Oligosaccharyltransferase Structure, Function, and Chemo-Enzymatic Glycan Synthesis

Comprehensive practice flashcards defining key vocabulary, enzymes, structural motifs, and reaction intermediates involved in oligosaccharyltransferase biology, human OST-A/OST-B architecture, and the GLYCO-BUILD chemo-enzymatic synthesis pipeline.

Oligosaccharyltransferase (OST)

A membrane-bound enzyme complex in the endoplasmic reticulum (ER) that catalyzes the transfer of an oligosaccharide from a lipid carrier onto asparagine residues within the consensus sequon $\text{N-X-(S/T)}$ (where $\text{X} \neq \text{P}$) of nascent or secretory proteins.

STT3

The conserved central catalytic subunit of eukaryotic oligosaccharyltransferase complexes that contains $13$ transmembrane (TM) spans and the active site responsible for binding both the donor LLO and acceptor polypeptide.

OST3 and OST6 Complexes (Yeast OST Isoforms)

The two native hetero-octameric isoforms of Saccharomyces cerevisiae OST that differ solely by incorporating either the noncatalytic oxidoreductase subunit Ost3p or Ost6p; both complexes exhibit similar substrate affinities ($K_M$) for peptide and LLO substrates in vitro, but the OST3 complex displays a $5$- to $6$-fold higher turnover rate ($k_{\text{cat}}$).

TAMRA-YANATS

An optimized synthetic, fluorophore-labeled hexapeptide substrate containing a tyrosine residue at the $-2$ position relative to the glycosylation sequon, displaying an apparent $K_M$ of $55.6 \pm 6.5\,\mu\text{M}$ and over tenfold greater specificity for the yeast OST3 complex compared to TAMRA-DANYTK.

Compound Ia [GlcNAc2-(OH)CPP-(S)-citronellylneryl]

A synthetic nonhydrolyzable hydroxy-pyrophosphonate lipid-linked oligosaccharide analog containing a $\text{C}_{20}$ lipid tail that competitively inhibits yeast OST3 complex activity with an $\text{IC}_{50}$ value of $44\,\mu\text{M}$.

Compound Ib [GlcNAc2-CPP-(S)-citronellylfarnesyl]

A synthetic nonhydrolyzable pyrophosphonate lipid-linked oligosaccharide analog containing a $\text{C}_{25}$ lipid tail that inhibits the yeast OST3 complex with an $\text{IC}_{50}$ value of $198\,\mu\text{M}$.

Subcomplex Organization of Eukaryotic OST

The structural arrangement of the multimeric eukaryotic OST into three modular units: Subcomplex I (OST1/RPN1 and OST5/TMEM258), Subcomplex II (STT3, OST4, and Ost3/Ost6 or DC2/MAGT1/TUSC3), and Subcomplex III (WBP1/OST48, OST2/DAD1, and SWP1/RPN2).

Bipartite LLO Recognition

The mechanism by which eukaryotic OST selectively recognizes the fully assembled donor $\text{Dol-PP-GlcNAc}_2\text{Man}_9\text{Glc}_3$ substrate through specific, simultaneous interactions at the reducing-end $\text{GlcNAc}_2$ via STT3 and the non-reducing terminal $\text{Glc}_3$ via WBP1 and OST2, leaving the B- and C-branches flexible.

Terminal Glc3 Binding Pocket

A preformed pocket in eukaryotic OST assembled by luminal regions of WBP1 and OST2, in which the terminal $\alpha\text{-(1,2)-glucose}$ forms a $\text{CH-}\pi$ interaction with Tyr383 of WBP1 and the penultimate glucose forms a hydrogen bond with Asn75 of OST2.

External Loop 5 (EL5)

A conformationally mobile loop in the catalytic STT3 subunit that packs against the luminal domain along with TM9 to complete the active site tunnel, correctly positioning catalytic residues Glu350 (or Glu403 in STT3B) and Arg328 (or Arg459 in STT3B) across bound donor and acceptor substrates.

OST Reaction Cycle (Ordered Priming Mechanism)

A modular reaction model wherein OST can form either an LLO-primed state (EL5 and TM9 unengaged, preventing premature hydrolysis) or a peptide-primed state (only C-terminal EL5-C engaged), with both substrates ultimately inducing the fully ordered ternary complex required for catalysis.

WWD Motif

A universally conserved catalytic sequence motif (Trp-Trp-Asp, corresponding to Trp516-Trp517-Asp518 in yeast Stt3p) that forms hydrogen bonds with the side-chain hydroxyl group of the serine or threonine residue at the $+2$ position of the acceptor sequon.

DK / MI Motif

A conserved OST sequence motif (MI in eubacterial PglB, DK in archaebacterial AglB and eukaryotic STT3) that contacts the methyl group of threonine in the acceptor sequon, accounting for the higher affinity of OST for threonine over serine at the $+2$ position.

Stt3p N539 Glycan

A strictly conserved, highly ordered high-mannose glycan ($\text{Man}_8\text{GlcNAc}_2$) located in a protected cleft formed by Stt3p, Wbp1p, and Swp1p near the catalytic center in yeast OST; its removal (N539Q) is lethal to yeast cells.

Human OST-A Complex

A mammalian OST complex composed of catalytic subunit STT3A, DC2, KCP2, ribophorin-I, ribophorin-II, OST48, TMEM258, DAD1, and OST4 that binds directly to the Sec61 translocation channel and translating ribosomes to perform cotranslational N-glycosylation.

