The quinoprotein methylamine dehydrogenase (MADH), type I copper protein amicyanin, and cytochrome c-551i form a complex within which interprotein electron transfer occurs. It was known that complex formation significantly lowered the oxidation-reduction midpoint potential (Em) value of amicyanin, which facilitated an otherwise thermodynamically unfavorable electron transfer to cytochrome c-551i. Structural, mutagenesis, and potentiometric studies have elucidated the basis for this complex-dependent change in redox properties. Positively charged amino acid residues on the surface of amicyanin are known to stabilize complex formation with MADH and influence the ionic strength dependence of complex formation via electrostatic interactions. Altering the charges of these residues by site-directed mutagenesis had no effect on the Em value of amicyanin, ruling out charge neutralization as the basis for the complex-dependent changes in redox properties. The Em value of free amicyanin varies with pH and exhibits a pKa value for the reduced form of 7.5. The crystal structure of reduced amicyanin at pH 4.4 reveals that His95, which serves as a ligand for Cu2+, has rotated by 180 degrees about the Cbeta-Cgamma bond relative to its position in oxidized amicyanin and is no longer in the copper coordination sphere. At pH 7.7, the crystal structure of reduced amicyanin contains an approximately equal distribution of two active-site conformers. One is very similar to the structure of reduced amicyanin at pH 4.4, and the other is very similar to the structure of oxidized amicyanin at pH 4.8. Potentiometric analysis of amicyanin in complex with MADH indicates that its Em value is not pH-dependent from pH 6.5 to 8.5, and exhibits an Em value similar to that of free amicyanin at high pH. The structure of reduced amicyanin at pH 4.4, with His95 protonated and "flipped", was modeled into the structure of the complex of oxidized amicyanin with MADH. This showed that in the complex, the redox-linked pH-dependent rotation of His95 is hindered because it would cause an overlap of van der Waals' radii with residues of MADH. These results demonstrate that protein-protein interactions profoundly affect the redox properties of this type I copper protein by restricting a pH-dependent, redox-linked conformational change of one of the copper ligands.

No completely satisfying explanation has been provided for the first question. The attachment of an aminoacyl group to the 8␣-or 6-position of riboflavin does not confer any unusual properties on the flavin, either free in solution or in an enzyme, with the exception of the ultraviolet-visible spectrum of 6-Scysteinylriboflavin, which is quite different from that of other forms of free and bound aminoacyl flavins (2, 3). While the oxidation-reduction potentials (E m 7 ) are about 50 -60 mV more positive for aminoacyl flavins than for unmodified forms (2), this increase in potential can also be achieved by noncovalent interactions with protein. Additionally, enzymes with covalently bound flavins do not catalyze a unique or specific set of reactions.As to the second question, it is known that 2-electron reduction of protein-free 8␣-O-tyrosylriboflavin or 8␣-S-cysteinylsulfonylriboflavin cause expulsion of the aminoacyl groups, thus producing unmodified, oxidized riboflavin (2, 4). The principle of microscopic reversibility suggests that the reverse reaction could occur within an enzyme, with or without intervention of an external enzyme. A mechanism for covalent flavinylation, which has long been in the literature, is shown in Fig. 2 (5, 6). An analogous mechanism was proposed for nonenzymic basecatalyzed nucleophilic attack at the 8␣-carbon of riboflavin derivatives in organic solvents (7).It has been suggested that covalent tethering might require prior activation of the flavin or proteins, e.g. a high energy phosphate bond (8). Of several enzymes studied to date, no specific enzyme has been implicated in the covalent modification process, which contrasts with examples of nonflavin-cofactor covalent attachment to apoenzymes (9 -13).The structural genes of several bacterial enzymes containing covalently bound flavin have been cloned into vectors for expression in new hosts: succinate dehydrogenase, fumarate reductase (14, 15), sarcosine oxidase (16), 6-hydroxy-D-nicotine oxidase (6-HDNO) 1 from Arthrobacter aerogenes (15) (all containing 8␣-N 3 -histidyl-FAD), trimethylamine dehydrogenase

High-resolution X-ray diffraction data to d(min) = 1.31 A were collected on a Xuong-Hamlin area detector from crystals of the blue-copper protein amicyanin, isolated from P. denitrificans. With coordinates from the earlier 2.0 A structure determination as a starting point, simulated annealing and restrained positional and temperature factor refinements using the program X-PLOR resulted in a final R factor of 15.5%, based on 21 131 unique reflections in the range 8.0-1.3 A. Comparison of the 1.31 A structure with that at 2.0 A shows the same basic features. However, the high-resolution electron-density maps clearly reveal additional solvent molecules and significant discrete disorder in protein side chains and within the solvent structure. As a consequence of modelling side-chain disorder, several new hydrogen-bonding interactions were identified.

Long chain hydroxy acid oxidase (LCHAO) is a member of an FMN-dependent enzyme family that oxidizes L-2-hydroxy acids to ketoacids. LCHAO is a peroxisomal enzyme, and the identity of its physiological substrate is unclear. Mandelate is the most efficient substrate known and is commonly used in the test tube. LCHAO differs from most family members in that one of the otherwise invariant active site residues is a phenylalanine (Phe23) instead of a tyrosine. We now report the crystal structure of LCHAO. It shows the same beta8alpha8 TIM barrel structure as other structurally characterized family members, e.g., spinach glycolate oxidase (GOX) and the electron transferases yeast flavocytochrome b2 (FCB2) and Pseudomonas putida mandelate dehydrogenase (MDH). Loop 4, which is mobile in other family members, is visible in part. An acetate ion is present in the active site. The flavin interacts with the protein in the same way as in the electron transferases, and not as in GOX, an unexpected observation. An interpretation is proposed to explain this difference between GOX on one hand and FCB2 and LCHAO on the other hand, which had been proposed to arise from the differences between family members in their reactivity with oxygen. A comparison of models of the substrate bound to various published structures suggests that the very different reactivity with mandelate of LCHAO, GOX, FCB2, and MDH cannot be rationalized by a hydride transfer mechanism.

Flavocytochrome b(2) catalyzes the oxidation of L-lactate to pyruvate and the transfer of electrons to cytochrome c. The enzyme consists of a flavin-binding domain, which includes the active site for lacate oxidation, and a b(2)-cytochrome domain, required for efficient cytochrome c reduction. To better understand the structure and function of intra- and interprotein electron transfer, we have determined the crystal structure of the independently expressed flavin-binding domain of flavocytochrome b(2) to 2.50 A resolution and compared this with the structure of the intact enzyme, redetermined at 2.30 A resolution, both structures being from crystals cooled to 100 K. Whereas there is little overall difference between these structures, we do observe significant local changes near the interface region, some of which impact on amino acid side chains, such as Arg289, that have been shown previously to have an important role in catalysis. The disordered loop region found in flavocytochrome b(2) and its close homologues remain unresolved in frozen crystals of the flavin-binding domain, implying that the presence of the b(2)-cytochrome domain is not responsible for this positional disorder. The flavin-binding domain interacts poorly with cytochrome c, but we have introduced acidic residues in the interdomain interface region with the aim of enhancing cytochrome c binding. While the mutations L199E and K201E within the flavin-binding domain resulted in unimpaired lactate dehydrogenase activity, they failed to enhance electron-transfer rates with cytochrome c. This is most likely due to the disordered loop region obscuring all or part of the surface having the potential for productive interaction with cytochrome c.