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PubMed1975

Complexes between synthetic polymer ligands and ferri-delta and ferro-protoporphyrin IX.

The complexes of synthetic polymer ligands, i.e. poly-L-lysine, poly-4-vinyl-pyridine, poly-N-vinyl-2-methylimidazole and the higher branched polyethyleneimine, with ferri- or ferro-protoporphyrin IX were studied from the standpoint of polymer ligand effects by comparison with those of their monomeric model ligand complexes and poly-gamma-benzyl-L-glutamate containing an imidazole nucleus at the chain end. The coordination numbers and formation constants were determined optically and their structures were also estimated. The coordination number of a poly-L-lysine complex was two, but those of other polymer ligand complexes were one. One of the polymer effects, which was indicated by the large formation constants of the polymer complexes, was caused by the increment of the local ligand concentration around the polymer chain. Another was caused by the conformational effect of an alpha-helical structure in the poly-L-lysine complexes. The interaction of a poly-L-lysine-heme complex with molecular oxygen was also studied. An observed pseudo-allosteric phenomenon may be due to the specific structure of a poly-L-lysine complex which is different from those of other polymer ligand complexes.

Allosteric Site↗
PubMed1975

The role of tryptophan residues and hydrophobic interaction in the binding of riboflavin in egg-yolk flavoprotein.

Egg-yolk flavoprotein has 7.2 tryptophan residues exposed, while the apoprotein shows an apparent exposure of 80 percent of these (5.7 residues) with dimethylsulphoxide as the perturbant. In the apoprotein at pH 6.9 only 4 groups are perturbed to ethylene glycol, 3.2 to glycerol and 1.4 to sucrose. Diminishing estimates of exposure obtained with increasing molecular diameter of the perturbant suggests that part of indole chromophores of apoprotein are located in "crevices" of the protein molecule. The apoprotein was treated with 2-hydroxy-5-nitrobenzyl bromide, H2O2 and N-bromosuccinimide under conditions designed to accomplish modification of tryptophan residues. Five to six of the eight tryptophans present in the protein were modified. Under these conditions the apoprotein completely looses its capacity for binding riboflavin and the fluorescent intensity of the protein at 360 nm is quenched at the same time to about 80 percent of its initial value. The presence of nonpolar amino acid residues on the surface of the apoprotein suggested the importance of hydrophobic interactions as the dominant factor controlling the binding of riboflavin. The hydrophobic probes Indocyanine green and 4-benzoylamide-4-aminostilbene-2,2-disulphonic acid bound to the apoprotein giving equimolar complexes with dissocation constants, KD 6.5-10(-7) M and 1.8-10(-6) M, respectively, Addition of an equimolar amount of riboflavin quantitatively displaced these dyes from their complexes with apoprotein as shown by spectrophotometric and spectrofluorometric studies.

Amino Acids↗
PubMed1977

Occurrence of tryptophan in the enzymically active site of diphtheria toxin fragment A.

Tryptophan residues of diphtheria toxin fragment A have been modified by 2-hydroxy-5-nitrobenzyl bromide. Fragment A loses its ability to inactivate ADP-ribosylation of the elongation factor 2, as a function of the number of residues modified. Modification of one tryptophan residue provides a dramatic loss of enzymic activity suggesting the presence of one essential residue of this type in fragment A. Examination of fingerprint maps of chymotryptic peptides of fragment A after reaction with 2-hydroxy-5-nitrobenzyl bromide allows us to identify this tryptophan residue at position 153 of the amino acid sequence. ORD and CD experiments joined to immunochemical studies seem to exclude the possibility that the observed decrease in the enzymic activity may be attributed to a conformational change. Finally, results of nicotinamide-adenine dinucleotide-binding measurements suggest that tryptophan 153 would be concerned with the elongation factor 2 binding site or with the catalytic site itself.

Benzyl Compounds↗