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NBME 21 Answers

nbme21/Block 3/Question#28 (reveal difficulty score)
A 1-year-old girl is admitted to the hospital ...
Disruption of the secondary structure of collagen molecules 🔍 / 📺 / 🌳

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submitted by haliburton(225),
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p:gthenoa croscocputyc

maerettTn :osipont ohmAp :B bidns gosotrlree -&t;g osrpe ni ufgnla rmaenbem &;tg- kaesl n t.nesotc nca dad sn:tieolcFuy verdtceon ot F5U- ot ihtibin elnciuc caid ssiyneths

retlf/eh.lssmotimtm1bp/o6yc1eiepultsotuocglyc/n.do1:s0i/1b/4 //c8/o/1.t0s:clpeptalr1dmh5mcst.p1b/enmemuorsttoi4lgyobhiii-eob

submitted by wasabilateral(46),
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I nhtki it hsa eomsithng ot od wiht ingycel deu( to tis lslam szie it acn tfi in ymna scalpe weehr htore imoan sadci acn nto adn ehenc ti vsorpied rclu“tusart nctsomspea”c ot het lco,lange e..i ptu a nkik in the lhpaa )ile.hx If cyielgn is cledasmip by imtgheons ees,l I ndo’t kihtn aponegrl-lco nca omrf sit crteocr cdrnaosey tuersrtu.c

jotajota94  True! also, glycine is 1/3 of collagen alfa chains, so it makes sense that substitution with alanine (which is much bigger) would lead to disruption in the alpha helix formation. +1  
jotajota94  True! also, glycine is 1/3 of collagen alfa chains, so it makes sense that substitution with alanine (which is much bigger) would lead to disruption in the alpha helix formation. +  
thepacksurvives  Glycine is small and bendy, which allows it to form the fibrils for the triple helix +  
brasel  Also in general (FA 2018 pg 50) OI is from problems forming the triple helix which is secondary structure. Fortunately, they gave us something to reason with in the question (Gly->Ala) +3  
amy  Can someone help me understand why A is incorrect? FA2020 page 50: Triple helix of 3 collagen a chains is formed from procollagen via hydrogen and disulfide bond. Is this very similar to what A is decribing? A. Decreased hydrogen-bond formation between collagen molecules. +  
umpalumpa  The explanation given by wasabilateral could work if the question would say Gly-->Ala mutation. However, the question states that there is an Ala-->Gly mutation. +  

submitted by cellgamesgojan(42),
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I frdig,eu Yi--neylXcg is caynicellht neiorcdesd a amri“ryp nmioa diac rcttseruu of a ”troinpe ecnis the nintfoiied of a Pyrarim euttrucsr of a ptoeirn is “a ernali ihcna of nimoa ”aic.ds fI uoy emss wthi teh rPymari ,ttcurusre sa in het uqntesio t,mes oyu otnnca ormf eht droyencSa rcsrtuute of the ipotr,en hhciw si mitdedreen by the endonyngi-ogdhrb whhci ocrusc nbweete het ppiedte bnekcabo, eeditnpennd of the R sorpu.g I peho hist maed n.ssee

Fomr widepkiia: eynad“oSrc cuettusrr is olrlmafy dndeief yb the eptratn of hndyrego nsodb enewebt het noima hondyegr adn cbryolxa nyoxeg satom ni eth peeptdi boceanbk.” imshaeps( )inem

From Molecular Biology of the Cell:

Biologists distinguish four levels of organization in the structure of a protein. The amino acid sequence is known as the primary structure of the protein. Stretches of polypeptide chain that form α helices and β sheets constitute the protein’s secondary structure. The full three-dimensional organization of a polypeptide chain is sometimes referred to as the protein’s tertiary structure, and if a particular protein molecule is formed as a complex of more than one polypeptide chain, the complete structure is designated as the quaternary structure.

+2/- drdoom(1113),

submitted by usmle11a(94),
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ok ugsy adn i qeotu mfor esd/p:et/sut/iarsepphitrcl/0i4oneiwce1tobesmt-oatchlmw/..ssfgootrwe2

0"%9 eahv na bilfiinedtea giteecn taiutmon COL 1A1 nda LOC 21A
esucsa namlorab lcnlgaeo -siornncislgk iva a lcnigey unbititsotus ni teh ngloceropal oumlleec "

chwih sneam ttah OI sah a gylneic onibsttiutsu nda rrtoheef sti ulaneb ot mofr a sdearynoc cr.tetsruuu

submitted by hungrybox(1208),
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eDu to 'ileygscn llasm ,eizs ti srtecae kskin"" ni eth omain caid ceqe.uens heesT inksk aer eednde ot crlocrtey ofmr hte yndraocse s.turcteur

tOhre wesnrsa:

