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An amino acid combines carboxylic acid and amine functionality in the same molecule; the name alone does not specify their separation. An alpha-amino acid has its amino nitrogen attached to the carbon immediately adjacent to the carboxyl carbon, the amino acid alpha carbon. For example, ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ is glycine, whereas ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ has an extra intervening carbon and is not an alpha-amino acid. These chemical formulas describe connectivity, not the predominant protonation state in water.

Most common proteinogenic amino acids share the skeleton ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}(R)-\mathrm{COOH}$, with $R$ the amino acid side chain. In glycine, $R$ is hydrogen; in alanine, $R=\mathrm{C}{\mathrm{H}}_3$. Proline is a useful exception to the displayed primary-amine skeleton: its amino nitrogen is part of a ring and has a second carbon substituent. The shared backbone permits assembly into polypeptides, while different amino acid side chains give the resulting proteins diverse chemistry. ^[ref]

In a chemical solution, protonation changes the electric charges without changing this backbone. Free glycine near neutral pH is predominantly ${\mathrm{H}}_3{\mathrm{N}}^{+}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CO}{\mathrm{O}}^{-}$, a zwitterion. Its positive and negative groups are joined within one molecule; zero net electric charge does not mean absence of local electric charges, nor does it mean the uncharged drawing ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ predominates. Glycine protonation explains how lowering or raising pH changes the balance. ^[ref]

Building a protein joins the carboxyl carbon of one amino acid to the amino nitrogen of another through a peptide bond, an amide linkage. The formal net condensation loses the elements of water; cellular translation uses activated substrates at a ribosome, rather than simply mixing free amino acids in water. Incorporated units are amino acid residues. The interior amide groups no longer have the same acid/base behavior as the two free terminal groups of glycine, so its free-molecule glycine titration cannot be copied independently at every chain position. ^[ref]

Chirality distinguishes most common proteinogenic amino acids from glycine. The other nineteen members of the conventional twenty have a stereocenter at their amino acid alpha carbon; ordinary translation incorporates their L forms. Glycine's two identical hydrogen substituents make it achiral. Thus being an amino acid, or even an alpha-amino acid, does not by itself imply chirality. ^[ref]

A bacterial regulator whose expression can depend on EF-P-sensitive proline sequences.^[ref]

I would increase the supply of correctly matured translation elongation factor P in a bacterial production system whose target proteins contain EF-P-sensitive proline motifs. This is a concrete proposal to relieve a catalytic bottleneck while preserving the machinery that selects the correct aminoacyl-tRNA. Its benefit should be tested for the chosen products rather than asserted for every protein or organism.

The rationale is that adjacent prolines are difficult substrates for peptide-bond formation. EF-P binds the ribosome near the peptidyl-tRNA and assists productive peptide-bond formation at susceptible sequences. Lili K. Doerfel and colleagues used defined biological translation reactions and engineered substrates to connect EF-P with rapid synthesis through consecutive prolines; Susanne Ude and colleagues showed relief of polyproline-dependent stalling and physiological regulation of the bacterial protein CadC.^[ref][ref] Increasing functional EF-P is therefore mechanistically different from weakening decoding discrimination or accelerating rejection-sensitive steps indiscriminately.

I would first establish the limiting step in a reconstituted bacterial biological translation system, holding the mRNA, ribosome concentration, initiation factors, charged tRNA supply and energy regeneration constant. Titrate correctly modified EF-P against the unmodified or inactive factor. For an Escherichia coli production strain, coordinate EF-P expression with its native maturation enzymes: increasing an inactive, insufficiently modified pool may add burden without relieving stalls. Introduce the change in a tunable manner and verify both EF-P abundance and its modification state by targeted mass spectrometry. This is an engineering proposal based on demonstrated factor function; the cited experiments do not establish that overexpression will improve every industrial production strain.

