cAMP-dependent Tyrosine Phosphorylation of Subunit I Inhibits Cytochrome c Oxidase Activity

Abstract

Signaling pathways targeting mitochondria are poorly understood. We here examine phosphorylation by the cAMP-dependent pathway of subunits of cytochrome c oxidase (COX), the terminal enzyme of the electron transport chain. Using anti-phospho antibodies, we show that cow liver COX subunit I is tyrosinephosphorylated in the presence of theophylline, a phosphodiesterase inhibitor that creates high cAMP levels, but not in its absence. The site of phosphorylation, identified by mass spectrometry, is tyrosine 304 of COX catalytic subunit I. Subunit I phosphorylation leads to a decrease of V max and an increase of K m for cytochrome c and shifts the reaction kinetics from hyperbolic to sigmoidal such that COX is fully or strongly inhibited up to 10 M cytochrome c substrate concentrations, even in the presence of allosteric activator ADP. To assess our findings with the isolated enzyme in a physiological context, we tested the starvation signal glucagon on human HepG2 cells and cow liver tissue. Glucagon leads to COX inactivation, an effect also observed after incubation with adenylyl cyclase activator forskolin. Thus, the glucagon receptor/G-protein/cAMP pathway regulates COX activity. At therapeutic concentrations used for asthma relief, theophylline causes lung COX inhibition and decreases cellular ATP levels, suggesting a mechanism for its clinical action.Cytochrome c oxidase (COX), 1 the terminal enzyme of the mitochondrial respiratory chain, reduces oxygen to water and pumps protons across the inner mitochondrial membrane. COX contains 13 subunits per monomer, three of which are encoded by the mitochondrial genome. COX has been shown to be the rate-limiting enzyme of oxidative metabolism under physiological conditions in a variety of human cell types (1) and in a mouse cell line with a mutation in COX subunit I (2). The functional mammalian enzyme has been crystallized as a dimer (3) and shows three features, described below, that are found in key metabolic enzymes: allosteric regulation, isoforms, and phosphorylation. COX activity is regulated by small molecules such as ATP and ADP (4 -6), and the thyroid hormone T2 (7). COX contains skeletal muscle/heart ("heart-type") and nonskeletal muscle ("liver-type") isoforms of subunits VIa, VIIa, and VIII that have been known for the past 2 decades (reviewed in Ref. 8). We have recently discovered three additional isoforms: a lung-specific isoform of subunit IV, a third isoform of subunit VIII, and a testes-specific isoform of subunit VIb (9 -11). Although it is clear that protein kinases and phosphatases are crucial in cellular signaling, little is known about their role in the regulation of the respiratory chain complexes.There have been three studies of COX phosphorylation: Steenaart and Shore (12) examined mitochondrial proteins phosphorylated by endogenous kinases in the presence of [␥-32 P]ATP and identified COX subunit IV; Miyazaki et al. (13) showed that COX subunit II can be phosphorylated by nonreceptor tyrosine kinase c-Src in osteoblasts and found a positive ...

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