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Amino Acids and PeptidesAmino Acids and Peptides
Andy Howard
Introductory BiochemistryFall 2010, IIT
08/31/2010 Biochemistry: Amino Acids
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Acids, bases, amino acidsAcids, bases, amino acids
We begin looking at specific categories of small molecules by examining acid-base equilibrium, both in general and in amino acids
These simple molecules are inherently important, and they help illustrate some general principles
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PlansPlans
Review Acid-base
equilibrium Amino acid
structures Chirality Acid/base chemistry
Side-chain reactivity Peptides and
proteins Side-chain reactivity
in context Disulfides
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Review questionsReview questions
1. The correct form of the free energy equation is generally given as:– (a) H = G - TS– (b) PV = nRT– (c) G = H - TS– (d) S = H - G– (e) none of the above
(20 seconds for this one)
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Review questions, problem 2Review questions, problem 2
2. Suppose a reaction is at equilibrium with H = -6 kJ mol-1 andS = -0.02 kJ mol-1K-1.Calculate the temperature.– (a) 250K– (b) 280K– (c) 300K– (d) 310K– (e) 340K
45 seconds for this one
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Review questions, problem 3Review questions, problem 3
3. Suppose the reaction AB is endergonic with Go = 37 kJ/mol. What would be a suitable exergonic reaction to couple this reaction to in order to drive it to the right?– (a) hydrolysis of ATP to AMP + PPi
– (b) hydrolysis of glucose-1-phosphate– (c) hydrolysis of pyrophosphate– (d) none of the above
30 seconds for this one
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Acid-Base EquilibriumAcid-Base Equilibrium
In aqueous solution, the concentration of hydronium and hydroxide ions is nonzero
Define:– pH -log10[H+]– pOH -log10[OH-]
Product [H+][OH-] = 10-14 M2 (+/-) So pH + pOH = 14 Neutral pH: [H+] = [OH-] = 10-7M:
pH = pOH = 7.
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So what’s the equilibrium So what’s the equilibrium constant for this reaction?constant for this reaction?
Note that the equation isH2O H+ + OH-
Therefore keq = [H+][OH-] / [H2O]But we just said that
[H+] = [OH-] = 10-7MWe also know that [H2O] = 55.5M
(= (1000 g / L )/(18 g/mole))So keq = (10-7M)2/55.5M = 1.8 * 10-16M
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Alternative approachAlternative approach
Assume the protonated species is H3O+ rather than H+
Then the reaction is2 H2O H3O+ + OH-
keq = [H3O+][OH-] / ([H2O]2)At pH=7, [H3O+] = [OH-] = 10-7MDilute solution: [H2O] = 55.5M, so
keq = 10-14 M2/ [(55.5)2 M2] = 3.24*10-18
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Henderson-Hasselbalch Henderson-Hasselbalch EquationEquation
If ionizable solutes are present, their ionization will depend on pH
Assume a weak acid HA H+ + A-
such that the ionization equilibrium constant is Ka = [A-][H+] / [HA]
Define pKa -log10Ka
Then pH = pKa + log10([A-]/[HA])
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The Derivation is Trivial!The Derivation is Trivial!
Ho hum:pKa = -log([A-][H+]/[HA])
= -log([A-]/[HA]) - log([H+])= -log([A-]/[HA]) + pH
Therefore pH = pKa + log([A-]/[HA])
Often writtenpH = pKa + log([base]/[acid])
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How do we use this?How do we use this?
Often we’re interested in calculating [base]/[acid] for a dilute solute
Clearly if we can calculate log([base]/[acid]) = pH - pKa
then you can determine[base]/[acid] = 10(pH - pKa)
A lot of amino acid properties are expressed in these terms
It’s relevant to other biological acids and bases too, like lactate and oleate
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Reading recommendationsReading recommendations
If the material on ionization of weak acids isn’t pure review for you, I strongly encourage you to read the relevant sections in Garrett & Grisham
We won’t go over this material in detail in class because it should be review, but you do need to know it!
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So: let’s look at amino acidsSo: let’s look at amino acids
The building blocks of proteins are of the form H3N+-CHR-COO-;these are -amino acids.
