PROTEIN ENGINEERING CLASS NOTES (31/07/2026)
Definition & Core Scope of Protein Engineering

Definition: A field of study involving the creation (de novo synthesis) and
modification of proteins to impart novel structural or functional properties.

De Novo Synthesis: Generating completely novel proteins from scratch
without relying on naturally occurring wild-type protein sequences.

Protein Modification: Engineering existing wild-type proteins to tailor them
for industrial or clinical applications (e.g., eliminating cofactor requirements for
an enzyme).
Target Properties:
o
Functional: Catalytic mechanism, substrate specificity, binding affinity.
o
Structural: Matrix and scaffolding proteins (e.g., engineered collagen).
The Protein Folding Challenge & Central Dogma

Central Dogma Limit: DNA → mRNA → Polypeptide

Replication Bottleneck: Proteins cannot self-replicate; high-yield expression
requires inserting synthetic/modified DNA sequences into expression hosts
(e.g., E. coli yielding ∼ 10 mg of protein per 100 mL culture).

Yield vs. Functionality: Producing large quantities of a primary amino acid
sequence does not guarantee a functional protein.
Determinants of Protein Function:
o
Proper 3D folding into stable tertiary/quaternary structures
o
Correct disulfide bond formation and post-translational modifications
(PTMs, such as glycosylation).
o
Multimerization (assembly into functional dimers, trimers, or
complexes).
Environmental Dynamics & Folding Parameters

Lack of a Universal Folding Code: Unlike the genetic code, there is no
single master formula to predict a protein's folded 3D state solely from its
primary sequence.

Key Environmental Variables:
o
pH: Alters amino acid protonation states (e.g., Histidine, Tryptophan),
shifts the isoelectric point (pI), and changes overall molecular charge,
driving conformational changes. Example: The Bacillus thuringiensis
(Bt) Cry toxin activates via pH-dependent conformational changes in
the alkaline insect gut.

2+
2+
o
Cofactors & Metal Ions: Bound ions (Ca , Mg2+ , Co , Mn2+ , Zn2+ ) or
coenzymes (NAD+ ,FAD) are often required for both folding stability and
catalytic competence.
o
In Vivo Synthesis Kinetics: Expression rate dictated by promoter
strength (e.g., strong promoter/high IPTG induction vs. weak promoter)
directly impacts folding fidelity inside host cells. Overly rapid translation
can overwhelm chaperones, inducing aggregation into inclusion
bodies.
Computational Folding Limitations: In silico software (e.g., AlphaFold)
predicts backbones using Ramachandran plot dihedral angles (𝜙, 𝜓), but
generally cannot fully account for environmental variables, pH fluctuations,
metal cofactor coordination, or complex PTMs.
Comparative Analysis of Biological Engineering Disciplines
Engineering
Field
Level of
Manipulation
Primary Focus / Mechanism
Genetic
Engineering
Organism /
Genome Level
Direct gene manipulation in vivo (gene
knockouts, insertions, transgenic
lines).
Tissue
Engineering
Cellular / Tissue
Level
Assembling functional tissue
constructs using stem cells and
scaffolds to repair/replace organs.
Biomolecular
Engineering
Post-Synthesis
Molecular Level
Chemical/enzymatic modifications
applied after biomolecule synthesis
(e.g., biotinylation).
Biochemical
Engineering
Process / Reactor
Level
Optimization of unit operations,
bioreactors, additives, and culture
conditions to maximize yield.
Engineering
Field
Level of
Manipulation
Primary Focus / Mechanism
Protein
Engineering
Gene / Molecular
Level (In Vitro)
Sequence modification at the
DNA/PCR level prior to recombinant
expression to alter structure/function.
Applications of Protein Engineering

Therapeutics: Recombinant Erythropoietin (EPO), engineered insulin
analogs, monoclonal antibodies (e.g., targeting triple-negative breast cancer),
and epitope-focused subunit vaccines (minimizing dosage and off-target side
effects compared to whole attenuated viruses).

Diagnostics: Single-chain variable fragments (scFv, retaining antigen binding
while omitting unnecessary Fc effector domains) for ELISA assays; CRISPRbased nucleic acid detection platforms.

