Hæmin-Protein Binding in Peroxidase and Methæmalbumin

Aggregation Behavior of Giant Amphiphiles Prepared by Cofactor Reconstitution

We report on biohybrid surfactants, termed "giant amphiphiles", in which a protein or an enzyme acts as the polar head group and a synthetic polymer as the apolar tail. It is demonstrated that the modification of horseradish peroxidase (HRP) and myoglobin (Mb) with an apolar polymer chain through the cofactor reconstitution method yields giant amphiphiles that form spherical aggregates (vesicles) in aqueous solution. Both HRP and Mb retain their original functionality when modified with a single polystyrene chain, but reconstitution has an effect on their activities. In the case of HRP the enzymatic activity decreases and for Mb the stability of the dioxygen myoglobin (oxy-Mb) complex is reduced, which is probably the result of a disturbed binding of the heme in the apo-protein or a reduced access of the substrate to the active site of the enzyme or protein.

Haem propionates control oxidative and reductive activities of horseradish peroxidase by maintaining the correct orientation of the haem

The role of haem propionates in oxidative and reductive reactions catalysed by horseradish peroxidase (HRP) was studied after successful reconstitution of ferric protoporphyrin IX dimethyl ester (PPDME) into the apoperoxidase. The reconstituted enzyme oxidizes neither guaiacol (aromatic electron donor) nor iodide or thiocyanate (inorganic donor). Although the reconstituted enzyme binds guaiacol with a similar Kd (13 mM) to that of the native enzyme (10 mM), the Kd for SCN- binding (5 mM) is decreased 20-fold compared with that of the native enzyme (100 mM). This indicates that haem propionates hinder the entry or binding of inorganic anion to the active site of the native HRP. However, the reconstituted enzyme is catalytically inactive as it does not form spectroscopically detectable compound II with H2O2. CD measurements indicate a significant loss of haem CD spectrum of the reconstituted enzyme at 409 nm, suggesting a loss of asymmetry of the haem-protein interaction. Thus the inability of the reconstituted enzyme to form catalytic intermediates results from the change in orientation of the haem due to loss of interactions via the haem propionates. HRP also catalyses reductive reactions such as reduction of iodine (I+) in the presence of EDTA and H2O2. The reconstituted enzyme cannot catalyse I+ reduction because of the loss of I+ binding to the haem propionate. Since I+ reduction requires formation of the catalytically active enzyme-I+-EDTA ternary complex, the loss of reductive activity is primarily due to the loss of active enzyme formation. Haem propionates thus play a vital role in the oxidative and reductive reactions of HRP by favouring the formation of catalytic intermediates with H2O2 by maintaining the correct orientation of the haem with respect to the surrounding residues.

Lysis of malarial parasites and erythrocytes by ferriprotoporphyrin IX-chloroquine and the inhibition of this effect by proteins

Ferriprotoporphyrin IX(FP) lysed both erythrocytes and isolated Plasmodium falciparum as judged by decrease in turbidity of erythrocyte and parasite suspensions. The lytic effect of FP on erythrocytes was enhanced by chloroquine (CQ). In the presence of 2.5-20 microM CQ, 5 microM FP led to complete hemolysis within 45 min. However, the lytic effect of FP or FP-CQ on both erythrocytes and parasites was inhibited completely by proteins. The protein inhibition was non-specific. This finding, the failure of FP and FP--CQ to cause hemolysis and lysis of malarial parasites in a protein-containing medium, does not support the "FP--CQ complex hypothesis" for the antimalarial action of chloroquine.