A simple throughput calculation makes the scope explicit. If a ribosome needs $T_0$ for ordinary elongation and termination, and an additional $T_s$ at susceptible stalls, the completion capacity of $R$ productive ribosomes is approximatelyThis neglects initiation limitation, traffic and variation between ribosomes. As an illustrative calculation, reducing stall time from 20 s to 2 s when the remaining cycle takes 30 s raises the capacity by $50/32=1.56$ at unchanged quality. A product without substantial EF-P-sensitive stalls would show little direct benefit. Moreover, extra ribosomes do not necessarily increase output if they collide behind a slow site; relief of the bottleneck may free ribosomes as well as accelerate individual chains.

Preserving error control requires more than measuring total protein. aminoacyl-tRNA synthetase editing controls charging identity, while ribosomal initial selection and kinetic proofreading help discriminate cognate from near-cognate tRNAs. These should remain intact. If an independent per-residue missense probability is $ϵ$ for a chain of length $L$, the error-free fraction is approximately $(1-ϵ{)}^L$; even a small per-residue deterioration can matter for a long protein. EF-P acts principally on difficult peptidyl transfer, but altered dwell times can still change ribosomal frameshifting, folding and quality control. The proposal promises a testable route to low-error acceleration, not an automatic absence of side effects.

The central controls would use the same target sequence, promoter and untranslated regions with or without added active EF-P. An inactive or maturation-defective factor tests whether catalytic assistance, rather than nonspecific protein overexpression, accounts for the gain. A motif-free reporter tests specificity. A proline-substitution reporter can test the mechanism, but cannot substitute for testing the unchanged production target because it changes the product itself. Measure growth and resource burden and include several EF-P doses: excessive factor supply may become inhibitory or wasteful. Use matched biological replicates and establish that mRNA abundance or stability has not changed enough to explain the result.

Measure elongation using time-resolved synthesis and normalized ribosome profiling, looking for relief at the predicted stall sites and reduced collisions. Profiling alone does not measure completed protein flux. Pulse–chase measurements and calibrated mass spectrometry of full-length products should establish completion, while an activity assay tests correct folding. fluorescence alone is inadequate because fluorophore maturation and degradation also affect signal. Separate assays should quantify missense incorporation, stop-codon readthrough and frameshifts, with positive error controls and detection limits. Mass-spectrometric error measurements require standards because erroneous and correct peptides need not ionize equally.

Finally, compare soluble active yield, not just synthesis speed: some pauses aid cotranslational folding, and relieving them can lower useful output. If substrate or charged tRNA supply becomes limiting, rebalance those components rather than weakening kinetic proofreading. A successful result is a reproducible increase in completed, correctly folded target molecules per unit time, with no material increase in independently measured error classes. Extension to other sequence bottlenecks would require their own mechanistic diagnosis.

On releasing the tensile load, the extended chain first retracts towards less aligned conformations because they have much greater configurational multiplicity. Viscous friction and internal relaxation determine the time course; stored free energy is dissipated as heat. An unconstrained ideal chain does not shrink to a point: its mean vector is zero but its root mean square end-to-end length is $b\sqrt{N}\simeq 2.37\mathrm{nm}$ for the supplied ideal-chain parameters.

A real polypeptide also has sequence-dependent interactions. If the original native structure is the stable state in the prevailing solvent and the molecule remains intact and capable of folding, it can subsequently refold, gaining favourable intramolecular and solvent free energy. The folded state is not selected by chain entropy alone, and formation of native contacts need not be instantaneous. Kinetic traps, proline isomerization, protein aggregation, misfolding, surface attachments or chemical damage may prevent complete recovery or produce hysteresis. Thus reversible refolding is a physically motivated possibility, not guaranteed simply by removal of force. This distinction connects the recoil of a stretched polymer to the thermodynamics discussed in Question 6(c).

