Attachment of the saccharide concatenation to protein is one of the most important posttranslational alterations of eucaryotic proteins, particularly secreted and membrane proteins, which lead to the formation of a host of protein-bound oligosaccharides with diverse biological maps. There are 5 distinguishable types of sugar-peptide bonds i.e. N- and O-glycosylated, P-mannosylated, phophoglycation and glypiation ( Spiro 2002 ) .
The N- glycosidic bond between the glycan concatenation and Asn residue in the consensus motive ( Asn-X-Ser/Thr ) of a protein is most prevailing among all the types of sugar-amino acid/peptide bonds ( Spiro, 1973 ) . There are more than 16 enzymes involved in assorted stairss of glycosylation that occur in endoplasmic Reticulum ( ER ) , golgi setup, cytosol and karyon. One of such enzymes is treating alpha glucosidase-I ( glu-I ) , an ER inner membrane bound protein. I±-Glucosidase I commences paring of N-glycans by spliting the I±- ( 1,2 ) linked glucose unit from the freshly assembled Glc3Man9GlcNAc2 after en axis transportation to the nascent sequence ( Asn-X-Thr/Ser ) of proteins ( Grinna & A ; Robbins 1980 ) .
It has become apparent that the alterations that take topographic point in the ER reflect a spectrum of maps related to glycoprotein folding, quality control, screening, debasement, and secernment. In the ER, the Glu-I along with alpha-glucosidase-II dramas a major function in protein quality control system. Hence any damage in this tract leads to drastic effects on map of the secreted glycoprotein every bit good as the physiology of the being. The deficiency of glu-I in worlds leads to CDG-IIb [ CDG – congenital upsets of glycosylation ] ( De Praeter et Al. 2000 ) . Despite its critical function in the synthesis of glycoproteins, many facets of construction, catalytic mechanism of glu-I are still unknown.
In Saccharomyces cerevisiae, CWH41 cistron encodes glucosidase-I
Baker ‘s barm, Saccharomyces cerevisiae, a often used eucaryotic theoretical account being, merely possesses two sorts of protein glycosylation, N-glycosylation of asparagine residues and O-mannosylation of threonine and serine residues ( Lehle et al. 2006 ) . These two types of glycosylation are greatly conserved from barm to human. Furthermore, inborn upsets of glycosylation are associated with these two types of glycosylation. Hence, in the current survey, baker ‘s barm will be used as the theoretical account being to analyze the catalytic mechanism and structure-function relationship of glu-I.
In S.cerevisiae, glu-I is encoded by CWH41 cistron ( Romero et al. 1997 ) . It catalyses the remotion of distal I±-1,2 glucose residues from Glc3Man9GlcNAc2 ( G3M9GNA2 ) concatenation co-translationally attached to Asn residue in the consensus motif Asn-X-Ser/Thr of a nascent protein ( Kornfeld and Kornfeld 1985 ) . It belongs to glycosyl hydrolases household 63. This enzyme was foremost purified by Bause et Al. ( 1986 ) and reported to be a 95 kDa glycoprotein with optimal pH 6.8. Glu-I along with glucosidase-II
Based on old surveies Bause et al. 1986, it is apparent that the glu-I is a 94 kDa protein that is bound to inner membrane of ER. The omission of the N-terminal amino acid residues ( 24 residues ) resulted in soluble activity ( Faridmoayer ) . The proteolytic cleavage of glu-I resulted in assorted abbreviated signifiers among which a smallest 37kDa fragment showed highest catalytic activity, 1.9 times higher than that of the whole protein.
There is less information available sing the amino acid residues that play a function in contact action and substrate binding of glu-I. In the old surveies, site directed mutagenesis of glu-I was carried out ( Faridmoayer and Scaman 2007 ) . When E613 and D617 residues were replaced by alanine, glu-I wholly lost its map. This loss of map was either due to loss of active conformation or the loss of catalytic residues.
Processing I±-glucosidase I or mannosyl-oligosaccharide glucosidase ( EC 126.96.36.199 ) is a type-II membrane edge glycoprotein with 833 aminic acids. The omission of transmembrane part ( 1-24 amino acids at N-terminus ) resulted in soluble, glycosylated, functional glu-I where as omission of N-terminal 34 amino acids lead to the nonglycosylated catalytically active protein. This truncated catalytically active 94 kDa fragment of glu-I may be expressed in any strong procaryotic look systems like pet in E. coli due to the absence of glycosylation, to steer easy purification.
In general, the hydrolysis by glycosidases occurs via two major mechanisms giving rise to either an overall keeping or an inversion of constellation ( Koshland 1953 ) . The glu-I from S.cerevisiae was found to be an inverting glycosidase ( Palcic ) . Understanding the mechanism of glu-I can assist in development of mechanism based drug. The hydrolysis of glycosidic bond by glycosidases takes topographic point through acid- base contact action which requires two amino acids where one serves as a proton giver and the other as a nucleophile ( ref ) . Based on old findings on glu-I ( Faridmoayer and Scaman 2007 ) , we hypothesize that any two of the conserved carboxylic residues ( D601, E613, D617, D602, D670 and E804 ) play a important function in either contact action or substrate binding since these residues lie in catalytic part. However, homology surveies revealed that the carboxylic residues E613 and E804 are the possible catalytic residues.
Based on current apprehension, trypsin hydrolysis of glu-I at places Lys524 and Phe525 releases a 37 kDa fragment which is 1.9 times more active than the whole enzyme. The recombinant look of 37kDa fragment as an single protein in S.cerevisiae did non take to a catalytically active protein. This may be due to the demand of non-catalytic n-terminal sphere for the active conformation/ folding. Based on these surveies on glu-I, we can speculate that the N-terminal sphere of glu-I has two important functions i.e. aid in geting active conformation/folding and regulative map.