But there are others,e.g. beta-alanine:H3N+-CH2-CH2-COO-
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These are zwitterionsThese are zwitterions
Over a broad range of pH:– the amino end is protonated and is therefore
positively charged– the carboxyl end is not protonated and is
therefore negatively charged
Therefore both ends are charged Free -amino acids are therefore highly
soluble, even if the side chain is apolar
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At low and high pH:At low and high pH:
At low pH, the carboxyl end is protonated
At high pH, the amino end is deprotonated
These are molecules with net charges
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Identities of the R groupsIdentities of the R groups
Nineteen of the twenty ribosomally encoded amino acids fit this form
The only variation is in the identity of the R group (the side chain extending off the alpha carbon)
Complexity ranging from glycine (R=H) to tryptophan (R=-CH2-indole)
Note that we sometimes care about-amino acids that aren’t ribosomal—like ornithine ornithine
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Let’s learn the Let’s learn the ribosomal amino acids.ribosomal amino acids.
We’ll walk through the list of 20, one or two at a time
We’ll begin with proline because it’s weird
Then we’ll go through them sequentiallyYou do need to memorize these, both
actively and passively
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But first: a reminderBut first: a reminder
We often characterize a carbon atom by specifying how many hydrogens are attached to it
–CH3 is methyl
–CH2– is methylene–CH– is methine
|
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Special case: prolineSpecial case: proline
Proline isn’t an amino acid: it’s an imino acid
Hindered rotation around bond between amine N and alpha carbon is important to its properties
Tends to abolish helicity because of that hindered rotation
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The simplest amino acidsThe simplest amino acids
Glycine
AlanineCN+HHHHCCOO-HHH
CN+HHHHCOO-H
methyl
These are moderately nonpolar
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CN+HHHHCCOO-CCHHHHCHHHHH
CN+HHHHCCOO-CHCHHHHCHHHH
Branched-chain aliphatic aasBranched-chain aliphatic aas
Valine
Isoleucine
Leucine
CN+HHHHCCOO-CCHHHHHHH
isopropylSeriously nonpolar
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Hydroxylated, polar amino acidsHydroxylated, polar amino acids
Serine Threonine
CN+HHHHCCOO-OHHH
CN+HHHHCCOO-OCHHHHH
hydroxyl
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Amino acids with carboxylate Amino acids with carboxylate side chainsside chains
Aspartate Glutamate
CN+HHHHCCOO-CHHO-O
CN+HHHHCCOO-HHCHHCO-O
carboxylate
methylene
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Amino Acids with amide side Amino Acids with amide side chainschains
asparagine glutamineCN+HHHHCCOO-HHCHHCNOHH
CN+HHHHCCOO-HHCNOHH
Note: these are uncharged! Don’t fall into the trap!
amide
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Sulfur-containing amino acidsSulfur-containing amino acids
Cysteine Methionine
CN+HHHHCCOO-HHSH
CN+HHHHCCOO-HHSCCHHHHH
sulfhydryl
Two differences:(1) extra methylene(2) methylated S
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Positively charged side chainsPositively charged side chains
Lysine Arginine
CN+HHHHCCOO-HHCCCHHHHHHN+HHH CN+HHHHCCOO-HHCNCHHHHHCNN+HHHH
Guani-dinium
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Aromatic Amino AcidsAromatic Amino Acids
Phenylalanine TyrosineCN+HHHHCCOO-HHCCCCCCHHHHH
CN+HHHHCCOO-HHCCCCCCHHHHOH
phenyl
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Histidine: a special caseHistidine: a special case
Histidine
imidazole
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Tryptophan: the biggest of allTryptophan: the biggest of all
TryptophanCN+HHHHCCOO-HHCCCNCCHHCCHHHH
indole
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ChiralityChirality
Remember:any carbon with four non-identical substituents will be chiral