Research & Development Tools: Engineered restriction enzymes, highfidelity DNA polymerases, and modified Cas variants:

o
Cas9-HF1: High-fidelity variant engineered to prevent off-target
genomic cleavage.
o
SpRY: A near-PAM-less Cas9 variant capable of targeting virtually any
genomic site.
o
dCas9 (Dead Cas9): Catalytically inactive Cas9 used as a targeted
delivery vehicle for transcriptional regulators or fluorescent imaging
markers.
o
Cas13: Engineered variants for single-stranded RNA cleavage and
diagnostic processing.
Industrial & Environmental: Bioremediation enzymes, protein-based
biopesticides, and biomass utilization enzymes (e.g., engineering multidomain cellulases that combine endo-, exo-, and 𝛽-glucosidase activities into
a single polypeptide chain).
Engineering Strategies & Frameworks

Starting Point Strategies:
o
Ancestral Sequence Reconstruction (ASR): Highly specialized
modern enzymes are structurally rigid and resistant to further
evolutionary changes. Reconstructing ancestral nodes via phylogenetic
alignment yields ancestral proteins with greater structural flexibility and
higher "evolvability."
o
Enzyme Promiscuity: Exploiting broad substrate tolerance or
secondary/promiscuous catalytic activities of wild-type enzymes as a
baseline to evolve high-efficiency catalysts (𝑘𝑐𝑎𝑡 /𝐾𝑚 optimized up to
−1
106 − 107 M s−1).
o

Cofactor Swapping: Replacing active-site metal ions (e.g., swapping
2+
Zn2+ for Ca in SMP30/lactonase enzymes) to shift activity toward
new substrates (e.g., organophosphate degradation).
The Three Main Engineering Approaches:
1. Rational Design: Computer-aided modeling based on detailed 3D
structure and functional data, executed via site-directed mutagenesis.
2. Directed Evolution: Laboratory-guided natural selection using random
mutagenesis (error-prone PCR, DNA shuffling) coupled with highthroughput screening/selection, requiring no prior structural knowledge.
3. Semi-Rational Design: Targeted random mutagenesis focused on key
functional regions (e.g., active-site loops, binding pockets, sitesaturation mutagenesis) guided by structural models.
General Protein Engineering Workflow

Target Identification: Select wild-type, promiscuous, or ancestral gene
sequence.

Mutagenesis & Library Generation: Introduce point mutations, domain
shuffling, or insertions/deletions.

Cloning & Expression: Ligated library into expression vectors and transform
into host systems (E. coli, yeast, insect/mammalian cells).

High-Throughput Screening: Screen thousands of clones using colorimetric,
fluorometric, or cell-display selection systems.

Characterization: Isolate top variants and evaluate kinetic constants
(𝑘𝑐𝑎𝑡 , 𝐾𝑚 ), thermal tolerance (𝑇𝑚 ), pH stability, and protease resistance.
PROTEIN ENGINEERING CLASS NOTES: BACTERIAL EXPRESSION SYSTEMS
(4/08/2026)
Cloning vs. Expression Systems

Cloning Systems: Pair high-copy plasmids (e.g., pUC, pGEM, pTA) with
specialized strains (e.g., DH5alpha, XL1-Blue, DH10B). Designed to produce
large quantities of plasmid DNA while protecting unmethylated PCR inserts
from host restriction endonucleases.

Expression Systems: Pair low-to-medium copy vectors (e.g., pET, pBAD,
pGEX, pACYC) with dedicated expression hosts (e.g., BL21, Rosetta,
Origami, ArcticExpress). Formulated to prioritize transcript and protein
production over plasmid DNA replication.
BL21 Strain Physiology & DE3 Expression Mechanism

Non-Motile Mutation (𝒇𝒍𝒉𝑫 Deletion): Deletion of flagellar biosynthesis
genes prevents flagella formation. Eliminating motility conserves cellular
energy and amino acid pools for recombinant protein synthesis.

Reduced Acetate Stress: High constitutive acetyl-CoA synthetase activity
rapidly routes pyruvate into acetyl-CoA and the TCA cycle rather than
accumulating acetate. This prevents culture acidification (maintaining pH
7.5 − 8.5) and reduces acetate-induced growth inhibition.

