No abstract

The heart and those striated muscles that contract for long periods, having available almost limitless oxygen, operate in sustained steady states of low sarcoplasmic oxygen pressure that resist change in response to changing muscle work or oxygen supply. Most of the oxygen pressure drop from the erythrocyte to the mitochondrion occurs across the capillary wall. Within the sarcoplasm, myoglobin, a mobile carrier of oxygen, is developed in response to mitochondrial demand and augments the flow of oxygen to the mitochondria. Myoglobin-facilitated oxygen diffusion, perhaps by virtue of reduction of dimensionality of diffusion from three dimensions towards two dimensions in the narrow spaces available between mitochondria, is rapid relative to other parameters of cell respiration. Consequently, intracellular gradients of oxygen pressure are shallow, and sarcoplasmic oxygen pressure is nearly the same everywhere. Sarcoplasmic oxygen pressure, buffered near 0.33·kPa (2.5·torr; equivalent to approximately 4·µmol·l -1 oxygen) by equilibrium with myoglobin, falls close to the operational Km of cytochrome oxidase for oxygen, and any small increment in sarcoplasmic oxygen pressure will be countered by increased oxygen utilization. The concentration of nitric oxide within the myocyte results from a balance of endogenous synthesis and removal by oxymyoglobin-catalyzed dioxygenation to the innocuous nitrate. Oxymyoglobin, by controlling sarcoplasmic nitric oxide concentration, helps assure the steady state in which inflow of oxygen into the myocyte equals the rate of oxygen consumption.Key words: myoglobin, oxygen, facilitated diffusion, dimensionality in diffusion, heart, red skeletal muscle, nitric oxide, mitochondria, cytochrome oxidase, Krogh cylinder. Here, we describe myoglobin-augmented oxygen supply to heart and red muscle, taking into account their threedimensional structures and the elevated concentration of myoglobin in the cytoplasmic domain to which it is restricted and recognizing the large area of mitochondrial surface available for oxygen diffusion. [A mathematical formulation of oxygen diffusion in the cardiac myocyte will be presented elsewhere.] Heart and muscle, having available an almost unlimited supply of oxygen, actually operate at controlled low oxygen pressure, at or near 0.33·kPa (2.5·torr), where myoglobin is about half-saturated with oxygen. Partial saturation of myoglobin enables oxymyoglobin to play a pivotal role; by converting endogenous nitric oxide to the innocuous nitrate, oxymyoglobin controls the level of nitric oxide (NO) within the cell. This, in turn, may control both the rate of capillary oxygen delivery to the cell and the rate of oxygen utilization by cytochrome oxidase. Summary Introduction Review Myoglobin function reassessed Formulations of oxygen diffusion in musclePresent descriptions of oxygen diffusion/transport in tissues originate from the studies of Krogh, Hill and Jeffries Wyman, to whom this essay is dedicated. Krogh (1919a,b) and later Hill (1928) considered that oxyge...

Mycobacterium tuberculosis, the causative agent of human tuberculosis, and Mycobacterium bovis each express two genes, glbN and glbO, encoding distantly related truncated hemoglobins (trHbs), trHbN and trHbO, respectively. Here we report that disruption of M. bovis bacillus Calmette-Gué rin glbN caused a dramatic reduction in the NO-consuming activity of stationary phase cells, and that activity could be restored fully by complementing knockout cells with glbN. Aerobic respiration of knockout cells was inhibited markedly by NO in comparison to that of wild-type cells, indicating a protective function for trHbN. TyrB10, which is highly conserved in trHbs and interacts with the bound oxygen, was found essential for NO consumption. Titration of oxygenated trHbN (trHbN⅐O2) with NO resulted in stoichiometric oxidation of the protein with nitrate as the major product of the reaction. The second-order rate constant for the reaction between trHbN⅐O2 and NO at 23°C was 745 M ؊1 ⅐s ؊1 , demonstrating that trHbN detoxifies NO 20-fold more rapidly than myoglobin. These results establish a role for a trHb and demonstrate an NO-metabolizing activity in M. tuberculosis or M. bovis. trHbN thus might play an important role in persistence of mycobacterial infection by virtue of trHbNs ability to detoxify NO.

Two putative hemoglobin genes, glbN and glbO, were recently discovered in the complete genome sequence of Mycobacterium tuberculosis H37Rv. Here, we show that the glbN gene encodes a dimeric hemoglobin (HbN) that binds oxygen cooperatively with very high affinity (P 50 ‫؍‬ 0.013 mmHg at 20°C) because of a fast combination (25 M ؊1 ⅐s ؊1 ) and a slow dissociation (0.2 s ؊1 ) rate. Resonance Raman spectroscopy and ligand association͞dissociation kinetic measurements, along with mutagenesis studies, reveal that the stabilization of the bound oxygen is achieved through a tyrosine at the B10 position in the distal pocket of the heme with a conformation that is unique among the globins. Physiological studies performed with Mycobacterium bovis bacillus Calmette-Guérin demonstrate that the expression of HbN is greatly enhanced during the stationary phase in aerobic cultures but not under conditions of limited oxygen availability. The results suggest that, physiologically, the primary role of HbN may be to protect the bacilli against reactive nitrogen species produced by the host macrophage.