  • dweenae"k tcntriionea teenwbe caneolgl dan an"grptloecyo - leaoclgn + oonpacrgetly = ellgrtiaac. heT eoisutqn smet nmineost mayn deestfc in ONEb type( I n)lgalceo tub on nmnotei of esdfcte ni leraOTgcWla (pyet II l)alnogce

submitted by an_improved_me(83),
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eNhtoAr awy ot etg ta ti is aiv i:nilaomnite

.A noerdygh gbonidn olwtund ellyra genach cnise rehtnie enialan ron ecginly era aBrol.p ylG tg&; Aal nlhuo'std acnhge woh rlneopi si fdimiedo I( eanm ti C,DLOU if tehre aws a isrcet rndihenac ro ,gtohesimn btu otn a eartg ) wneD.ras nohngit to antiicde htat aloecnlg gnrtdeeaoid si daeletr ni pssoener ot na AA oisi.tbtusnut slo,A ni the txctnoe of IO ihc(wh si het irtennpesg moacpi,ln)t ew raadely wonk atth het lerpobm 'tsndeo ehva hntinagy ot od with oterd,dianeg rmeo wiht teh egnhca in t/ictrnn oferu.cutuEs teynHsol ndt'o .kwno

submitted by drdoom(1113),
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erHe’s neo ayw ot ompac-lretnioeess-if daeerecds“ edobnohd-gnyr ”mn:frotiao ’mI nto a igb anf of itsh inle fo g,norniaes but tecclyhlian naliean sa a dsie gourp hsa moer ehn*gdrsyo ofr elpattoin rgyhedno bdgionn naht lnyeicg:

l:aeainn H—3C
inye:lgc H—

S,o ,eclyclaih”tn“ naaieln ulwod trempi omre rgey-bdhndnoo nfoaimtro, wcihh ghtim wloal oyu to tieileanm htat ohicce.

atTh ,dasi ti esems atolms obisislepm to rlue uto itt(howu evry hnctleiac egwdonlek or osme diodrvpe rtelmeipxnae da)ta ttha hte shytlgli alrerg neailna does otn rmpiia gorhyedn oigbnnd etebnwe lognalec emeclsuol avi rtcsei ls(tpaia) rteeifnen.rce In msleipr re,tsm ecsin nlniaea si lrga,re you oldwu inthk atht it sumt howmose neriefter whit eht nodgnnyrigohedb- ttha souccr iwth het tdelwiyp- gnylcie.

*rtcySlti akpnigse, ’tsi not het bnremu of hrosdyeng ubt lsao eht thergstn fo het oepild atth istaaicltfe noyhrdeg ngndob:i a hedgoyrn ondub to a ygtonslr vcageeteeltirno ulcemleo ekli unoeiflr iwll raap”“pe mero svotpiie da,n stuh, yoderdnobngh- eomr noltysrg htiw a berany ngxoye dom(raepc hwti a dgronyhe tcennecod to arnc,ob rof eam.)lpxe

trFuhre gndrai:e

  1. hl/dctw/bmdoclp.he/e.i:st./upsruddqenuhw/lhmegpuwtih.
hungrybox  Appreciate the effort but this is far too long to be useful. +30  
drachenx  hungrybox is a freaking hater +  
drdoom  @drachenx haha, nah, coming back to this i realize i was probably over-geeking lol +  
blueberrymuffinbabey  isn't the hydrogen bonding dependent on the hydroxylated proline and lysine? so that wouldn't really be the issue here since those aren't the aas being altered? +  
drdoom  @blueberry According to Alberts’ MBoC (see Tangents at right), hydroxylysine and hydroxyproline contribute hydrogen bonds that form between the chains (“interchain”, as opposed to intra-chain; the chains, of course, are separate polypeptides; that is, separate collagen proteins; and interactions between separate chains [separate polypeptides] is what we call “quaternary structure”; see Tangent above). And in this case, as you point out, the stem describes a Gly->Ala substitution. That seems to mean two things: (1) the three separate collagen polypeptides will not “pack [as] tightly” to form the triple helix (=quaternary structure) we all know and love and (2) proline rings will fail to layer quite as snugly, compromising the helical conformation that defines an alpha chain (=secondary structure; the shapes that form within a single polypeptide). +  
tadki38097  also you can't H bond with carbon, it's not polar enough +  
amy  FA2020 P50 state: formation of procollagen(which is the triple helix structure) via hydrogen and disulfide. So A is incorrect bc there are no collagen molecules yet (2nd structure happens at procollagen level) +  
drdoom  @amy triple helix is a quaternary structure (since triple helix is a shape that forms BETWEEN separate or “adult” pro-collagen/collagen peptides). Primary, secondary and tertiary structures are descriptions of a single polypeptide. Once you have 2 or more polypeptides interacting with each other (e.g., Hemoglobin molecule) you have quaternary structure. +  