DE3 Induction Pathway:
1. The host genome contains the 𝜆DE3 lysogen carrying T7 RNA
polymerase under control of the lacUV5 promoter.
2. IPTG binds and inactivates the LacI repressor.
3. T7 RNA polymerase is transcribed and translated.
4. T7 RNA polymerase selectively binds the plasmid's T7 promoter to
drive high-level transcription of the gene of interest (GOI).
Specialized Expression Hosts

Rosetta Series: Contains a chloramphenicol-resistant plasmid
(pRARE/pRARE2) supplying tRNAs for rare E. coli codons (AGA, AGG, AUA,
CUA, GGA, CCC). Overcomes eukaryotic codon bias without requiring gene
codon optimization.

Origami Series: Carries mutations in thioredoxin reductase (trxB) and
glutathione reductase (gor). Shifts the cytoplasmic redox state from reducing
to oxidizing, permitting direct cytoplasmic disulfide bond (−S − S −) formation.

ArcticExpress Series: Co-expresses cold-adapted chaperonins (Cpn10 and
Cpn60 from Oleispira antarctica). Enables functional protein folding at low
growth temperatures (4∘ C − 12∘ C), preventing inclusion body formation in
hydrophobic or aggregation-prone targets.
Vector Architecture & Copy Number Regulation


Origin of Replication (𝒐𝒓𝒊):
o
Wild-Type ColE1: Maintains 15 − 20 copies/cell.
o
High-Copy Mutation (pUC): Loss/mutation of the rop (repressor of
priming) gene removes negative feedback control, increasing yield to >
100 − 500 copies/cell.
o
Low-Copy Origins (p15A, pSC101): Maintain 5 − 12 copies/cell (e.g.,
pACYC, pBAD). Essential for expressing toxic proteins or integral
membrane proteins to avoid cell lysis and metabolic collapse.
Duet Vectors: Feature two distinct multiple cloning sites (MCS), each
preceded by an independent T7 promoter and ribosome binding site (RBS),
allowing balanced co-expression of protein complexes or multi-subunit
heterodimers.
Antibiotic Selection Mechanisms
Antibiotic
Target / Mode
of Action
Ampicillin
Irreversibly
inhibits
transpeptidase
, blocking
peptidoglycan
Resistance Gene
& Enzyme
Mechanism of
Resistance
𝛽-lactamase (bla)
Secreted
enzyme cleaves
the 𝛽-lactam
ring, degrading
Antibiotic
Target / Mode
of Action
Resistance Gene
& Enzyme
Mechanism of
Resistance
cell wall crosslinking.
the antibiotic in
the media.
Kanamycin
Binds 30S
ribosomal
subunit
proteins,
causing
translational
misreading
and arrest.
Aminoglycoside
phosphotransferas
e (aph)
Intracellular
phosphorylation
of kanamycin,
preventing
ribosomal
binding.
Chloramphenico
l
Binds 23S
rRNA
(A2451/A2452)
of 50S subunit,
inhibiting
peptidyl
transferase.
Chloramphenicol
acetyltransferase
(cat)
Covalent
acetylation of
chloramphenicol
, inactivating the
drug inside the
host cell.
PROTEIN ENGINEERING CLASS NOTES: BACTERIAL EXPRESSION &
VECTOR DESIGN (7/08/2026)
Promoter Architecture in Bacterial Vectors

lac Promoter: Wild-type promoter regulated by lactose/IPTG induction.
Naturally subject to catabolite repression by glucose via the cAMP-CRP/CAP
system.

lacUV5 Promoter: A mutated variant of the lac promoter that significantly
reduces sensitivity to cAMP-CRP catabolite repression, enabling strong IPTGinduced transcription even in glucose-rich media.

trp Promoter: Tryptophan operon promoter, chemically induced using indole3-acrylic acid (IAA).

