Functional role of the Ti plasmid-encoded catabolic mannopine cyclase in mannityl opine catabolism by Agrobacterium spp

Opine biosynthesis in naturally transgenic plants: Genes and products

The plant pathogen Agrobacterium transfers DNA into plant cells by a specific transfer mechanism. Expression of this transferred DNA or T-DNA leads to crown gall tumors or abnormal, hairy roots and the synthesis of specific compounds, called opines. Opines are produced from common plant metabolites like sugars, amino acids and α-keto acids, which are combined into different low molecular weight structures by T-DNA-encoded opine synthase enzymes. Opines can be converted back by Agrobacterium into the original metabolites and used for agrobacterial growth. Recently it has been discovered that about 7% of Angiosperms carry T-DNA-like sequences. These result from ancient Agrobacterium transformation events, followed by spontaneous regeneration of transformed cells into natural genetically transformed organisms (nGMOs). Nearly all nGMOs identified up to date carry opine synthesis genes, several of these are intact and potentially encode opine synthesis. So far, only tobacco and cuscuta have been demonstrated to contain opines. Whereas opines from crown gall and hairy root tissues have been studied for over 60 years, those from the nGMOs remain to be explored.

Import pathways of the mannityl-opines into the bacterial pathogen Agrobacterium tumefaciens: structural, affinity and in vivo approaches

Agrobacterium tumefaciens pathogens use specific compounds denoted opines as nutrients in their plant tumor niche. These opines are produced by the host plant cells genetically modified by agrobacteria. They are imported into bacteria via solute-binding proteins (SBPs) in association with ATP-binding cassette transporters. The mannityl-opine family encompasses mannopine, mannopinic acid, agropine and agropinic acid. Structural and affinity data on mannopinic acid bound to SBPs are currently lacking while those of the three others mannityl opines are available. We investigated the molecular basis of two pathways for mannopinic acid uptake. MoaA was proposed as the specific SBP for mannopinic acid import in mannityl opines-assimilating agrobacteria, which was validated here using genetic studies and affinity measurements. We structurally characterized the mannopinic acid-binding mode of MoaA in two crystal forms at 2.05 and 1.57 Å resolution. We demonstrated that the non-specific SBP MotA, so far characterized as mannopine and Amadori compound importer, was also able to transport mannopinic acid. The structure of MotA bound to mannopinic acid at 2.2 Å resolution defines a different mannopinic acid-binding signature, similar to that of mannopine. Combining in vitro and in vivo approaches, this work allowed us to complete the characterization of the mannityl-opines assimilation pathways, highlighting the important role of two dual imports of agropinic and mannopinic acids. Our data shed new light on how the mannityl-opines contribute to the establishment of the ecological niche of agrobacteria from the early to the late stages of tumor development.

Structural basis for two efficient modes of agropinic acid opine import into the bacterial pathogen Agrobacterium tumefaciens

Agrobacterium tumefaciens pathogens genetically modify their host plants to drive the synthesis of opines in plant tumors. The mannityl-opine family encompasses mannopine, mannopinic acid, agropine and agropinic acid. These opines serve as nutrients and are imported into bacteria via periplasmic-binding proteins (PBPs) in association with ABC transporters. Structural and affinity data on agropine and agropinic acid opines bound to PBPs are currently lacking. Here, we investigated the molecular basis of AgtB and AgaA, proposed as the specific PBP for agropine and agropinic acid import, respectively. Using genetic approaches and affinity measurements, we identified AgtB and its transporter as responsible for agropine uptake in agropine-assimilating agrobacteria. Nonetheless, we showed that AgtB binds agropinic acid with a higher affinity than agropine, and we structurally characterized the agropinic acid-binding mode through three crystal structures at 1.4, 1.74 and 1.9 Å resolution. In the crystallization time course, obtaining a crystal structure of AgtB with agropine was unsuccessful due to the spontaneous lactamization of agropine into agropinic acid. AgaA binds agropinic acid only with a similar affinity in nanomolar range as AgtB. The structure of AgaA bound to agropinic acid at 1.65 Å resolution defines a different agropinic acid-binding signature. Our work highlights the structural and functional characteristics of two efficient agropinic acid assimilation pathways, of which one is also involved in agropine assimilation.

