Coupled spectrofluorometric assay for aminoglycoside phosphotransferases

Purification and characterization of aminoglycoside phosphotransferase APH(6)-Id, a streptomycin-inactivating enzyme

As part of an overall project to characterize the streptomycin phosphotransferase enzyme APH(6)-Id, which confers bacterial resistance to streptomycin, we cloned, expressed, purified, and characterized the enzyme. When expressed in Escherichia coli, the recombinant enzyme increased by up to 70-fold the minimum inhibitory concentration needed to inhibit cell growth. Size-exclusion chromatography gave a molecular mass of 31.4 ± 1.3 kDa for the enzyme, showing that it functions as a monomer. Activity was assayed using three methods: (1) an HPLC-based method that measures the consumption of streptomycin over time; (2) a spectrophotometric method that utilizes a coupled assay; and (3) a radioenzymatic method that detects production of (32)P-labeled streptomycin phosphate. Altogether, the three methods demonstrated that streptomycin was consumed in the APH(6)-Id-catalyzed reaction, ATP was hydrolyzed, and streptomycin phosphate was produced in a substrate-dependent manner, demonstrating that APH(6)-Id is a streptomycin phosphotransferase. Steady-state kinetic analysis gave the following results: K(m)(streptomycin) of 0.38 ± 0.13 mM, K(m)(ATP) of 1.03 ± 0.1 mM, V(max) of 3.2 ± 1.1 μmol/min/mg, and k(cat) of 1.7 ± 0.6 s(-1). Our study demonstrates that APH(6)-Id is a bona fide streptomycin phosphotransferase, functions as a monomer, and confers resistance to streptomycin.

Enzymatic analysis of a thermostabilized mutant of an Escherichia coli hygromycin B phosphotransferase

An Escherichia coli hygromycin B phosphotransferase (HPH) and its thermostabilized mutant protein, HPH5, containing five amino acid substitutions, D20G, A118V, S225P, Q226L, and T246A (Nakamura et al., J. Biosci. Bioeng., 100, 158-163 (2005)), obtained by an in vivo directed evolution procedure in Thermus thermophilus, were produced and purified from E. coli recombinants, and enzymatic comparisons were performed. The optimum temperatures for enzyme activity were 50 and 55 degrees C for HPH and HPH5 respectively, but the thermal stability of the enzyme activity and the temperature for protein denaturation of HPH5 increased, from 36 and 37.2 degrees C of HPH to 53 and 58.8 degrees C respectively. Specific activities and steady-state kinetics measured at 25 degrees C showed only slight differences between the two enzymes. From these results we concluded that HPH5 was thermostabilized at the protein level, and that the mutations introduced did not affect its enzyme activity, at least under the assay conditions.

Branched aminoglycosides: biochemical studies and antibacterial activity of neomycin B derivatives

The C5''-OH group in neomycin B was glycosylated with a variety of mono- and di-saccharides to probe the effect of introduction of additional binding elements on antibacterial activity and interaction with the aminoglycosides modifying enzyme APH(3')-IIIa. The designed structures show antibacterial activity superior to that of neomycin B against pathogenic and resistant strains, while in parallel they demonstrate poor substrate activity with APH(3')-IIIa.

Enhanced catalytic efficiency of aminoglycoside phosphotransferase (3')-IIa achieved through protein fragmentation and reassembly

Many monomeric proteins can be split into two fragments, yet the two fragments can associate to make an active heterodimer. However, for most locations in a protein such a conversion is not feasible, presumably due to inefficient assembly or improper folding of the fragments. For some locations, this can be overcome by fusion of the fragments to dimerization domains that facilitate correct assembly. A variety of heterodimers of aminoglycoside phosphotransferase (3')-IIa (Neo) were created in which the Neo fragments required fusion to a pair of leucine zippers for activity in vivo. However, the ability of these heterodimers to confer kanamycin resistance to Escherichia coli cells was impaired compared to wild-type Neo, primarily due to poor production of soluble protein. The mutations R177S and V198E restored the kanamycin resistance to wild-type levels while maintaining the dependence on leucine zippers for activity. These mutations restored high levels of kanamycin resistance not through an improvement in the production of soluble protein but rather by conferring a large improvement in k(cat)/K(m), surpassing that of Neo. Furthermore, whereas R177S and V198E served to improve k(cat)/K(m) 60-fold in the context of the heterodimer, the same mutations in the context of wild-type Neo had a ninefold negative effect on k(cat)/K(m). This demonstrates the possibility that enzymes with improved catalytic properties can be created through a process involving fragmentation and fusion to domains that facilitate assembly of the fragments.