tac Promoter: A synthetic hybrid promoter combining the −35 region of the
trp promoter with the −10 (Pribnow box) region of the lac promoter. Yields ∼
10-fold stronger expression than lacUV5.
Promoter Consensus Elements:
o
−10 Region (Pribnow Box): Consensus sequence TATAAT (∼ 69%
conserved T).
o
−35 Region: Consensus sequence TTGACA. Direct interaction
between these hexameric sequences and RNA polymerase dictates
promoter strength.
Mechanisms to Control Leaky (Basal) Expression
Unwanted basal transcription prior to induction can cause host toxicity or plasmid
instability, especially with membrane proteins. Three primary regulatory strategies
eliminate leaky expression:
1. lacIq Repressor Overexpression: The lacIq promoter mutation increases
repressor protein synthesis ∼ 10-fold, ensuring complete operator saturation
across high- or medium-copy plasmid populations.
2. T7 Lysozyme Expression (pLysS / pLysE Plasmids): Expresses low levels
of T7 lysozyme, which specifically binds and inhibits basal T7 RNA
polymerase molecules prior to IPTG induction.
3. Double-Lock / Operator Insertion System: Inserting a lac operator
sequence downstream of the T7 promoter on the expression vector. LacI
repressor simultaneously blocks T7 RNA polymerase transcription at both the
chromosome (host T7 RNAP gene) and plasmid levels.
Translation Control: Initiation & Termination

Ribosome Binding Site (RBS / Shine-Dalgarno): Conserved sequence
(UAAGGAGG) located 6 − 11 nucleotides upstream of the AUG start codon.
Crucial for mRNA-ribosome alignment and essential when designing PCR
primers for cell-free protein synthesis (CFPS).

Stop Codon Preference: UAA (ochre) is the most efficient and predominant
termination codon in E. coli.

Ribosome Processivity Limits: Translation fidelity and efficiency drop by ∼
50% for every 1000 amino acids synthesized. Ribosome stalling or stopcodon read-through can yield truncated or extended proteins.

Dual Stop Codons: Including tandem stop codons (e.g., TAA TAA) in reverse
cloning primers prevents suppressor-tRNA-mediated read-through.
Endogenous Proteolytic Processing in E. coli

Signal Peptidases: SPase I cleaves N-terminal signal peptides following AlaX-Ala motifs during periplasmic translocation.

Methionine Aminopeptidase (MAP): Cleaves the N-terminal initiator
Methionine (Met) if the side-chain radius of the second amino acid is small (<
4 AA, e.g., Ala, Gly, Ser, Pro). Met is retained if the adjacent side chain is
bulky (> 4 AA).
Affinity Tags & Fusion Partners
Tag /
Fusion
Partner
Size /
Type
Binding
Matrix /
Ligand
Elution Agent /
Mechanism
Primary
Function
PolyHistidine
(His6 )
Small
Peptid
e
(6 aa)
Ni2+ or
2+
Co IMAC
Resin
Imidazole
Affinity
Purification
PolyArginine
(Arg5 )
Small
Peptid
e
Cation
Exchange
Resin
NaCl Salt
Gradient
Charge-based
Purification
FLAG / cMyc / STag
Small
Epitop
e (8 −
15 aa)
Monoclonal
Antibodies
(mAb)
Low pH /
Competitor
Peptide
Immunodetectio
n & Tracking
Tag /
Fusion
Partner
Size /
Type
Binding
Matrix /
Ligand
MaltoseBinding
Protein
(MBP)
Large
Partner
(∼
40 kDa
)
Glutathion
e STransferas
e (GST)
Thioredoxi
n (TRX)
Elution Agent /
Mechanism
Primary
Function
Amylose
Resin
Maltose
Solubilization &
Periplasmic
Folding
Large
Partner
(∼
26 kDa
)
GlutathioneSepharose
Reduced
Glutathione
Solubilization &
Affinity Capture
Large
Partner
(∼
12 kDa
)
Phenylarsin
e Oxide
Resin
𝛽Mercaptoethan
ol
Cytoplasmic
Disulfide
Formation
Tag Removal Strategies


Chemical Cleavage — Cyanogen Bromide (CNBr):
o
Site: Cleaves C-terminal to Methionine residues.
o
Utility: Cost-effective chemical cleavage functional under denaturing
conditions (6 M Guanidine-HCl or 70% Formic Acid); requires target
protein to lack internal Met residues.
Enzymatic Cleavage (Specific Endoproteases):
o
Thrombin: Recognizes Leu-Val-Pro-Arg $\downarrow$ Gly-Ser;
cleaves after Arg.
o
Factor Xa: Recognizes Ile-(Glu/Asp)-Gly-Arg $\downarrow$; cleaves
C-terminal to Arg.
o
TEV Protease: Tobacco Etch Virus protease recognizing Glu-Asn-LeuTyr-Phe-Gln $\downarrow$ (Gly/Ser); highly specific cleavage after
Gln.