Glycoprotein is a kind of protein containing oligosaccharide chain, which is connected by covalent bond.The oligosaccharide chains are usually linked to proteins through the glycosylation in the process of co translation modification or post translation modification.
Glycoprotein polypeptide chains often carry many short heterosaccharide chains.They usually include N-acetylhexfurfuramide and hexose (often galactose or mannose, but less glucose).The end member of the chain is often sialic acid or L-fucose.This oligosaccharide chain is often branched, rarely containing more than 15 monomers, and generally contains 2-10 monomers, with a molecular weight equivalent to 540-3200.The number of sugar chains also varies greatly.
oligosaccharideThere are two ways to combine with protein: (1) Semi acetal of sugarhydroxylAnd amino acids containing hydroxyl groups（serine、threonine, hydroxylysine, etc.) with O-glycoside bond;(2) SugarySemiacetal hydroxylAnd winteramideThe amide group of is bound by N-glycoside bond.It is widely distributed in nature.Sixty or seventy species studiedPlasma proteinMost of them are glycoproteins.Some enzymes and hormones are glycoproteins.So is glycoproteinCell plasma membrane、Intercellular substance, plasma mucus, etc.More and more glycoproteins are isolated from plants, especiallylectin。Now from vertebrates andinvertebrateMany kinds of lectins have been isolated from many microorganisms, and most of them belong to glycoproteins.Glycoprotein has species specificity. One kind of protein exists in the form of glycoprotein in one animal, but it is different in another animal.Even if they are the same glycoprotein, their sugar content may be different, such as that of cattle, sheep and pigsPancreatic ribonucleaseBoth are glycoproteins, but the sugar content is 9.4%, 9.8% and 38% respectively;This enzyme in rats does not contain sugar.At the same time, glycoprotein is also a kind ofBinding protein, glycoprotein is composed of shortOligosaccharide The overall properties of molecules covalently linked with proteins are closer to proteins.The sugar and protein are mainly composed of proteincovalent bondConnected with several short oligosaccharide chains, these oligosaccharide chains are often branched heterosaccharide chains, and do not show duplicationdisaccharideThe series is generally composed of 2-10 monomers (less than 15)=, and the end members are usuallysialic acidOr L-fucose.Generally, each molecule contains less sugar (about 4%).Some glycoproteins contain only one or several sugar groups, while others contain multiple linear or branchedoligosaccharideSide chains.Glycoproteins are usually secreted into body fluids or membrane proteins, which are located outside cells and have corresponding functions.Glycoproteins include enzymes, hormones, carrierslectin, antibodies, etc.Compositionβ-D-GrapeSugar (Glc)α-D-mannose（Man） α-D-Galactose（Gal）α-D-xylose (Xyl)α-D-Arabinose（Ara） α-L-fucose (Fuc)Glucuronic acid(GlcuA) IduAN-acetylglucosamine(GlcNAc) N-acetylGalactosamine（GalNAc）N-acetylneuraminic acid(NeuNAc) is sialic acid (Sia).
② O-glycoside bond type: oligosaccharide chain (GalNACα-Hydroxy) and Ser, Thr and hydroxylysinehydroxyprolineThe hydroxyl group of is connected.
③ S-glycoside bond type:CysteineIs the glycopeptide bond of the connection point.
④ Ester glycoside bond type: AspartateglutamateThe free carboxyl group of is the junction point.
Structure of sugar chain
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The sugar chains in glycoproteins vary greatly and contain abundant structural information.Oligosaccharide It is often receptor and enzymeRecognition site。
1. N-glycosidic bond type (N-linked) N-glycosidic bond type mainly includes three types of oligosaccharide chains:
① HighmannoseType, consisting of GlcNAc and mannose;
N-Glycopeptide bond, such asβ- GlcNAc Asn and O-glycopeptide chains, such asα-GalNAc-Thr/Ser，β-Gal-Hyl，β-L-Arafat Hyp, N-connectedOligosaccharide (N-sugar chain) all contain a common structural pattern called core pentasaccharide or trisaccharidemannoseThe N-sugar chain can be divided into three types: complex type, high mannose type and heterozygous type. Their differences are mainly in the peripheral chain.
O-sugar chainIts structure is simpler than that of N-sugar chain, but its connection forms are more than that of N-sugar chain.[2]
glycoproteinOligosaccharide Terminalsialic acidResidues, which determine whether a protein exists in the blood stream or is removed by the liver.
A. VertebrateCeruloplasmin。Hepatocytes can degrade ceruloplasmin that has lost sialic acid, and the elimination of sialic acid may be one of the ways to mark the "old" protein in the body.
B. Red blood cells.The content of sialic acid on the membrane of newborn red blood cells is much higher than that of mature red blood cells.useSialidaseThe newborn red blood cells were treated and reinjected into the body, and disappeared after a few hours.However, red blood cells not treated with enzymes can still survive in vivo after several days of reinjection.
