Glycoconjugation and Glycan Synthesis

Comprehensive practice flashcards covering N-linked and O-linked glycosylation, O-GlcNAcylation, and GPI anchors based on the lecture transcript.

What is glycoconjugation?

The covalent attachment of sugars to proteins or lipids.

Approximately what percentage of all eukaryotic proteins are modified by glycoconjugation?

approximately $50\%$

What is the lipid anchor used in the synthesis of N-linked glycans?

dolichol phosphate

How many isoprene units span the dolichol phosphate anchor?

$15$ to $19$ isoprene units

On which side of the endoplasmic reticulum (ER) does N-linked glycan synthesis begin?

The cytosolic side

What complex transfers the pre-assembled sugar block to an asparagine residue in N-linked glycosylation?

The oligosaccharyltransferase (OST) complex

What is the specific sequence motif recognized for N-linked glycosylation?

Asn-X-Ser/Thr

Which amino acid residue nitrogen atom receives the N-linked glycan?

asparagine

What is the function of the glucose residues on the N-linked glycan precursor in the ER?

They act as specific binding signals for chaperone proteins to assist in protein folding.

Which two chaperone proteins bind to glucose residues to assist in glycoprotein folding?

calnexin and calreticulin

Which enzymes act as biological timers for protein folding in the ER by cleaving mannose residues?

mannosidase enzymes

What happens to a permanently misfolded protein in the ER after mannose cleavage prevents chaperone binding?

It is tagged for ER-Associated Degradation (ERAD).

What is the composition of the highly conserved core of N-glycans after trimming in the Golgi?

5 mannose and 2 N-acetylglucosamine (GlcNAc) residues

What serves as a trafficking tag to direct hydrolases to lysosomes?

mannose-6-phosphate

What molecules serve as biological timers to govern a protein's half-life in the bloodstream when placed at the terminal end of N-glycans?

terminal sialic acids

How does the synthesis of O-linked glycans differ from N-linked glycans regarding their 'block' transfer?

O-linked glycans are synthesized in a stepwise fashion, whereas N-linked glycans use en-bloc transfer.

Where are O-linked glycans synthesized?

entirely post-translationally in the Golgi apparatus

To which amino acid side chain atoms are O-linked glycans attached?

The oxygen atom of serine or threonine residues

Which sugar typically initiates the synthesis of O-linked glycans?

N-acetylgalactosamine (GalNAc)

What defines the O-linked glycosylation 'recognition' compared to N-linked?

There is no strict consensus sequence; synthesis is driven by a competitive environment of glycosyltransferases.

What are mucins?

Heavily O-glycosylated proteins bearing short, intensely sialylated glycans.

How do the sialic acids on mucins contribute to tissue lubrication?

Their dense negative charge causes the chains to repel each other and trap water.

In which antibodies do O-glycans provide structural rigidity to flexible hinge regions?

IgA and IgG antibodies

Which proteoglycan in cartilage uses O-xylose linked chains of chondroitin sulfate?

aggrecan

What physical property does the chondroitin sulfate on aggrecan provide to cartilage?

shock-absorbing resilience by trapping and releasing large volumes of water

FGF signaling requires heparan sulfate chains with what specific pattern?

alternating acidic and neutral sugars with precise sulfation patterns

How many EGF-like repeats are found on the Notch transmembrane receptor?

$36$ EGF-like repeats

Which GlcNAc transferase extends the initial fucose on the Notch receptor?

Fringe

What are the two possible ligands for the Notch receptor at the tissue midline?

Serrate or Delta

What is intracellular O-GlcNAcylation?

The dynamic, reversible addition of a single N-acetylglucosamine sugar to serine or threonine side chains.

Which enzyme is responsible for the addition of O-GlcNAc?

O-GlcNAc transferase (OGT)

Which enzyme is responsible for the removal of O-GlcNAc?

O-GlcNAcase (OGA)

What biological function does O-GlcNAcylation serve within the cell?

It acts as a global cellular nutrient and stress sensor.

Between which two types of protein modifications does direct crosstalk occur on serine and threonine?

O-GlcNAcylation and phosphorylation

How does reduced glucose metabolism lead to Tau protein toxicity in Alzheimer's disease?

Lowered O-GlcNAcylation leaves sites open for the toxic hyperphosphorylation of Tau.

What does GPI stand for?

Glycosylphosphatidylinositol

What is the function of a GPI anchor?

To physically tether soluble proteins to cell membranes.

On what lipid is the GPI precursor built outside the ER?

phosphatidylinositol

Which sugars elaborate the GPI precursor after it is flipped into the ER lumen?

mannose and glucosamine

What enzyme complex attaches the protein to the GPI anchor?

transamidase enzyme complex

To which specific group on the GPI anchor is the protein's C-terminus attached?

ethanolamine phosphate group

Which pathogen uses GPI anchors for its surface glycoproteins to evade the host immune system?

Trypanosomes

What is 'en-bloc' synthesis?

The transfer of a large, pre-assembled oligosaccharide block to a protein.

What happens to the lipid-linked oligosaccharide once a specific intermediate is formed on the cytosolic side of the ER?

It is 'flipped' into the ER lumen.

What sugar is used to 'activate' the sugars added during the cytosolic phase of N-linked synthesis?

nucleotide-activated sugars

Which type of glycosylation is described as armor against circulating proteases?

O-linked glycans on antibody hinge regions

What is the primary sugar involved in the 'Fringe' modification of the Notch receptor?

GlcNAc (added to a fucose)

What is the specific biosynthetic role of O-xylose in cartilage?

It acts as the link for chondroitin sulfate chains to the protein core.

Why is the GPI anchor canopy important for Trypanosomes?

It shields conserved transport proteins while allowing the pathogen to evade the immune system.

In N-linked synthesis within the ER lumen, which type of activated sugars are used to complete the precursor?

dolichol-phosphate-activated sugars