View the backbone along N–C${}_{\alpha }$ to display $\varphi =\angle (C_{i-1}^{\prime },N_i,C_{\alpha i},C_i^{\prime })$ in a Newman projection. The front amide nitrogen carries the preceding carbonyl and H, while the back C${}_{\alpha }$ carries the next carbonyl, H and side chain. Rotation changes the contacts among these groups. A corresponding view along C${}_{\alpha }$–C${}^{\prime }$ displays $\psi$. The peptide C${}^{\prime }$–N bond itself is approximately planar and is not the freely rotating bond in either definition.

glycine replaces the side chain by a second hydrogen. Removing C${}_{\beta }$ removes many contacts that exclude backbone arrangements for other residues, so its allowed $\varphi ,\psi$ region is much broader and includes substantial positive-$\varphi$ space. Its alpha carbon is also achiral, so the steric map is approximately mirror-symmetric between positive and negative torsions in the simple isolated-residue model.

proline closes a five-membered ring from C${}_{\alpha }$ through three methylenes back to nitrogen. In the N–C${}_{\alpha }$ projection, the nitrogen's side-chain arm and C${}_{\alpha }$'s C${}_{\beta }$ arm are tied together, preventing unrestricted relative rotation. Its $\varphi$ is consequently confined near the usual negative-$60^{\circ }$ region rather than exploring the broad range of an acyclic amino acid. proline also lacks a backbone N–H donor when incorporated into an ordinary internal peptide. Ring closure restricts $\varphi$ strongly, but does not freeze every backbone or ring degree of freedom. These differences are naturally represented by distinct Ramachandran plots; the original torsion/steric analysis established the basis for such comparisons.^[ref]

For hydrogen-bond acceptance, the three strongest groups are the terminal carboxylate, the Cys–CO–N(Pro) tertiary-amide carbonyl, and the Pro–CO–NH(Tyr) secondary-amide carbonyl, in that qualitative order. Negative charge makes carboxylate a particularly strong acceptor. Both amide oxygens accept strongly; greater nitrogen alkyl donation generally makes a tertiary amide oxygen more basic than the comparable secondary-amide oxygen. This last ordering is a functional-group estimate, not a precise calculation of a folded tripeptide's site parameters.

For donation, the order is terminal ammonium $>$ tyrosine phenolic OH $>$ the peptide NH between Pro and Tyr. The thiol is a weaker donor. Protonated ammonium has no available lone pair to accept; the proline amide nitrogen has no N–H bond to donate, and amide nitrogen lone pairs are delocalized into carbonyl groups. The carboxylate has no O–H donor in the drawn protonation state.

For a carbonyl oxygen of residue $i$ accepting from the NH of a later residue, the possibilities are:

The alpha helix provides a favourable combination of allowed backbone torsions, reasonably linear hydrogen bonds, and side-chain packing. Its successive side chains advance by about $100^{\circ }$, rather than lining up in the three columns of the narrower 3-10 helix. Smaller repeating rings impose more severe backbone constraints; the wider pi helix gives a different, generally less favourable packing arrangement. Short 3-10 and pi-helical segments can still occur in proteins, so this is a preference for a common extended secondary structure, not a prohibition of the alternatives. Proline and sequence-specific interactions can disrupt the idealized backbone donor pattern.

acetone first forms a proline enamine by reversible iminium formation and alpha-deprotonation. The enamine attacks the aromatic aldehyde in a hydrogen-bonded arrangement involving the catalyst's carboxylic acid. hydrolysis of the resulting iminium releases the beta-hydroxyketone and regenerates proline. The usual L-proline arrangement gives the $R$ secondary alcohol.

The sterically crowded aryl–amide axis is selected at the same bond-forming step: rapid rotation in the aldehyde permits the catalyst to react preferentially with the conformer that puts the smaller carbonyl-oxygen side of the amide towards the approaching aldol transition structure and the bulky diisopropylamino side away. A chair-like enamine–aldehyde model minimizes the interaction of the amide with the catalyst-side axial hydrogen; reversing the amide orientation creates the less favourable diastereomeric transition state. Replacing planar CHO by tetrahedral CH(OH)CH${}_2$COMe then raises the rotation barrier and traps the axis. This is a dynamic kinetic resolution, not resolution of two configurationally stable starting atropisomers.

