Studies on the heme and H2-uptake reaction from Azotobacter vinelandii bacterial ferritin

Preparation and representation of recombinant Mn-ferritin flower-like spherical aggregates from marine invertebrates

Ferritin has important functions in the transition and storage of toxic metal ions, but its regulation and function in many invertebrate species are still largely unknown. In our previous work, the cDNA sequence of Sinonovacula constricta, Apostichopus japonicas and Acaudina leucoprocta were constructed and efficiently expressed in E. Coli BL21 under IPTG induction. In this follow-up study, the recombinant ferritins were exposed to heavy metal manganese. The manganese concentration levels in three recombinant ferritins were greater than horse spleen ferritin (HSF). Compared with HSF, the amount of manganese enrichment in the three recombinant ferritins was 1.75-fold, 3.25-fold and 2.42-fold increases in ScFER, AjFER, and AlFER, respectively. After phosphate stimulation, the concentration of manganese increased and was higher than the ordinary dialysis control groups. The ScFER was four times its baseline value. The AjFER and AlFER were 1.4- and 8-fold higher, respectively. The AlFER sample stimulated by phosphate was 22-fold that of HSF. The morphologies of the resulting Mn-Ferritin from different marine invertebrates were characterized with scanning electron microscopy. Surface morphologies were lamella flower-like and are consistent with changes in surface morphologies of the standard Mn-HSF. Invertebrate recombinant ferritin and HSF both can uptake manganese. We found that the structure of A. leucoproctarecombinant Mn-Ferritin aggregate changed over time. The surface formed lamella flower-like aggregate, but gradually merged to create a relatively uniform plate-like phase of aggregate spherically and fused without clear boundaries.

Kinetics of iron release from pig spleen ferritin with bare platinum electrode reduction

Several anaerobic electrochemical cells were employed to study the kinetics of iron release from pig spleen ferritin (PSF) at a bare platinum electrode. Controlled potential microcoulometry (CPM) is the principal technology used to investigate the kinetics in the absence of a mediator. A kinetic study of iron release by microcoulometry has revealed that ferritin undergoes direct electron transfer at the electrode in the absence of a mediator, indicating that ferritin is an electroactive protein. Several experiments failed to show that alpha'alpha-bipyridyl has the capacity to reduce hydrolyzed Fe(3+) within the ferritin core after it has been reduced by the electrode at -600 mV vs. NHE in the absence of mediator. PSF is known to bind heme to generate a hemeoprotein, named pig spleen hemeoferritin (PSF(ho)). The rate of iron release is accelerated by the heme binding to PSF(ho) without the need for small mediators. Under similar conditions, two kinetic processes for iron release from PSF and bacterial ferritin of Azoaobacter vinelandii (AvBF) were studied and both fit a zero-order law. In addition, the rate of iron release in PSF can be accelerated two-fold by a specific reduction system consisting of ascorbic acid (AA) and the bare platinum electrode at -600 mV. However, this kinetic process does not follow zero-, half-, first, or second-order rate laws. A model is proposed to explain a mechanism of direct electron transfer between ferritin and the electrode is derived to describe the kinetics of iron release.

Effect of redox mediators on nitrogenase and hydrogenase activities in Azotobacter vinelandii

In bioelectrochemical studies, redox mediators such as methylene blue, natural red, and thionine are used to studying the redox characteristics of enzymes in the living cell. Here we show that nitrogenase activity in Azotobacter vinelandii is completely inhibited by oxidized methylene blue (MBo) when the concentration of this mediator in the medium is increased up to 72 microM. This activity in A. vinelandii is somewhat inhibited by a coenzyme, ascorbic acid (AA). However, the nitrogenase activity within the A. vinelandii cell is unchanged even for a high concentration of oxidized natural red (NRo) alone. Interestingly, these mediators and AA do not have the capacity to inhibit the H2 uptake activity of the hydrogenase in A. vinelandii. Average active rates of 66 nM H2 evolved/mg cell protein/min from the nitrogenase and 160 nM H2-uptake/mg cell protein/min from the hydrogenase in A. vinelandii are found in aid of the activities of the enzymes for H2 evolution and for H2 uptake are compared. The activities of both enzymes in A. vinelandii are strongly inhibited by thionine having high oxidative potential. Mechanisms of various mediators acting in vivo for both enzymes in A. vinelandii are discussed.

Construction of a ferritin reactor: an efficient means for trapping various heavy metal ions in flowing seawater

An apparatus consisting of two pumps, a mixer, a ferritin reactor, and a spectrophotometer was constructed to study the ability to trap various heavy metal ions (M2+) and the dynamics of a reconstituted ferritin reactor in flowing seawater. Reconstituted pig spleen ferritin (PSFr) is assembled from apo-protein shell to form a reconstituted iron core. The main components of the PSFr are its core, which contains an Fe2+:Pi stoichiometry of 6.0 +/- 0.5, reconstituted from pig spleen apoferritin (apo PSF), Fe2+, inorganic phosphate (Pi), and O2 (0.6 atm). The Fe3+-Pi clusters within the PSFr core exhibit resistance to salt ranging from 1% to 6% NaCl. The ferritin reactor consists of PSFr and an oscillating bag. Using the reactor, M2+ ions such as Cd2+, Zn2+, Co2+, and Mn2+ are directly trapped by the ferritin. We found a 1:2 +/- 0.2 stoichiometry of the trapped M2+ to the released iron as measured by chemical analysis or atomic absorption spectrometry; nontransient elements such as Na+, K+, Ca2+, etc., were scarcely trapped by the reactor. This study provides basic conditions for establishing a ferritin reactor and a convenient means for monitoring the pollution of heavy metal ions in seawater.

Role of phosphate and kinetic characteristics of complete iron release from native pig spleen ferritin-Fe

The kinetics for complete iron release showing biphasic behavior from pig spleen ferritin-Fe (PSFF) was measured by spectrophotometry. The native core within the PSFF shell consisted of 1682 hydroxide Fe3+ and 13 phosphate molecules. Inhibition kinetics for complete iron release was measure by differential spectrophotometry in the presence of phosphate; the process was clearly divided into two phases involving a first-order reaction at an increasing rate of 46.5 Fe3+/PSFF/min on the surface of the iron core and a zero-order reaction at a decreasing rate of 6.67 Fe3+/PSFF/min inside the core. The kinetic equation [C(PSFF-Fe3+)max - C(PSFF-Fe3+)t](1/2) = Tmax - Tt gives the transition time between the two rates and represents the complex kinetic characteristics. The rate was directly accelerated twofold by a mixed reducer of dithionite and ascorbic acid. These results suggest that the channel of the PSFF shell may carry out multiple functions for iron metabolism and storage and that the phosphate strongly affects the rate of iron release.