Quorum-dependent transfer of the opine-catabolic plasmid pAoF64/95 is regulated by a novel mechanism involving inhibition of the TraR antiactivator TraM

We previously described a plasmid of Agrobacterium spp., pAoF64/95, in which the quorum-sensing system that controls conjugative transfer is induced by the opine mannopine. We also showed that the quorum-sensing regulators TraR, TraM, and TraI function similarly to their counterparts in other repABC plasmids. However, traR, unlike its counterpart on Ti plasmids, is monocistronic and not located in an operon that is inducible by the conjugative opine. Here, we report that both traR and traM are expressed constitutively and not regulated by growth with mannopine. We report two additional regulatory genes, mrtR and tmsP, that are involved in a novel mechanism of control of TraR activity. Both genes are located in the distantly linked region of pAoF64/95 encoding mannopine utilization. MrtR, in the absence of mannopine, represses the four-gene mocC operon as well as tmsP, which is the distal gene of the eight-gene motA operon. As judged by a bacterial two-hybrid analysis, TmsP, which shows amino acid sequence relatedness with the TraM-binding domain of TraR, interacts with the antiactivator. We propose a model in which mannopine, acting through the repressor MrtR, induces expression of TmsP which then titrates the levels of TraM thereby freeing TraR to activate the tra regulon.

Structural Basis for High Specificity of Amadori Compound and Mannopine Opine Binding in Bacterial Pathogens

Agrobacterium tumefaciens pathogens genetically modify their host plants to drive the synthesis of opines in plant tumors. Opines are either sugar phosphodiesters or the products of condensed amino acids with ketoacids or sugars. They are Agrobacterium nutrients and imported into the bacterial cell via periplasmic-binding proteins (PBPs) and ABC-transporters. Mannopine, an opine from the mannityl-opine family, is synthesized from an intermediate named deoxy-fructosyl-glutamine (DFG), which is also an opine and abundant Amadori compound (a name used for any derivative of aminodeoxysugars) present in decaying plant materials. The PBP MotA is responsible for mannopine import in mannopine-assimilating agrobacteria. In the nopaline-opine type agrobacteria strain, SocA protein was proposed as a putative mannopine binding PBP, and AttC protein was annotated as a mannopine binding-like PBP. Structural data on mannityl-opine-PBP complexes is currently lacking. By combining affinity data with analysis of seven x-ray structures at high resolution, we investigated the molecular basis of MotA, SocA, and AttC interactions with mannopine and its DFG precursor. Our work demonstrates that AttC is not a mannopine-binding protein and reveals a specific binding pocket for DFG in SocA with an affinity in nanomolar range. Hence, mannopine would not be imported into nopaline-type agrobacteria strains. In contrast, MotA binds both mannopine and DFG. We thus defined one mannopine and two DFG binding signatures. Unlike mannopine-PBPs, selective DFG-PBPs are present in a wide diversity of bacteria, including Actinobacteria, α-,β-, and γ-proteobacteria, revealing a common role of this Amadori compound in pathogenic, symbiotic, and opportunistic bacteria.

Ecological dynamics and complex interactions of Agrobacterium megaplasmids

As with many pathogenic bacteria, agrobacterial plant pathogens carry most of their virulence functions on a horizontally transmissible genetic element. The tumor-inducing (Ti) plasmid encodes the majority of virulence functions for the crown gall agent Agrobacterium tumefaciens. This includes the vir genes which drive genetic transformation of host cells and the catabolic genes needed to utilize the opines produced by infected plants. The Ti plasmid also encodes, an opine-dependent quorum sensing system that tightly regulates Ti plasmid copy number and its conjugal transfer to other agrobacteria. Many natural agrobacteria are avirulent, lacking the Ti plasmid. The burden of harboring the Ti plasmid depends on the environmental context. Away from diseased hosts, plasmid costs are low but the benefit of the plasmid is also absent. Consequently, plasmidless genotypes are favored. On infected plants the costs of the Ti plasmid can be very high, but balanced by the opine benefits, locally favoring plasmid bearing cells. Cheating derivatives which do not incur virulence costs but can benefit from opines are favored on infected plants and in most other environments, and these are frequently isolated from nature. Many agrobacteria also harbor an At plasmid which can stably coexist with a Ti plasmid. At plasmid genes are less well characterized but in general facilitate metabolic activities in the rhizosphere and bulk soil, such as the ability to breakdown plant exudates. Examination of A. tumefaciens C58, revealed that harboring its At plasmid is much more costly than harboring it's Ti plasmid, but conversely the At plasmid is extremely difficult to cure. The interactions between these co-resident plasmids are complex, and depend on environmental context. However, the presence of a Ti plasmid appears to mitigate At plasmid costs, consistent with the high frequency with which they are found together.