Human OST-B Complex

A mammalian OST complex containing catalytic subunit STT3B and redox chaperone MAGT1 or TUSC3 (lacking DC2 and KCP2) that does not associate with the translocon and mediates posttranslocational glycosylation of skipped or extreme C-terminal sequons on partially folded proteins.

DC2 (Translocon Adaptor Protein)

A noncatalytic subunit specific to human OST-A that shares structural homology with TM2–4 of Ost3p/MAGT1 and mediates the primary attachment of OST-A to the Sec61 complex (Sec61$\alpha$, Sec61$\beta$, and Sec61$\gamma$) via an expanded luminal loop (L1/2).

Ribophorin-I Four-Helix Bundle

A cytoplasmic domain present in human OST-A stabilized by interactions with TMEM258 (specifically Glu470 of ribophorin-I interacting with Tyr46 and Lys45 of TMEM258) that directly contacts large ribosomal subunit protein eL28 and 28S rRNA; it is disordered in OST-B due to structural tilting and a 56-residue N-terminal extension on STT3B.

MAGT1 / TUSC3

Paralogous noncatalytic oxidoreductase subunits containing an active-site CXXC thioredoxin domain incorporated in a mutually exclusive manner into human OST-B (homologous to yeast Ost3p/Ost6p) that form mixed disulfides with nascent proteins to slow oxidative folding and expose glycosylation sequons.

Malectin

An ER-resident membrane-anchored lectin specific for diglucosylated high-mannose glycans ($\text{GlcNAc}_2\text{Man}_9\text{Glc}_2$) that co-purifies in substoichiometric amounts with mammalian OST via interactions with ribophorin-I and TMEM258.

GLYCO-BUILD Pipeline

A modular in vitro chemo-enzymatic platform that uses the commercially available diterpene alcohol phytol as an alternative to dolichol, utilizing recombinant bacterial, archaeal, and eukaryotic enzymes to synthesize defined N-glycans from $\text{GlcNAc}_2$ to $\text{GlcNAc}_2\text{Man}_9\text{Glc}_3$.

SmUdpK (Streptococcus mutans Undecaprenol Kinase)

A recombinant bacterial kinase employed in GLYCO-BUILD to catalyze the ATP-dependent phosphorylation of phytol to produce phytyl-phosphate (Phy-P) with an efficiency comparable to its native undecaprenol substrate.

SaAglH and SaAgl24

Recombinant enzymes from the thermophilic archaeon Sulfolobus acidocaldarius utilized in GLYCO-BUILD Pot 1 to transfer UDP-GlcNAc onto Phy-P and attach a second GlcNAc residue, respectively, yielding homogeneous phytyl-pyrophosphate-chitobiose ($\text{Phy-PP-GlcNAc}_2$).

PfDPMS (Pyrococcus furiosus Dolichyl-Phosphate Mannose Synthase)

A single-subunit recombinant archaeal enzyme used in GLYCO-BUILD Pot 3a and Pot 4 to synthesize phytyl-phosphate-mannose (Phy-P-Man) or phytanyl-phosphate-glucose (Phytanyl-P-Glc) from GDP-sugars, facilitating continuous in situ donor regeneration.

Golgi-Man5 LLO (Pot 3b)

A hybrid N-glycan precursor ($\text{Phy-PP-GlcNAc}_2\text{Man}_5$) synthesized directly in vitro by extending $\text{Phy-PP-GlcNAc}_2\text{Man}_3$ with ScALG3, GgALG12, and Phy-P-Man, bypassing cellular trimming and serving as an efficient acceptor substrate for human MGAT1.

Phytanol

A fully saturated $20$-carbon diterpene alcohol required to synthesize the donor substrate phytanylphosphate-glucose in GLYCO-BUILD Pot 4 because eukaryotic glucosyltransferase ALG8 cannot process donor substrates retaining an unsaturated bond at the first isoprene unit.

TbSTT3A

A single-subunit oligosaccharyltransferase from Trypanosoma brucei that can be recombinantly overexpressed in insect cells, exhibiting a strong donor preference for $\text{GlcNAc}_2\text{Man}_5$ LLO and acceptor sequons flanked by acidic amino acids.

TbSTT3B

A single-subunit oligosaccharyltransferase from Trypanosoma brucei that preferentially transfers fully extended oligomannose donor substrates ($\text{GlcNAc}_2\text{Man}_9$) onto acceptor sequons flanked by neutral or basic amino acids.

E2-N415 and E2-N417

Synthetic 15-amino acid peptide variants of antigenic site 412 (AS412) from Hepatitis C virus glycoprotein E2 that model resistance-conferring glycan shifting, successfully modified with homogeneous $\text{Man}_5$ or $\text{Man}_9$ glycans using GLYCO-BUILD and TbSTT3A/TbSTT3B.

V3-mini-optimized

An engineered 35-residue synthetic peptide representing the HIV-1 gp120 V3 loop containing two functional sequons (N301 and N332) tailored for efficient processing by TbSTT3A, achieving $>95\%$ double glycosylation with $\text{Man}_9$ oligomannose glycans.

NS1-N207

A 15-amino acid peptide corresponding to the conserved immunogenic Asn207 sequon of Dengue virus nonstructural protein 1 (NS1), converted to $>95\%$ homogeneous $\text{Man}_9$ glycopeptide using GLYCO-BUILD and TbSTT3A.