen activator. Previously we have reported that the lysine residue at position Aα-157 is crucial. During our further investigations on Aα-157 we found that lysine at position Aα-157 may be replaced by glutamic acid. This unexpected finding prompted us to re-investigate the requirements of this position. We prepared analogues of Aα-(148-160) in which the lysine residue at position Aα-157 was replaced by lysine derivatives (acetyl-lysine, benzyloxycarbonyl-lysine and methanesulphonylethyloxycarbonyl-lysine), acidic residues (aspartic acid and glutamic acid), basic residues (arginine and ornithine), polar residues (glutamine and methanesulphonylethyloxycarbonylornithine), apolar residues (alanine, valine, norleucine and glutamic acid 4-nitrobenzyl ester) and glycine. These analogues were tested for their stimulatory activity. When aspartic acid, glutamic acid 4-nitrobenzyl ester or norleucine is present at position Aα-157 in Aα-(148-160) virtually all stimulatory capacity is lost. With valine at position Aα-157 the stimulatory activity is marginal. None of the other replacements at position Aα-157 caused loss of rate-enhancing properties. From these results we conclude that for the rate-enhancing effect of Aα-(148-160) the side chain of the amino acid residue at position Aα-157 must fulfil certain requirements: there must be one (as in alanine) or no (as in glycine) carbon atom in the side chain, or at least two carbon atoms and a polar group (charged or uncharged) to which a rather bulky group (such as the benzyloxycarbonyl group) or a polar group (such as the methanesulphonylethyloxycarbonyl group) may be attached. The highest activity [even higher than native Aα-(148-160)] was obtained with ornithine, methanesulphonylethyloxycarbonylornithine or methanesulphonylethyloxycarbonyl-lysine at position Aα-157. Chemicals/CAS: Fibrin, 9001-31-4; Fibrinogen, 9001-32-5; Plasminogen, 9001-91-6; Tissue Plasminogen Activator, EC 3.4.21.68