Interaction of human serum albumin with hematoporphyrin and its Zn(2)+-and Fe(3)+-derivatives

Human serum albumin at pH values above 6.8 has one strong binding site for hematoporphyrin; the stability constant of the 1:1 complex is about 10(6) M-1 as determined by Scatchard plot after estimation of the bound hematoporphyrin-induced quenching of the fluorescence emitted by the single tryptophanyl residue of the protein. Determination of the tryptophan-to-hematoporphyrin energy transfer efficiency yields a Förster parameter R0 of 6.2 - 6.9 nm, depending on the value chosen to represent the donor-acceptor mutual orientation, and a tryptophan-to-hematoporphyrin distance of about 1.7 nm. Zn2+- and Fe3+-hematoporphyrin also give a 1:1 complex with albumin, probably binding at the same site as hematoporphyrin, as shown by the identity of the energy transfer parameters; however, the metal ions do not appear to be involved in the formation of the albumin-porphyrin complex. The albumin-hematoporphyrin interaction is drastically affected by the pH of the medium; below pH 6.5 we find a large number of binding sites with weak affinity for hematoporphyrin, which disappear upon increasing the pH. The main site, below pH 6.5, has an affinity comparable with that of the secondary sites. Circular dichroism studies show that the pH effect is due to a change in the protein conformation leading to different interactions between bound porphyrin and specific amino acid side chains.

Studies on the uptake of porphyrin by isolated rat liver mitochondria

1. The uptake of deuteroporphyrin by isolated rat liver mitochondria proceeds by two different mechanisms, a passive binding, and a mechanism sensitive to CCCP plus valinomycin, with different pH, temperature and time dependencies. 2. The CCCP plus valinomycin-sensitive uptake of deuteroporphyrin parallels the transmembrane potassium gradient ([K+in]/[K+out]). 3. Only that deuteroporphyrin taken up in parallel to the transmembrane potassium gradient is accessible to ferrochelatase. 4. The uptake of deuteroporphyrin at high concentrations is followed by series of damaging effects on the mitochondria: uncoupling, dissipation of the mitochondrial energy potential, increased ion permeability and leakage of endogenous potassium. 5. The detrimental effects of porphyrins at high concentrations on mitochondrial structure might explain the apparently unrelated metabolic aberrations characteristic of certain porphyric diseases.

The binding of porphyrins by ligandin

Spectrophotometric and equilibrium-dialysis measurements show that ligandin (glutathione S-transferase B, EC 2.5.1.18) binds monomeric porphyrins at a single site with association constants in the range 10(4)-10(6) litre/mol at pH 7.0. Binding affinities are paralleled by the tendencies of the porphyrins to aggregate, increasing in the order: uroporphyrins I and III less than coproporphyrins I and III approximately haematoporphyrin less than protoporphyrin IX. From this it is deduced that the hydrophobic effect is the predominant driving-force for binding. The porphyrins can be displaced from their binding site on ligandin by bromosulphophthalein and oestrone sulphate. In enzyme inhibition studies, 50% inhibition was brought about by 8 micron-haematoporphyrin and by 1 micron-protoporphyrin IX. In the analysis of the haemotoporphyrin-ligandin system the self-association of haematoporphyrin was studied in detail. It was found to be limited to dimerization in the concentration range 0-200 micron at pH 7.0, 25 degrees C and a dimerization constant of 1.9 x 10(5) litre/mol was determined. Coproporphrin III has a dimerization constant of 5.2 x 10(5) litre/mol under the same conditions.

Interaction of rabbit hemoplexin with copro- and uroporphyrins

Rabbit hemopexin forms equimolar complexes in vitro with the I and III isomers of both coproporphyrin and uroporphyrin. The apparent dissociation constants (Kd) of these complexes are estimated to be 4-10(-7) M for coproporphyrin-hemopexin and 10(-6) M for uroporphyrin-hemopexin by equilibrium dialysis and quenching of protein fluorescence. Results of competitive binding experiments suggest that all four porphyrins bind at the heme-binding site of hemopexin, and that the relative affinity of rabbit hemopexin for these porphyrins is: deuteroheme greater than coproporphyrin I or III greater than uroporphyrin I or III. These findings provide further evidence that hemopexin may function as a transport protein for circulating coproporphyrins as well as for heme.