Quorum-dependent mannopine-inducible conjugative transfer of an Agrobacterium opine-catabolic plasmid

The Ti plasmid in Agrobacterium tumefaciens strain 15955 carries two alleles of traR that regulate conjugative transfer. The first is a functional allele, called traR, that is transcriptionally induced by the opine octopine. The second, trlR, is a nonfunctional, dominant-negative mutant located in an operon that is inducible by the opine mannopine (MOP). Based on these findings, we predicted that there exist wild-type agrobacterial strains harboring plasmids in which MOP induces a functional traR and, hence, conjugation. We analyzed 11 MOP-utilizing field isolates and found five where MOP induced transfer of the MOP-catabolic element and increased production of the acyl-homoserine lactone (acyl-HSL) quormone. The transmissible elements in these five strains represent a set of highly related plasmids. Sequence analysis of one such plasmid, pAoF64/95, revealed that the 176-kb element is not a Ti plasmid but carries genes for catabolism of MOP, mannopinic acid (MOA), agropinic acid (AGA), and the agrocinopines. The plasmid additionally carries all of the genes required for conjugative transfer, including the regulatory genes traR, traI, and traM. The traR gene, however, is not located in the MOP catabolism region. The gene, instead, is monocistronic and located within the tra-trb-rep gene cluster. A traR mutant failed to transfer the plasmid and produced little to no quormone even when grown with MOP, indicating that TraRpAoF64/95 is the activator of the tra regulon. A traM mutant was constitutive for transfer and acyl-HSL production, indicating that the anti-activator function of TraM is conserved.

Convergent evolution of Amadori opine catabolic systems in plasmids of Agrobacterium tumefaciens

Deoxyfructosyl glutamine (DFG, referred to elsewhere as dfg) is a naturally occurring Amadori compound found in rotting fruits and vegetables. DFG also is an opine and is found in tumors induced by chrysopine-type strains of Agrobacterium tumefaciens. Such strains catabolize this opine via a pathway coded for by their plasmids. NT1, a derivative of the nopaline-type A. tumefaciens strain C58 lacking pTiC58, can utilize DFG as the sole carbon source. Genes for utilization of DFG were mapped to the 543-kb accessory plasmid pAtC58. Two cosmid clones of pAtC58 allowed UIA5, a plasmid-free derivative of C58, harboring pSa-C that expresses MocC (mannopine [MOP] oxidoreductase that oxidizes MOP to DFG), to grow by using MOP as the sole carbon source. Genetic analysis of subclones indicated that the genes for utilization of DFG are located in a 6.2-kb BglII (Bg2) region adjacent to repABC-type genes probably responsible for the replication of pAtC58. This region contains five open reading frames organized into at least two transcriptional soc (santhopine catabolism) groups: socR and socABCD. Nucleotide sequence analysis and analyses of transposon-insertion mutations in the region showed that SocR negatively regulates the expression of socR itself and socABCD. SocA and SocB are responsible for transport of DFG and MOP. SocA is a homolog of known periplasmic amino acid binding proteins. The N-terminal half of SocB is a homolog of the transmembrane transporter proteins for several amino acids, and the C-terminal half is a homolog of the transporter-associated ATP-binding proteins. SocC and SocD could be responsible for the enzymatic degradation of DFG, being homologs of sugar oxidoreductases and an amadoriase from Corynebacterium sp., respectively. The protein products of socABCD are not related at the amino acid sequence level to those of the moc and mot genes of Ti plasmids responsible for utilization of DFG and MOP, indicating that these two sets of genes and their catabolic pathways have evolved convergently from independent origins.