Every amino acid except glycine is chiral at its alpha carbon
Two amino acids (ile and thr) have a second chiral carbon: C
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All have the same handedness at the alpha carbon
The opposite handedness gives you a D-amino acid– There are D-amino acids in many organisms– Bacteria incorporate them into structures of their
cell walls– Makes those structures resistant to standard
proteolytic enzymes, which only attack amino acids with L specificity
Ribosomally encoded amino Ribosomally encoded amino acids are L-amino acidsacids are L-amino acids
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The CORN mnemonicThe CORN mnemonicfor L-amino acidsfor L-amino acids
Imagine you’re looking from the alpha hydrogen to the alpha carbon
The substituents are, clockwise:C=O, R, N:
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Abbreviations for the amino Abbreviations for the amino acidsacids
3-letter and one-letter codes exist– All the 3-letter codes are logical– Most of the 1-letter codes are too
6 unused letters, obviously– U used for selenocysteine– O used for pyrrollysine– B,J,Z are used for ambiguous cases:
B is asp/asn, J is ile/leu, Z is glu/gln– X for “totally unknown”
http://www.chem.qmul.ac.uk/iupac/AminoAcid/A2021.html
CN+HHHHCOO-CHHSeH
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Acid-base propertiesAcid-base properties
-amino acids take part in a variety of chemical reactivities, but the one we’ll start with is acid-base reactivity
The main-chain carboxylate and amine groups can undergo changes in protonation
Some side chains can as well
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Letters A-F: acid-base propertiesLetters A-F: acid-base properties
AminoAcid
Side-chain
3-lettabbr.
1-let
pKa,COO-
pKa, NH3
+
alanine CH3 ala A 2.4 9.9
* asx Bcysteine CH2SH cys C 1.9 10.7aspartate CH2COO- asp D 2.0 9.9glutamate (CH2)2COO- glu E 2.1 9.5phenyl-alanine
CH2-phe phe F 2.2 9.3
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Letters G-LLetters G-L
AminoAcid
Side-chain
3-lettabbr.
1-let
pKa,COO-
pKa, NH3
+
glycine H gly G 2.4 9.8histidine -CH2-
imidazolehis H 1.8 9.3
isoleucine CH(Me)Et ile I 2.3 9.8
Ile/leu * lex? J 2.3 9.7-9.8lysine (CH2)4NH3
+ lys K 2.2 9.1leucine CH2CHMe2 leu L 2.3 9.7
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Letters M-SLetters M-S
methionine (CH2)2-S-Me met M 2.1 9.3
asparagine CH2-CONH2 asn N 2.1 8.7
pyrrol-lysine
see above pyl O 2.2 9.1
proline (CH2)3CH (cyc) pro P 2.0 10.6glutamine (CH2)2CONH2 gln Q 2.2 9.1
arginine (CH2)3-guanidinium
arg R 1.8 9.0
serine CH2OH ser S 2.2 9.2
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Letters T-ZLetters T-Z
threonine CH(Me)OH thr T 2.1 9.1
seleno-cysteine
CH2SeH Sec U 1.9 10.7
valine CH(Me)2 val V 2.3 9.7tryptophan CH2-indole trp W 2.5 9.4unknown Xaa X
tyrosine CH2-Phe-OH tyr Y 2.2 9.2
glu/gln (CH2)2-COX glx Z
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Remembering the abbreviationsRemembering the abbreviations
A, C, G, H, I, L, M, P, S, T, V easy F: phenylalanine sounds like an F R: talk like a pirate D,E similar and they’re adjacent N: contains a nitrogen W: say tryptophan with a lisp Y: second letter is a Y Q: almost follows N, and gln is like asn You’re on your own for K,O,J,B,Z,U,X
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Do you need to memorize these Do you need to memorize these structures?structures?
Yes, for the 20 major ones(not B, J, O, U, X, Z)
The only other complex structures I’ll ask you to memorize are:– DNA, RNA bases
– Ribose, glucose, glyceraldehyde
– Cholesterol, stearate, palmitate
– A few others I won’t enumerate right now.
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How hard is it to How hard is it to memorize the memorize the structuresstructures??
Very easy: G, A, S, C, VRelatively easy: F, Y, D, E, N, QHarder: I, K, L, M, P, THardest: H, R, WAgain, I’m not asking you to memorize
the one-letter codes, but they do make life a lot easier.
