Antibodies and Antigens

Transcription

1 Antibdies nd Antigens Yur bdy s s nturl defense system Antibdies The frml chemicl nme fr ntibdies is immunglbulins. Immunglbulins re prtein mlecules which defend yur bdy frm freign txins knwn s ntigens. Immunglbulin G, IgG, is the mjr ntibdy in serum (bld) nd will be wht we use fr exmple purpses. Immunglbulin Structure IgG, is cmprised f 3 plypeptide chins 4 light chins, L 25-kd chins (shwn in yellw) 2 hevy chin, H 50-kd chin (shwn in blue) A disulfide bnd links L t H. S S H chins link t ech ther by r mre disulfide bnds.

2 Immunglbulin Clevge IgG is cleved by n enzyme clled ppin int 3 frgments, F. F b = ntigen binding frgment F c = cnstnt frgment (des nt bind t ntigens) The disulfide bnds cnnecting F b nd F c re flexible nd clled hinges This flexibility llws fr ne ntibdy t bind t ntigens with multiple bnding sites. Antigen Crss-linking Since ech IgG cntins tw F b chins it cn crss- link multiple ntigens Other Immunglbulins IgA externl secretins: sliv, ters, mucus, etc. IgM ppers first in serum upn intrductin f ntigen, 0 binding sites llws fr excellent binding t multiple receptr ntigens IgD rle nt knwn IgE prtectin frm prsites, cn ls cuse llergic rectins 2

3 Vrible Regins Mechnism f Antibdy Actin The end terminus f the F b chins re HIGHLY vrible. This is why ur bdies re ble t frm ntibdies t nerly ll ntigens. V L = vrible light V H = vrible hevy C L = cnstnt light C H = cnstnt hevy Why cn they vry s widely? V L nd V H Chemistry the REAL BASICS! V L nd V H re cmplex prteins which dpt cmmn structure clled the immunglbulin fld. The immunglbulin fld is cnstructed frm tw β-sheets. The bet sheets cntin nti- prllel strnds. These strnds surrund hydrphbic cre. The tw sheets re cnnected vi disulfide bnd. ey spect: Three lps t the end re hypervrible, hypervrible, mening tht their min cid sequences cn be vried in mny, mny wys. These lps re the vrible regins we sme in the previus slide. This regin is ls knwn s the cmplementrity determining regin r CDR. Big Picture S if V H cn vry immensely nd V L cn very immensely then: i,j Σ (V L ) i + (V H ) j = i,j= Ok, s mre like relly big number! 3

4 Hw d d he d tht? S then yu sy Hw cn ur bdy mke ll these different ntibdies? I sy : Tht is wht B-cells B re fr! B-cells nd T-cellsT Bth develp in the bne mrrw B-cells mture in the Bne ne mrrw s well wheres, T-cells T mture in the Thymus. hymus. B-cells prduce ntibdies with the help f T-cells. T Sme T-cells T cn ct s ntibdies themselves (i.e., ginst viruses but we wn t t get int tht) Why d we cre nd hw the heck des this relte t Chemistry? # Everything reltes t CHEMISTRY!!! #2 Yu shuld cre becuse this is hw yur bdy prtects itself. #3 One f the mny wys this reltes t Chemistry is the reltinship between ntigens nd ntibdies bth kineticlly nd in terms f equilibrium. 4

5 Antibdy/Antigen Affinity A very imprtnt spect f the reltinship between ntibdies nd ntigens is their ffinities fr ech ther. Sme ntibdy/ntigen pirs hve reltively high ffinity fr ech ther nd sme hve smewht lwer ffinity fr ech ther. Equilibrium Think bck t when we studied equilibrium. We shwed tht when species re sscited with ech ther we cn mesure the degree f sscitin with n???? Equilibrium Cnstnt, Exmple We ll cll ntigens G nd ntibdies A. d stnds fr Equilibrium f disscitin GA G + A d = [ G][ A] [ GA] 5

6 Try these prblems! Suppse tht the disscitin cnstnt f n b -hpten cmplex is 3 x 0-7 M t 25 C. F b A) Immunlgist ften spek f ffinity ( ), the reciprcl f the disscitin cnstnt, in cmpring ntibdies. Wht is the ffinity f this F b? B) Wht is the stndrd free energy f binding? C) The rte cnstnt f relese f hpten frm the cmplex is 20 s -. Wht is the rte cnstnt f sscitin? A. = 6 = = M M d B. = RT ln f fr binding this wuld be J ml f = = 3724 J f = 37kJ f ( 298 ) 6 ln(3.3 0 M ) 6

7 rte relese = equilibrium : C. rtesscitin = k rte relese k k k k relese sscitin sscitin sscitin relese [ GA] [ G][ A] sscitin = rtesscitin [ GA] = k [ G][ A] sscitin k [ GA] relese = = k relese [ G][ A] 6 = 20s ( M ) 8 = M s Here is nther prblem! The stndrd free energy f binding f F b derived frm n ntivirl IgG is -7 kcl/ml t 25 C. A) Clculte the disscitin cnstnt f this interctin. B) Predict the disscitin cnstnt f the IgG, ssuming tht bth cmbining sites f the ntibdy cn interct with virl epitpes nd tht the free-energy energy cst f ssuming fvrble hinge ngle is +3 kcl/ml. A. 7kcl 000cl 4.8J = J ml kcl cl ml = RT ln f ln = RT = e = e ( )( ) =.3 0 M f f RT 6 = = M d M 7

8 f ln = RT = e = e ( )( ) =.38 0 M B. 4kcl 000cl 4.8J = 6720 J ml kcl cl ml nw duble it becuse 2 sites : J = J ml ml = RT ln f f RT 5 = = M d M References This presenttin ws prepred with the id f the fllwing texts: Grrett nd Grishm, 2002 Berg, Tymczk, nd Stryer, r.sectin.5576 The fllwing website ws ls used: lgypges/b/b_nd_tcells.html 8