Intracellular accumulation of mannopine, an opine produced by crown gall tumors, transiently inhibits growth of Agrobacterium tumefaciens

pYDH208, a cosmid clone from the octopine-mannityl opine-type tumor-inducing (Ti) plasmid pTi15955 confers utilization of mannopine (MOP) and agropine (AGR) on Agrobacterium tumefaciens strain NT1. NT1 harboring pYDH208 with an insertion mutation in mocC, which codes for MOP oxidoreductase, not only fails to utilize MOP as a sole carbon source, but also was inhibited in its growth by MOP and AGR. In contrast, the growth of mutants with insertions in other tested moc genes was not inhibited by either opine. Growth of strains NT1 or UIA5, a derivative of C58 that lacks pAtC58, was not inhibited by MOP, but growth of NT1 or UIA5 harboring pRE10, which codes for the MOP transport system, was inhibited by the opine. When a clone expressing mocC was introduced, the growth of strain NT1(pRE10) was not inhibited by MOP, although UIA5(pRE10) was still weakly inhibited. In strain NT1(pRE10, mocC), santhopine (SOP), produced by the oxidation of MOP by MocC, was further degraded by functions encoded by pAtC58. These results suggest that MOP and, to a lesser extent, SOP are inhibitory when accumulated intracellularly. The growth of NT1(pRE10), as measured by turbidity and viable cell counts, ceased upon the addition of MOP but restarted in a few hours. Regrowth was partly the result of the outgrowth of spontaneous MOP-resistant mutants and partly the adaptation of cells to MOP in the medium. Chrysopine, isochrysopine, and analogs of MOP in which the glutamine residue is substituted with other amino acids were barely taken up by NT1(pRE10) and were not inhibitory to growth of the strain. Sugar analogs of MOP were inhibitory, and those containing sugars in the D form were more inhibitory than those containing sugars in the L form. MOP analogs containing hexose sugars were more inhibitory than those containing sugars with three, four, or five carbon atoms. Mutants of NT1(pRE10) that are resistant to MOP arose in the zone of growth inhibition. Genetic and physiological analyses indicate that the mutations are located on pRE10 and abolish uptake of the opine.

Opine catabolic loci from Agrobacterium plasmids confer chemotaxis to their cognate substrates

Opines are carbon compounds produced by crown galls and hairy roots induced by Agrobacterium tumefaciens and A. rhizogenes, respectively. These novel condensation products of plant metabolic intermediates are utilized as nutritional sources by the Agrobacterium strains that induced the growths. Thus, opines are thought to favor the propagation of agrobacteria in the tumorsphere. Certain Agrobacterium strains were chemoattracted to opines. The chemotactic activities to octopine, to nopaline, to mannopine, and to agrocinopines A + B were dependent on the type of the Ti plasmid present in the bacterium. The determinants for chemotaxis to these opines were localized to the regions of the octopine- and nopaline-type Ti plasmids coding for transport and catabolism of that opine. An insertion in accA, which encodes the periplasmic binding protein for agrocinopines A + B, abolished chemotaxis while an insertion in accC, which encodes a component of the transport system, and an insertion in accF, which encodes a function required for agrocinopine catabolism, did not affect chemotaxis to this opine. Thus, transport and catabolism of these opines are not required for the chemotactic activity. Analyses of subclones of the acc region confirmed that accA is the only gene required from the Ti plasmid for chemotaxis to agrocinopines A + B.

Octopine-type Ti plasmids code for a mannopine-inducible dominant-negative allele of traR, the quorum-sensing activator that regulates Ti plasmid conjugal transfer

Conjugal transfer of Agrobacterium tumefaciens Ti plasmids is regulated by two hierarchical signalling systems. Transfer is dependent on a subset of opines produced by the plant tumours induced by the bacterium. Induction also requires an acyl-homoserine lactone signal, called AAI, that is produced by the bacteria themselves. AAI is the co-inducer for TraR, the transcriptional activator required for expression of the tra regulon. Octopine induces conjugation of the octopine-mannityl opine-type Ti plasmids by regulating the expression of traR via OccR, the octopine-dependent activator of the opine regulon. We have discovered a second traR-like gene, trlR, on the octopine-mannityl opine-type Ti plasmids pTi15955 and pTiR10. This gene is located in an operon coding for a mannopine transport system and is expressed as part of the mannityl opine regulon. Sequence analysis indicated that trlR is a frameshift allele of traR, and the resulting protein lacks the carboxy-terminal domain thought to constitute the DNA-binding region of TraR. Expression of trlR inhibited octopine-induced conjugation of pTi15955 and pTiR10 by suppressing the TraR-mediated transcription of the tra and trb operons. Although TrlR had no effect on the expression of traR, TraR activated the expression of trlR. Southern hybridizations indicated that several other Ti and opine-catabolic plasmids contain more than one copy of genes homologous to traR. We propose that trlR is a dominant negative allele of traR and that TrlR inhibits conjugation by forming inactive heteromultimers with TraR.