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Another review questionAnother review question
What amino acids are in ELVIS?(a) asp - lys - val - ile - ser(b) asn - lys - val - ile - ser(c) glu - leu - val - ile - ser(d) glu - lys - val - ile - ser(e) Thank you very much.
(25 seconds)
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… … and anotherand another
How many of the twenty plentiful, ribosomally encoded amino acids have exactly one chiral center?– (a) zero– (b) one– (c) seventeen– (d) eighteen– (e) twenty
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Main-chain acid-base chemistryMain-chain acid-base chemistry
Deprotonating the amine group: H3N+-CHR-COO- + OH- H2N-CHR-COO- + H2O
Protonating the carboxylate:H3N+-CHR-COO- + H+ H3N+-CHR-COOH
Equilibrium far to the left at neutral pH First equation has Ka=1 around pH 9 Second equation has Ka=1 around pH 2
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Why does pWhy does pKKaa depend on the depend on the
side chain?side chain? Opportunities for hydrogen bonding or
other ionic interactions stabilize some charges more than others
More variability in the amino terminus, i.e. the pKa of the carboxylate group doesn’t depend as much on R as the pKa of the amine group
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When do these pWhen do these pKKaa values values
apply?apply? The values given in the table are for the free
amino acids The main-chain pKa values aren’t relevant for
internal amino acids in proteins The side-chain pKa values vary a lot
depending on molecular environment:a 9.4 here doesn’t mean a 9.4 in a protein!
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How do we relate pHow do we relate pKKaa to to
percentage ionization?percentage ionization?Derivable from Henderson-Hasselbalch
equationIf pH = pKa, half-ionized
One unit below:– 90% at more positive charge state,– 10% at less + charge state
One unit above: 9% / 91%
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Don’t fall into the trap!Don’t fall into the trap!
Ionization of leucine:pH 1.3 2.3 3.3 8.7 9.7 10.7
%+ve 90 50 10 0 0 0
% neutral 10 50 90 90 50 10
%-ve 0 0 0 10 50 90
Main species
NH3+-
CHR-COOH
NH3+C
HR-COO-
NH3+
CHR-COO-
NH2-
CHR-COO-
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Side-chain reactivitySide-chain reactivity
Not all the chemical reactivity of amino acids involves the main-chain amino and carboxyl groups
Side chains can participate in reactions:– Acid-base reactions– Other reactions
In proteins and peptides,the side-chain reactivity is more important because the main chain is locked up!
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Acid-base reactivity Acid-base reactivity on side chainson side chains
Asp, glu: side-chain COO-:– Asp sidechain pKa = 3.9– Glu sidechain pKa = 4.1– That means that at pH = 5.1, a glutamate
will be ~90.9% chargedLys, arg: side-chain nitrogen:
– Lys sidechain –NH3+ pKa = 10.5
– Arg sidechain =NH2+ pKa = 12.5
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Acid-base reactivity in histidineAcid-base reactivity in histidine
It’s easy to protonate and deprotonate the imidazole group
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Cysteine: a special caseCysteine: a special case
The sulfur is surprisingly ionizableWithin proteins it often remains
unionized even at higher pHCN+HHHHCOO-CHHSHCN+HHHHCOO-CHHS-H+H+pKa = 8.4
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Ionizing hydroxylsIonizing hydroxyls
X–O–H X–O- + H+ Tyrosine is easy, ser and thr hard:
– Tyr pKa = 10.5
– Ser, Thr pKa = ~13
Difference due to resonance stabilization of phenolate ion:
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Resonance-stabilized ionResonance-stabilized ion
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Other side-chain reactionsOther side-chain reactions
Little activity in hydrophobic amino acids other than van der Waals
Sulfurs (especially in cysteines) can be oxidized to sulfates, sulfites, …
Nitrogens in his can covalently bond to various ligands
Hydroxyls can form ethers, estersSalt bridges (e.g. lys - asp)
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PhosphorylationPhosphorylation
ATP donates terminal phosphate to side-chain hydroxyl of ser, thr, tyr
ATP + Ser-OH ADP + Ser-O-(P) Often involved in activating or inactivating
enzymes Under careful control of enzymes called
kinases and phosphatases This is an instance of post-translational
modification
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Amino acid frequencies and Amino acid frequencies and importance in active sitesimportance in active sites
