Pima subjects homozygous for the Trp64Arg beta 3-adrenergic-receptor mutation have an earlier onset of NIDDM and tend to have a lower resting metabolic rate. This mutation may accelerate the onset of NIDDM by altering the balance of energy metabolism in visceral adipose tissue.

Two prototropic forms of glucose oxidase undergo aerobic oxidation reactions that convert FADH ؊ to FAD and form H2O2 as a product. Limiting rate constants of kcat͞KM(O2) ‫؍‬ (5.7 ؎ 1.8) ؋ 10 2 M ؊1 ⅐s ؊1 and kcat͞KM(O2) ‫؍‬ (1.5 ؎ 0.3) ؋ 10 6 M ؊1 ⅐s ؊1 are observed at high and low pH, respectively. Reactions exhibit oxygen-18 kinetic isotope effects but no solvent kinetic isotope effects, consistent with mechanisms of rate-limiting electron transfer from flavin to O2. Site-directed mutagenesis studies reveal that the pH dependence of the rates is caused by protonation of a highly conserved histidine in the active site. Temperature studies (283-323 K) indicate that protonation of His-516 results in a reduction of the activation energy barrier by 6.0 kcal⅐mol ؊1 (0.26 eV). Within the context of Marcus theory, catalysis of electron transfer is attributed to a 19-kcal⅐mol ؊1 (0.82 eV) decrease in the reorganization energy and a much smaller 2.2-kcal⅐mol ؊1 (0.095 eV) enhancement of the reaction driving force. An explanation is advanced that is based on changes in outer-sphere reorganization as a function of pH. The active site is optimized at low pH, but not at high pH or in the H516A mutant where rates resemble the uncatalyzed reaction in solution. F lavins are highly versatile enzyme cofactors that undergo electron and proton-coupled electron transfer reactions (1-3). As a result, flavoenzymes are involved in an array of chemical and photochemical processes from COH oxidations (4) to electron transport (5) to repair of cross-linked DNA (6). Glucose oxidase (GO) is a homodimeric protein found predominantly in fungi (National Center for Biotechnology Information, www.ncbi.nlm.nih.gov). Each protein subunit contains an equivalent of noncovalently bound FAD Ϸ15 Å below the surface (7). GO mediates net hydride transfer from the anomeric COH bond of glucose to FAD in the reductive half-reaction (8, 9) and the oxidation of reduced cofactor (FADH Ϫ ) by O 2 in the oxidative half-reaction, forming H 2 O 2 as a product. All evidence points toward a rate-limiting electron transfer step during FADH Ϫ oxidation as shown in Eq. 1 (10). This reaction involves the transfer of negative charge from cofactor to superoxide ion with no net change in charge at the active site.To date, most mechanistic studies of O 2 activation have focused on metalloenzymes. In such reactions, electron transfer and electrostatic stabilization often occur within a single step (ref. 11 and references therein), causing rates to approach the diffusion limit (12). Additionally, ligands control metal coordination geometries and modulate redox potentials, thereby tuning reactivity toward O 2 (13). Enzymes that use organic cofactors do not enjoy such advantages, but may use specialized protein environments to help overcome the kinetic and thermodynamic barriers associated with activation of O 2 .The physical characterization of the protein dielectric is an area of growing interest (14-18) and may provide the key to understanding how proteins like GO facilita...

The transfer of a hydrogen atom-a proton and an electron-is a fundamental process in chemistry and biology. A variety of hydrogen atom transfer reactions, involving iron complexes, phenols, hydroxylamines, tBuOOH, toluene, and related radicals, are shown to follow the Marcus cross relation. Thus, the Marcus theory formalism based on ground-state energetics and self-exchange rates, originally developed for electron transfer processes, is also valuable for hydrogen atom transfer. Compounds that undergo slow proton transfer (C-H bonds) or slow electron transfer (cobalt complexes) also undergo slow hydrogen atom transfer. Limitations of this approach are also discussed.

Self-exchange reactions between high-spin iron complexes of 2,2′-bi-imidazoline (H 2 bim) have been investigated by the dynamic NMR line-broadening technique. Addition of the ferric complex [Fe III (H 2 bim) 3 ] 3+ causes broadening of the 1 H NMR resonances of the ferrous analogue, [Fe II (H 2 bim) 3 ] 2+ . This indicates electron self-exchange with k e -) (1.7 ( 0.2) × 10 4 M -1 s -1 at 298 K in MeCN-d 3 (µ ) 0.1 M). Similar broadening is observed when the deprotonated ferric complex [Fe III (Hbim)(H 2 bim) 2 ] 2+ is added to [Fe II (H 2 bim) 3 ] 2+ . Because these reactants differ by a proton and an electron, this is a net hydrogen atom exchange reaction. Kinetic and thermodynamic results preclude stepwise mechanisms of sequential proton and then electron transfer, or electron and then proton transfer. Concomitant electron and proton (H • ) transfer occurs with bimolecular rate constant k H • ) (5.8 ( 0.6) × 10 3 M -1 s -1 . This is a factor of 3 smaller than k e -under the same conditions. The H-atom exchange reaction exhibits a primary kinetic isotope effect k NH /k ND ) 2.3 ( 0.3 at 324 K, whereas no such effect is detected in the electron exchange reaction. Proton self-exchange between the two ferric complexes, [Fe III (Hbim)(H 2 bim) 2 ] 2+ and [Fe III (H 2 bim) 3 ] 3+ , has also been investigated and is found to be faster than both the electron and H-atom transfer reactions. From kinetic analyses and the application of simple Marcus theory, an order of intrinsic reaction barriers λ H • > λ e -> λ H + is derived. The reorganization energies are discussed in terms of their inner-sphere and outer-sphere components.