A T-DNA gene required for agropine biosynthesis by transformed plants is functionally and evolutionarily related to a Ti plasmid gene required for catabolism of agropine by Agrobacterium strains

The mechanisms that ensure that Ti plasmid T-DNA genes encoding proteins involved in the biosynthesis of opines in crown gall tumors are always matched by Ti plasmid genes conferring the ability to catabolize that set of opines on the inducing Agrobacterium strains are unknown. The pathway for the biosynthesis of the opine agropine is thought to require an enzyme, mannopine cyclase, coded for by the ags gene located in the T(R) region of octopine-type Ti plasmids. Extracts prepared from agropine-type tumors contained an activity that cyclized mannopine to agropine. Tumor cells containing a T region in which ags was mutated lacked this activity and did not contain agropine. Expression of ags from the lac promoter conferred mannopine-lactonizing activity on Escherichia coli. Agrobacterium tumefaciens strains harboring an octopine-type Ti plasmid exhibit a similar activity which is not coded for by ags. Analysis of the DNA sequence of the gene encoding this activity, called agcA, showed it to be about 60% identical to T-DNA ags genes. Relatedness decreased abruptly in the 5' and 3' untranslated regions of the genes. ags is preceded by a promoter that functions only in the plant. Expression analysis showed that agcA also is preceded by its own promoter, which is active in the bacterium. Translation of agcA yielded a protein of about 45 kDa, consistent with the size predicted from the DNA sequence. Antibodies raised against the agcA product cross-reacted with the anabolic enzyme. These results indicate that the agropine system arose by a duplication of a progenitor gene, one copy of which became associated with the T-DNA and the other copy of which remained associated with the bacterium.

Detection and Enumeration of a Tagged Pseudomonas fluorescens Strain by Using Soil with Markers Associated with an Engineered Catabolic Pathway

Previously we described a novel gene tagging method, using the moc (mannityl opine catabolism) region from the Agrobacterium tumefaciens Ti plasmid pTi15955, to identify microorganisms destined for release into the environment. Here, we used the engineered strain Pseudomonas fluorescens PF5MT12 carrying the moc region integrated into the bacterial chromosome to demonstrate the usefulness of the markers for detection and direct selection of marked organisms present in soil samples. Using this system, we routinely detected population levels as low as 10(sup2) CFU per g of soil sampled. In addition to direct selection, we developed an immunologically based assay using MOP cyclase, a unique enzyme associated with moc, as the epitope for detecting the tagged organism. The colony immunoblot assay proved to be highly specific and without any false-positive signals when used to identify organisms cultured from soil on nonselective medium. The numbers of colonies that were immunoreactive with the anti-MOP cyclase antibody were essentially equal to those that grew out on selection plates. This indicates that MOP cyclase can be used as a marker and that we can use nonselective medium to retrieve the marked genetically engineered microorganisms and then identify them by using colony immunoblot assays. These direct selection and colony immunoblot methods provide a sensitive and accurate strategy for identifying and enumerating marked organisms recovered from soil samples. We also developed a rapid assay for MOP cyclase that does not require cell permeabilization with toluene. This assay can be used to verify tagged organisms isolated by other methods or to screen large numbers of colonies for the tag following nonselective isolation.

Ti plasmid-encoded genes responsible for catabolism of the crown gall opine mannopine by Agrobacterium tumefaciens are homologs of the T-region genes responsible for synthesis of this opine by the plant tumor