Polar amino acids, particularly S, H, D, E, K, are at the heart of most active sites of enzymes and other globular proteins
Yet they’re relatively uncommon overall in proteins
Nonpolar amino acids (V, L, I, A) occur with higher frequencies overall
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Peptides and proteinsPeptides and proteins
Peptides are oligomers of amino acidsProteins are polymersDividing line is a little vague:
~ 50-80 aa.All are created, both formally and in
practice, by stepwise polymerizationWater eliminated at each step
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Growth of oligo- or polypeptideGrowth of oligo- or polypeptide
CN+HHHHCOO-R1CN+HHHCOO-+H2OCN+HHHHCOR1CNCOO-HR2HR2H
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The peptide bondThe peptide bond
The amide bond between two successive amino acids is known as a peptide bond
The C-N bond between the first amino acid’s carbonyl carbon and the second amino acid’s amine nitrogen has some double bond character
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Double-bond character of Double-bond character of peptidepeptide
CN+HHHHCOR1NCHR2HCOCN+HHHHCO-R1N+CHR2HCO
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The result: planarity!The result: planarity!
This partial double bond character means the nitrogen is sp2 hybridized
Six atoms must lie in a single plane:– First amino acid’s alpha carbon– Carbonyl carbon– Carbonyl oxygen– Second amino acid’s amide nitrogen– Amide hydrogen– Second amino acid’s alpha carbon
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Rotations and flexibilityRotations and flexibility
Planarity implies = 180º, where is the torsion angle about N-C bond
Free rotations are possible about N-C and C-C bonds– Define = torsional rotation about N-C– Define = torsional rotation about C-C
We can characterize main-chain conformations according to ,
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Ramachandran anglesRamachandran angles
G.N. Ramachandran
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Preferred Values of Preferred Values of and and
Steric hindrance makes some values unlikely
Specific values are characteristic of particular types of secondary structure
Most structures with forbidden values of and turn out to be errors
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How far from 180º can How far from 180º can vary? vary?
Remember what we said about the partial double bond character of the C-N main-chain bond
That imposes planarityIn practice it rarely varies by more than
a few degrees from 180º.
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Ramachandran plotRamachandran plot
Cf. figures in text If you submit a
structure to the PDB with Ramachandran angles far from the yellow regions, be prepared to justify them!
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How are oligo- and polypeptides How are oligo- and polypeptides synthesized?synthesized?
Formation of the peptide linkages occurs in the ribosome under careful enzymatic control
Polymerization is endergonic and requires energy in the form of GTP (like ATP, only with guanosine):
GTP + n-length-peptide + amino acid GDP + Pi + (n+1)-length peptide
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What happens at the ends?What happens at the ends?
Usually there’s a free amino end and a free carboxyl end:
H3N+-CHR-CO-(peptide)n-NH-COO-
Cyclic peptides do occurCyclization doesn’t happen at the
ribosome: it involves a separate, enzymatic step.
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Reactivity in peptides & proteinsReactivity in peptides & proteins
Main-chain acid-base reactivity unavailable except on the ends
Side-chain reactivity available but with slightly modified pKas.
Terminal main-chain pKavalues modified too
Environment of protein side chain is often hydrophobic, unlike free amino acid side chain
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Another iClicker questionAnother iClicker question
What’s the net charge on ELVIS at pH 7?
(a) 0(b) +1(c) -1(d) +2(e) -2
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DisulfidesDisulfides
In oxidizing environments, two neighboring cysteine residues can react with an oxidizing agent to form a covalent bond between the side chains
CHHSHCHHSH+(1/2)O2SSHCHHCHH2O
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What could this do?What could this do?
Can bring portions of a protein that are distant in amino acid sequence into close proximity with one another
This can influence protein stability