To specify the predicted major axis without relying on a flat bond-line formula, look from the aromatic ipso carbon towards the amide carbonyl carbon. Put the higher-priority aryl path, the trimethylsilyl-bearing ortho side, at nine o'clock. The preferred model puts the carbonyl oxygen at approximately six o'clock and the diisopropylamino group at twelve o'clock. This is $S_a$ using the near-to-far high-priority convention; oxygen outranks nitrogen at the far end. Coupled with the alcohol assignment, the model predicts $(R,S_a)$. The drawing defines the configuration even if an alternative viewing direction is used. The dynamic-resolution mechanism is established by the primary aldol study; the product-axis assignment here is a stereochemical-model prediction for the printed substrate.^[ref]

Proline is a cyclic alpha-amino acid whose amine nitrogen is joined to two carbon positions within a five-membered ring. This explains the exception to ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}(R)-\mathrm{COOH}$ in amino acid. ^[ref]

An enamine formed reversibly between proline and an enolizable carbonyl compound attacks an electrophilic carbonyl. The carboxylic acid group can orient and activate the electrophile; hydrolysis releases the aldol product and regenerates proline.

A proteinogenic amino acid is incorporated into proteins during translation. The conventional twenty include glycine, alanine, methionine and proline; they are a subset of amino acids, not a definition of the whole family. ^[ref]

A Ramachandran plot displays backbone $\varphi ,\psi$ angles. glycine has a broader sterically allowed region because its side chain is hydrogen, whereas proline ring closure strongly restricts $\varphi$.

A bacterial factor that assists peptide-bond formation at susceptible sequences, including consecutive prolines. Functional maturation of the factor and substrate sequence affect its activity.^[ref]

Write a peptide's protein primary structure from its N-terminus toward its C-terminus. The usual simplified trypsin cleavage specificity rule places a cut immediately after K (lysine) or R (arginine), unless the next amino acid residue is P (proline). The following amino acid residue is essential: a terminal K or R has no following peptide bond to cut. The inspected institutional documentation describes proline as usually blocking cleavage, and explicitly lists sequence contexts in which that blocking is negligible. It also warns that experimental results can differ from sequence-based predictions. Thus “never before proline” is too strong.^[ref]

For an original worked example, stipulate complete cleavage at all sites allowed by that simplified rule in the twelve-amino acid residue sequenceA denotes alanine, G denotes glycine, and K, R and P have the meanings above. Cuts follow positions $3$, $7$ and $10$. Position $5$ is R, but its following P suppresses that cut in the stipulated rule; position $12$ is terminal K. Notice that the R in ARPK is retained inside the fragment, despite being a usually favoured trypsin target.

Near neutral pH, the lysine and arginine amino acid side chains undergo predominant protonation and carry positive electric charge. This helps distinguish those amino acid residues from the uncharged letter symbols used to write the sequence; those symbols specify identities, not whole-fragment electric charges.^[ref]

Fragment accounting checks the prediction: lengths $3+4+3+2=12$ conserve the twelve amino acid residues. The original linear chain has eleven peptide bonds; the four fragments retain $2+3+2+1=8$, so three were hydrolysed. Three water molecules are consumed per original chain in this formal stoichiometry. More generally, cutting $m$ distinct bonds of one linear peptide makes $m+1$ fragments: each cut splits one connected piece into two. This argument assumes an unbranched chain without additional links joining the pieces.

A missed trypsin cleavage at position $7$ would instead give $\mathrm{AGK}|\mathrm{ARPKAGR}|\mathrm{AK}$, merging two neighbouring fragments without changing their order. Even a correct cleavage preference does not guarantee complete proteolysis; predicted fragment boundaries describe a model, while actual boundaries require evidence from the digest.