ABSTRACT
Thediscoveryofthetwoquintessentialretinoid-bindingproteins,CRBPand CRABP,providednewopportunitiestostudyretinoidmetabolismandfunction (87--89).Theirwidespreaddistributioninmultipletissuesandcelltypesarguesfor anessentialandcentralroleinretinoidbiologyandposesseveralnonmutually exclusivepossibilities(90,91).Theycouldserveasvehiclestosolubilizeand transportretinoids;assubstratesforretinoidmetabolismbyuniqueenzymes(as wellasforpreventingretinoidsfromundergoingmetabolismbyenzymesthat cannotrecognizetheholoproteincomplex);asregulatoryproteinswhichactivate orinhibitpathwaystodeterminethedirectionofmetabolicflux(retinoidstorage, catabolism,orretinoicacidsynthesis),dependingontheirstate(apoorholo).The workingmodelusedinmylabhasthetwoproteinsholo-CRBPandholo-CRABP mediatingthesynthesisandmetabolismofretinoicacidthroughprotein-protein interactions.Thispostulatesthatspecificenzymesrecognizethebolo-binding proteins(the"cassette")assubstrates,obviatingobligatorydissociationofthe retinoidfromthebindingproteinandrandomdiffusionoftheretinoidthroughthe aqueousmediumto"find"anenzyme.Theformulaspresentthedistinction betweentherandomdiffusionandprotein-proteininteractionmodels:
1.RandomDiffusion BP-retinoid+Enz=="'BP+retinoid+Enz=="'BP+Enz-retinoid-+product
2.Protein-ProteinInteraction a.DirectTransfer BP-retinoid+Enz=="'BP-retinoid-Enz::;;=:::Bp+retinoid-Enz-+product OR b.MetabolisminSitu BP-retinoidsub+Enz=="'BP-retinoidsub-Enz->
BP-retinoidproo-Enz-+BP-retinoidprod+Enz
Oneoftwomechanismscoulddescribethebindingprotein(BP)-enzymeinter~ action.TheenzymecouldrecognizeandformacomplexwiththeBP,perhaps alteringtheconformationoftheBPtoloweritsaffinityforretinoidandfacilitate directtransferoftheretinoidfromthebindingpocketoftheBPtotheactivesite ofanenzyme.Alternatively,metabolisminsitucouldoccurwhiletheretinoid remainsboundtothebindingprotein,withultimatereleaseofthefrom thebindingprotein.Directtransferratherthanmetabolisminsituseemsmore reasonableasafirstapproximationbasedontheavailabledataonthestructuresofBPsanddehydrogenases(92-95).Regardlessoftheprecisemechanism, atnotimeisdiffusionrequiredforthetransferofretinoidsbetweenproteins.
Protein-protein interaction provides a mechanism for selectively during retinoid metabolism by allowing only enzymes that recognize retinoid BPs to metabolize retinoids and by denying enzymes access to retinoids that do not recognize the ligand-specific retinoid BPs. Channeling retinoids between proteins also provides the advantage of protecting sensitive cells from the toxicity of free retinoids. Figure 4 depicts the protein-protein interaction mtodel with the direct transfer mechanism.