We hypothesize that the N-terminal fragment, due to its regulative map, decreases the activity by strictly specific adhering to its natural substrate. The 37 kDa truncated signifier, which is non regulated by N-terminal sphere, may demo relaxed specificity for sugars ( footing or back uping information needed. YGJK may be an illustration for this but homologya). We besides hypothesise that N-terminal sphere of glu-I shows lectin-like activity which facilitates specific adhering to glycan concatenation.
The chief obstruction in analyzing the construction and catalytic mechanism of glu-I is the measure of the active protein. Purification of ER membrane bound glu-I is boring and besides outputs were really low ( ref ) . In order to acquire the big measures of glu-I for farther word picture surveies, the over look of glu-I will be carried out in E.coli since N-glycosylation is non of import for the map of glu-I.
By and large, the mechanism of action of glycosyl hydrolases involves engagement of two carboxylic residues. Based on old findings ( Faridmoayer and Scaman 2007 ) , we hypothesize that any two of the conserved carboxylic residues ( D601, E613, D617, D602, D670 and E804 ) play a important function in either contact action or substrate binding since these residues lie in catalytic part. However, homology surveies revealed that the carboxylic residues E613 and E804 are the possible catalytic residues. Using site directed mutagenesis, aminic acids at these places will be replaced by their aminoalkane signifiers. Further mutagenesis surveies will besides be carried out in order to understand the catalytic mechanism.
The pH and temperature optima and the kinetic parametric quantities ( substrate specificity, kilometer, Vmax, and Ki against inhibitors like deoxynojiromycin ) of the catalytically active discrepancies will besides be carried out. Functional look of abbreviated signifiers of CWH41 will be carried out for farther apprehension of the functions of assorted spheres.
Three dimensional constructions of eucaryotic glu-I have non yet been reported although they were studied extensively. The surveies to find the three-dimensional construction of glu-I are being carried out at a confederate ‘s research lab. However, readyings will be carried out for crystallographic surveies of C-terminal 37kDa fragment and mutant signifiers of glu-I.
E. coli expresses a protein called ygjK which is homologous to glu-I ( Kurakata et al. 2008 ) . Due to its more relaxed substrate specificity, mutagenesis surveies will besides be carried out on ygjK to understand the of import residues involved in substrate binding and contact action.
His pull down to find interaction between n- and c-terminal spheres
The co-immunoprecipitation is normally used to prove whether two protein interact with each other. The n- and c-terminal spheres will be expressed as single proteins. The purified single his labeled N-terminal sphere will be treated with HRV3C peptidase and purified utilizing IMAC affinity chromatography to take its 6xhis ticket. The N-terminal sphere ( without his ticket ) therefore obtained will be incubated with his-tagged C-terminal sphere which is immobilized onto Ni2+ affinity column. The edge protein will be eluted and eluted fractions will be analyzed on SDS-PAGE.
If any interaction is observed between the c-terminal and n-terminal spheres, farther experiments will be carried out to find exact location or part that is responsible for interaction.
Determination of lectin activity of glu-I
N-terminal part may move as a sugar adhering sphere ( lectin like ) and regulates activity of glu-I. The lectin-like activity of glu-I will be determined by utilizing microtiter home bases that are impregnated with assorted glycan ironss.
Structural word picture of discrepancies of glu-I
The construction of glu-I will be determined utilizing crystallography and X-ray diffraction methods which will be carried out at confederate ‘s lab. The crystals will be grown utilizing hanging bead vapour diffusion method. The crystals therefore formed will be used for diffraction and subsequent analysis for the finding of 3D construction.
The biochemical activity of the glu-I is critical for the normal development and map of many beings including worlds ( De Praeter et Al. 2000 ) . Understanding of assorted facets of this enzyme will assist in understanding legion disease provinces that are straight or indirectly associated with its map.
Inhibitors of N-linked glycosylation tract have been extensively tested for antiviral activity since viral membrane proteins are synthesized and modified post-translationally by host machinery. The inhibitors of glu-I reportedly inhibited the reproduction of many viruses, which require to a great extent glycosylated proteins ( which strongly depend on canexin/calreticulin lectin chaperone system ) for their assembly and map, including human immune lack virus ( Dwek et al. 2002 ) .
Understanding the construction and catalytic mechanism of glu-I will significantly assist in developing mechanism- based drugs that are utile in intervention of assorted disease conditions. Presently many glu-I inhibitors likea are used as drugs to handle viral infections. Mechanism-based inactivators/drugs provide same possible clinical advantages as antecedently observed with inhibitor based drugs. The dissociation rate for mechanism based drugs, which covalently modify their mark enzyme, is really low or peers to zero whereas interaction of inhibitors with their enzyme opposite numbers is reversible. The mechanism-based drugs rely on chemical science of enzyme active site and extremely selective to aim enzyme which provide ultimate specificity for usage as drugs.
Understanding of glu-I will besides heighten the cognition of N-glycosylation, which is necessary for use of glycan ironss or glycoengineering for heterologic production of assorted medically or commercially of import proteins.
The processiong I±-glucosidase-I dramas a important function in quality control of protein N-glycosylation. The current survey will heighten the apprehension of glu-I catalytic mechanism and structural penetrations. The site-directed mutagenesis of glu-I will supply more inside informations about the amino acid residues that play a function in contact action.