Metal-Calayzed Oxidation Of Proteins

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Metal-catalyzed Oxidation of Proteins
PHYSIOLOGICALCONSEQUENCES
Earl R. StadtmanS andCynthia N. Oliver4

THEJOURNAL BIOLOGICAL OF CHEMISTRY Vol. 266, No. 4, Issue of February 5 , pp. 2005-2008. 1991 Printed in U.S.A.

0,

CATALASE

pox

~

From the Laboratory of Biochemistry, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland20892

PROTEASE

T

AMINO ACIDS

Fe3'

MnZt

FIG.1. Mechanism of enzyme oxidation and degradation. SOD, superoxide dismutase; P , protein; Pox,oxidized protein. The symbols 8 and 8 indicate stimulation and inactivation, respectively.

Detailed studies of the oxidative modification of glutamine synthetase (GS), in extracts of Escherichia coli (1, 3) and Klebsiellaaerogenes (2, 4)established that the oxidation is dependent upon NAD(P)H, O,, and Fe(II1) or Cu(I1). The requirements for 0, and an auxiliary electron donor ( i e . a donor other than GS itself) indicated that the oxidation is promoted by a mixed function oxidation (MFO)' type mechanism. In the meantime, it found that many enzymes are was readily inactivated by MFO systems (5-7) and that this renders them susceptible toproteolytic degradation by a variety of exogenous and endogenous proteinases (1,8-12). Itbecame evident also that the oxidation proteins islikely implicated of in the killing of bacteria by neutrophils(13),andinthe of generation of altered (inactiveor less active) forms enzymes that accumulate during animal aging (6, 14-16) and under several pathological conditions (17-23). It is the intent thisof review to summarize the results studies on the mechanism of of proteinoxidation by MFOsystemsandthe biological implications of such oxidations.

the case of enzymes they are converted to catalytically inactive or less active, more thermolabile forms (14).

Site-specific Nature f the MCO-catalyzed Reactions o

According to Fig. 1, the MCO-catalyzed oxidation of proteins isasite-specificprocessinvolving the interaction of H202 and Fe(I1) at metal binding sites on the protein. The site-specific nature of the reaction is indicated by the following facts. ( a ) The inactivation of enzymes by MCO systems is relatively insensitive to inhibition free radical scavengers by (formate, ethanol, and mannitol)(5,6, 27). ( b )Only one or at mostonlya few amino acidresiduesina proteincan bemodified by MCO systems (25, 26) whereas almost all amino acid residues can be modified when proteins are subjected to free radicals obtained by radiolysis (28-30). ( c ) Most of the enzymes that are highly sensitive to modification by MCO systems require metal ions for catalytic activity. Therefore, they must containa metal ion binding site. ( d ) In the case of The Mechanismo f Metal Ion-catalyzedProtein E . coli glutaminesynthetase,the loss of catalyticactivity Oxidation correlates with the loss of a single histidyland a single arginyl residue per subunit, bothof which are situated inclose proxThe current view of how MCO' systems mediate protein imity to one two divalent metal binding sites on enzyme of the oxidation is illustrated in Fig. 1. In this representation, the electron donorsystem needed only to catalyze the reduction (31, 32).3 is A plausible mechanism for the site-specific modification of of O2 to H202and of Fe(II1) to Fe(I1). Depending upon the a lysyl residue atthemetalbindingsite of a protein is electron donor system used, the reduction of 0, can proceed reduction of Fe(II1) by a two-electron mechanism yielding H,O, directly, or by illustrated in Fig. 2. In thismechanism the way of two sequential one-electron transferprocesses leading (step a ) is followed by binding of the Fe(I1) to the enzyme (step b ) to form a coordination complex in which the e-amino first to superoxide anion followed by its dismutation toH,O, serves as one and 0,. Similarly, the reduction Fe(II1) to Fe(I1) can occur group of a lysyl residue at the metal binding site of of several...