Agrobacterium tumefaciens NT1 harboring pSaB4, which contains the 14-kb BamHI fragment 4 from the octopine/mannityl opine-type Ti plasmid pTi15955, grew well with agropine (AGR) but slowly with mannopine (MOP) as the sole carbon source. When a second plasmid encoding a dedicated transport system for MOP was introduced, these cells grew well with both AGR and MOP. Transposon insertion mutagenesis and subcloning identified a 5.7-kb region of BamHI fragment 4 that encodes functions required for the degradation of MOP. DNA sequence analysis revealed seven putative genes in this region: mocD (moc for mannityl opine catabolism) and mocE, oriented from right to left, and mocRCBAS, oriented from left to right. Significant identities exist at the nucleotide and derived amino acid sequence levels between these moc genes and the mas genes that are responsible for opine biosynthesis in crown gall tumors. MocD is a homolog of Mas2, the anabolic conjugase encoded by mas2'. MocE and MocC are related to the amino half and the carboxyl half, respectively, of Mas1 (MOP reductase), the second enzyme for MOP biosynthesis. These results indicate that the moc and mas genes evolved from a common origin. MocR and MocS are related to each other and to a putative repressor for the AGR degradation system encoded by the rhizogenic plasmid pRiA4. MocB and MocA are homologs of 6-phosphogluconate dehydratase and glucose-6-phosphate dehydrogenase, respectively. Mutations in mocD and mocE, but not mocC, are suppressed by functions encoded by the chromosome or the 450-kb megaplasmid present in many Agrobacterium isolates. We propose that moc genes derived from genes located elsewhere in the bacterial genome and that the tumor-expressed mas genes evolved from the bacterial moc genes.

A Ti plasmid-encoded enzyme required for degradation of mannopine is functionally homologous to the T-region-encoded enzyme required for synthesis of this opine in crown gall tumors

The mocC gene encoded by the octopine/mannityl opine-type Ti plasmid pTi15955 is related at the nucleotide sequence level to mas1' encoded by the T region of this plasmid. While Mas1 is required for the synthesis of mannopine (MOP) by crown gall tumor cells, MocC is essential for the utilization of MOP by Agrobacterium spp. A cosmid clone of pTi15955, pYDH208, encodes mocC and confers the utilization of MOP on strain NT1 and on strain UIA5, a derivative of NT1 lacking the 450-kb cryptic plasmid pAtC58. NT1 or UIA5 harboring pYDH208 with an insertion mutation in mocC failed to utilize MOP as the sole carbon source. Plasmid pSa-C, which encodes only mocC, complemented this mutation in both strains. This plasmid also was sufficient to confer utilization of MOP on NT1 but not on UIA5. Computer analysis showed that MocC is related at the amino acid sequence level to members of the short-chain alcohol dehydrogenase family of oxidoreductases. Lysates prepared from Escherichia coli cells expressing mocC contained an enzymatic activity that oxidizes MOP to deoxyfructosyl glutamine (santhopine [SOP]) in the presence of NAD+. The reaction catalyzed by the MOP oxidoreductase is reversible; in the presence of NADH, the enzyme reduced SOP to MOP. The apparent Km values of the enzyme for MOP and SOP were 6.3 and 1.2 mM, respectively. Among analogs of MOP tested, only N-1-(1-deoxy-D-lyxityl)-L-glutamine and N-1-(1-deoxy-D-mannityl)-L-asparagine served as substrates for MOP oxidoreductase. These results indicate that mocC encodes an oxidoreductase that, as an oxidase, is essential for the catabolism of MOP. The reductase activity of this enzyme is precisely the reaction ascribed to its T-region-encoded homolog, Mas1, which is responsible for biosynthesis of mannopine in crown gall tumors.

Purification and characterization of catabolic mannopine cyclase encoded by the Agrobacterium tumefaciens Ti plasmid pTi15955

Catabolic mannopine (MOP) cyclase encoded by certain Agrobacterium Ti and Ri plasmids lactonizes MOP to agropine (AGR). The enzyme, purified to homogeneity from a recombinant clone, has a molecular mass of 45 kDa as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chromatography. The enzyme catalyzed the lactonization of MOP to AGR without the need for any cofactors. The enzyme also converted AGR to MOP with the lactonizing activity being predominant over the reverse reaction. MOP cyclase is specific for imine conjugates of D-hexose and L-glutamine and was not inhibited by sugars or amino acids. The enzyme lactonized deoxyfructosyl glutamine, a natural intermediate of MOP synthesis and catabolism, to a product indistinguishable from chrysopine, a newly discovered crown gall opine. The enzyme also lactonized N-l-(1,2-dideoxy-D-mannityl)-L-glutamine, indicating that a hydroxyl group at carbon atom 2 of the sugar moiety is not required for the enzymatic reaction.