Refolding and purification of non-fusion HPT protein expressed in Escherichia coli as inclusion bodies

The gene encoding hygromycin B phosphotransferase (hpt) is a widely used selectable marker in the production of genetically engineered crops. To facilitate the safety assessment of this protein, the non-fusion hpt expression plasmid was constructed and introduced into Escherichia coli to produce enough quantity of the HPT protein. High level expressed HPT was achieved but most of the expressed protein aggregated as inclusion bodies. The inclusion bodies were washed, separated from the cells, and solubilized by 0.3% Sarkosyl. The protein was renatured by dilution and dialysis, and then purified by anion-exchange chromatography. The activity is 8 U/mg protein and the purity is about 95%. Further studies showed that the microbially produced HPT protein had comparable molecular weight, immuno-reactivities, N-terminal amino acid sequences, and biological activities with those of the HPT produced by transgenic rice harboring hpt gene. All these results demonstrated the validity of utilizing the microbially produced HPT to assess the safety of the HPT protein produced in genetically engineered rice.

Purification and characterization of aminoglycoside 3'-phosphotransferase type IIa and kinetic comparison with a new mutant enzyme

Aminoglycoside 3'-phosphotransferase [APH(3')s] provide an important means for high-level resistance to neomycin- and kanamycin-type aminoglycoside antibiotics. A four-step purification which affords milligram quantities of homogeneous APH(3') type IIa [APH(3')-IIa] is described. The kinetic parameters for the turnover of five substrates by the enzyme were determined, and the pH dependence and metal activation for catalysis were investigated. All five cysteines in the amino acid sequence of the enzyme exist in their reduced forms; hence, there are no disulfide bonds in the protein. Modification of the cysteine thiols by S-cyanylation showed essentially no effect on the enzymatic activity. A mutant enzyme derived from APH-3'-IIa, which possesses a conservative Glu-182-Asp point mutation and which provides diminished resistance to G418 (R. L. Yenofsky, M. Fine, and J. W. Pellow, Proc. Natl. Acad. Sci. USA 87:3435-3439, 1990), was also purified to homogeneity. Kinetic analysis of this mutant protein indicated an increase of approximately ninefold in the Km for Mg2+ ATP. Insofar as Km may approximate Ks, this finding argues for the involvement of residue 182 in the binding of Mg2+ ATP. Thus, purified APH(3')-IIa and a point mutant derivative enzyme were characterized enzymologically, and the roles of metal cofactors and the five reduced cysteine residues were probed in the wild-type enzyme.

Purification and characterization of microbially expressed neomycin phosphotransferase II (NPTII) protein and its equivalence to the plant expressed protein

The gene encoding neomycin phosphotransferase II (NPTII) has been used routinely as a selectable marker in the production of genetically engineered crops. To facilitate the safety assessment of this protein, the same coding sequence used for plant transformation was introduced into Escherichia coli to produce gram quantities of this protein. A unique, simple, rapid and efficient purification method was developed to purify thirty grams of NPTII protein. The microbially produced NPTII was shown to be chemically and functionally equivalent to the NPTII protein expressed in and purified from genetically engineered cotton seed, potato tubers and tomato fruit. Microbially produced and plant produced NPTII proteins have comparable molecular weights, immuno-reactivities, epitope structures, amino terminal amino acid sequences, biological activities and both lack glycosylation. Demonstrating the equivalence of NPTII protein from these sources establishes the validity of using the microbially produced NPTII to assess the safety of the NPTII protein produced in genetically engineered crops.

An immunoaffinity immobilized enzyme assay for neomycin phosphotransferase II in crude cell extracts

An immunoaffinity immobilized enzyme assay for neomycin phosphotransferase II (NPT II) has been developed. This method combines affinity purification with an enzyme-catalyzed reaction. The assay is mechanically simple and can be semiautomatable since all steps are performed in a microtiter plate. An immunoaffinity step separates NPT II from endogenous kinases, which may produce false positive results, and from endogenous phosphatases and inhibitors, which decrease the apparent NPT activity. This method thus exploits two modes of specificity: antigen-antibody specificity and enzyme catalysis specificity. This gives a high degree of specificity and allows quantitation of 0.1 ppm NPT in crude plant protein extracts. The catalytic ability of the NPT is not significantly hampered by its attachment to the gel, in the Km values for ATP and neomycin and the catalytic number for immobilized NPT are comparable to those for the NPT in solution.