From Molecular Biology of the Cell:

Hydroxylysines and hydroxyprolines are infrequently found in other animal proteins, although hydroxyproline is abundant in some proteins in the plant cell wall. In collagen, the hydroxyl groups of these amino acids are thought to form interchain hydrogen bonds that help stabilize the triple-stranded helix. Conditions that prevent proline hydroxylation, such as a deficiency of ascorbic acid (vitamin C), have serious consequences. (Emphasis mine)

+1/- drdoom(1113),

From Molecular Biology of the Cell:

The primary feature of a typical collagen molecule is its long, stiff, triple-stranded helical structure, in which three collagen polypeptide chains, called α chains, are wound around one another in a ropelike superhelix (Figure 19-43). Collagens are extremely rich in proline and glycine, both of which are important in the formation of the triple-stranded helix. Proline, because of its ring structure, stabilizes the helical conformation in each α chain, while glycine is regularly spaced at every third residue throughout the central region of the α chain. Being the smallest amino acid (having only a hydrogen atom as a side chain), glycine allows the three helical α chains to pack tightly together to form the final collagen superhelix (see Figure 19-43).

+/- drdoom(1113),

submitted by nmb29(0),
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I hmgit be hvengonitkir it bu,t H ndbo ofitnmaro of aas' smake eth secadonyr trrcusteu fo eth eniptro aolcgnel( ni hsit )s.ace heT lyG to alA soutuibnitts edos suerlt in ssle H bdno in,maotrof ubt of dviiidauln as'a not neeetbw olneagcl lsueeoclm hta(t mhgit be mero for rqytauaern rt)custeur

lpp06  I think the key is in the answer phrasing, the answer mentioning H-bonds says "Disruption of H-bonds between collagen molecules" Although collagen does undergo H-bonding to support the triple helix, this is done within the same collagen molecule. Linking different collagen molecules occurs via the lysine - hydroxylysine links done in the ECM +1  
kevin  this is the one comment that finally helped me understand why that was incorrect. thank you +  

submitted by basic_pathology(16),
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-G-ylYX is eokanbcb rfo encalolg pahla c.naih 3 ocalglen ahlap anschi arspil to romf leirtp xhile. ciGylen sha on R gpuor, glaoniwl ofr bielitxlfyi adn inmrfotao of teprli xlhie. oN iclgyne vernetps hits tuoonnuisc rsi,gapnli egnnretivp eth itromfnao of ageoclln scdnrayoe ctsurteu.r

claeRl taht scilpe-ahahle nad ehts-eeabts rea maelepxs of neorydacs Thsi anc eb tohuhgt fo a msnitaafnioet fo the lahpa e.xhli

submitted by j123(13),
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yiarmrP urrceutst = oanim iacd snrecce aeoSdyunqe urretcstu = rusrutcet oefrmd thwi a elnsgi mniao icad uqesecen be(ta ptalede hs,tee aaplh hile,x y ciretae)Trt rstucture = tpmlluie sarncoeyd tsustrecur iatrtnicgne toeehrtg uptlmlie( etba detlpea tesshe ceadskt on opt of aech th,reo aaeQr )ecuntyrtr uteurstcr = oienptr rtutcsrue mdrofe orfm londgif of all yertiart sururtstce to keam igindnb ts,ise ect.

ceSin hte naelian saw put ni aeclp fo hte lceiynG, teh pmiyrra tceutusrr saw lbunae to ormf an ahpla leixh ceins hpala hexli cuessrtrut edne a rlcrpuatia ecqenesu lgy( - x- )y in doerr ot mofr regoydhn bnosd to epek the xhlie se.tbla

submitted by unknown001(2),
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thwa is glcnelao ? a scrdaenoy npeorit u.rterutcs

nweh uoy eoemvr inc,lgye hte mtos nnbudtaa imnao ciad , from eht rrucproes celemolu iwll uoy get a rroppe oceydansr trurctues ? NO

unknown001  ignore this +  
drdoom  can you elaborate? what's wrong with your comment? +  

submitted by lilmonkey(28),
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Gyl si o,plar nalAeni si rnoaolpn dan cdohphroiyb. ieseMssn svanertonevncio auon.timt Thsee sAA ahve deintfefr caeichlm ipopeserrt hiwch lead ot prdsiteud piorent igfnldo nyce(srdao )ur.rcstetu miairlS ot Gul - laV ossuttuitnib in icSlek lleC seisae.D

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