The key role of oligosaccharide chain
lymphocyteNormally, it should return to the spleen and be cut offsialic acidAfter that, it unexpectedly returned to the liver.stayProkaryoteExpressed inEukaryotic gene, cannotGlycosylation。Glycoproteins can be either cytosolic or membrane-bound, and can exist in cells or inIntercellular substanceMedium.Glycoproteins are typical in animals and plants, especially in vertebrates, such as metalsTransporter（transferrin）Blood ceruloplasmin,Coagulation factor、Complement system, some hormones, follicle stimulating hormone (FSH), RNase, membraneBinding protein(e.gAnimal cellNa of membrane+-K+-ATPase）、Major histocompatibility antigen（major histocompatibility antigen，cell surfaceThe marker that mediates the cross matching between donor organs and recipient organs).Most glycoproteinoligosaccharideIt is the functional center of glycoprotein.Some glycoprotein sugars play a protective or lubricating role for the glycoprotein itself. For example, bovine RNaseB (glycoprotein) is more resistant to heat than RNaseA, and a large amount of sialic acid can enhance theviscosityThis enhances the lubricity of saliva.Antarctic fishAntifreeze proteinThe sugar component of C can form hydrogen bond with water to prevent the formation of ice crystals, thus improving the frost resistance.Glycoprotein plays a more complex role in intercellular signal transmission.The target cell binding protein GP120 of HIV is a glycoprotein that can bind to the CD4 receptor on the surface of human target cells to attach to the surface of target cells. If the sugar part of GP120 is removed, it cannot bind to the CD4 receptor to lose its infectivity.Glycoproteins on the cell surface form cellularSugar calyx(sugar coating), participate in cell adhesion, which plays a key role in the growth, development and differentiation of embryos and tissues.[3]
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Glycoprotein formation in cells
Glycoprotein is a protein containing sugarOligosaccharide And certain in peptide chainAmino acid residueIt is covalently linked by glycoside bond.Whose main biological function is cell or moleculeBiometricsFor example, when the egg is fertilized, the sperm needs to recognize the corresponding glycoprotein on the egg cell membrane.recipientProtein and tumor cell surface antigen are also glycoproteins.Glycoproteins widely exist in animals, plants and microorganisms, with a wide variety of functions.It can be divided into three categories according to the way of existence:
2. The peptide chain of membrane binding glycoprotein is composed of hydrophobic peptide segment and hydrophilic peptide segment.Hydrophobic peptide can be one to several, and can be passed throughHydrophobic interactionEmbedded in the membrane lipid bilayer.The hydrophilic peptide was exposed outside the membrane.sugar chainThey are attached to hydrophilic peptides and have strict directionality.stayplasma membraneThe sugar chains on the surface all face outward;stayCell intimaIt generally faces the cavity surface.Membrane binding glycoproteins include enzymesrecipient, lectin, carrier protein, etc.These glycoproteins are often involved inCell recognitionAnd can be used as the surface marker or surface antigen of specific cells or cells at specific stages.
Microscopic glycoprotein
3. Structural glycoproteinExtracellular matrixInsolubility inmacromoleculeGlycoproteins, such ascollagen And various non gelatinous glycoproteins（Fibronectin、LamininEtc.).Their function is not only to support, connect and buffer the extracellular matrix, but also to participate in cell recognition, adhesion and migration, and regulate cell proliferation and differentiation.
Oligosaccharide It usually refers to 2~10monosaccharideA polymer that is linked by glycoside bonds.The oligosaccharide chain of glycoprotein is mostly branched.Because the terminal carbon (heterocephalic carbon) atom of monosaccharide hasα、βTwo configurations, and there are multiplehydroxyl, therefore,sugar chainThe diversity of structures exceedsPolynucleotideAnd peptide chain.Enough identification information can be stored in the sugar chain structure, so thatmolecular recognitionandCell recognitionPlay a decisive role in.The physiological functions of glycoprotein include coagulation, immunity, secretionEndocytosis、Material transportRegulation of information transmission, nerve conduction, growth and differentiationcell migrationCell homing, wound repair and regeneration.The sugar chain of glycoprotein is also involved in maintaining its peptide chain in the presence ofbiological activityAnd stable peptide chain structure, and endow the whole glycoprotein molecule with specific physical and chemical properties (such as lubricity, viscoelasticity, anti heat inactivation, anti protease hydrolysisFrost resistanceEtc.).
glycoprotein
Glycoprotein and many diseases such as infection, tumorCardiovascular disease, liver disease, kidney diseasediabetesAnd the occurrence and development of some genetic diseases.also,cell surfaceGlycoprotein andGlycolipidsThey can "fall off" to the surrounding environment or enter the blood circulation, and they can be used as signs of abnormalitiesclinical diagnosisProvide information;Glycoproteins in body fluids are also often found in patients with certain diseasesSpecificityStrong or weak changes, which can help diagnosis or prognosis.Glycoprotein is also increasingly involved in treatment.For example, antibodies that target specific sugar structures on specific cell surfaces can be used as targeted vectors for targeting therapeutic drugs.Utilization of sugars (monosaccharidesoligosaccharideorGlycopeptide）Anti infection and antiTumor metastasisIt has also come to the fore.[1]
