A5 - Glycan metabolism by the Microbiota I

What are glycans?

It’s an umbrella term for polysaccharides, oligosaccharides and glycoconjugates

Why are simple sugars from our diet, not available to the gut microbiota?

Because the human body is very efficient at absorbing them meaning the gut microbiota isn’t able to access them as easily

Xylans and beta-glucans are examples of what?

Plant cell wall polysaccharides

Most CAZymes are _________ which cleave _______ bond using _____.

Most are glycoside hydrolases which cleave glycosidic bonds using water

Approximately, how many CAZymes does the human genome encode?

Around 17 enzymes.

• This is significantly less than the hundreds of CAZymes which some bacteria encode

What are the three main roles of glycan metabolism in host heath?

• Production of Short Chain Fatty Acids

• Production of the mucus barrier

• Metabolism drives and maintains microbiota diversity

Short Chain Fatty Acids are a waste product of what?

A waste product of anaerobic fermentation of glycans by gut bacteria

Only around __% of SCFAs are excreted as faeces. This suggests __% is used by humans and other gut bacteria.

Only around 5% is excreted as faeces. This suggests 95% is used by humans and other gut bacteria

What are the three main types of SCFA produced by the gut microbiota?

• Acetate

• Propionate

• Butyrate

Which gut bacteria produce Acetate?

Most gut bacteria produce Acetate.

What is the role of Acetate in host health?

• Prevents colonisation by some pathogens by lowering the pH

• Increases Butyrate production

Which gut bacteria produce Propionate?

Mostly produced by Bacterodota.

What is the role of Propionate in host health?

• Maintains glucose homeostasis

• Reduces appetite

Which bacteria produce Butyrate?

Mostly produced by Firmicutes

What is the role of Butyrate in host health?

• Main energy source of colonocytes

• Anti-cancer role

  • Suppresses cell proliferation and induces apoptotic cell death

All three SCFA types stimulate pro-inflammatory pathways.

True or False?

False.

All three stimulate anti-inflammatory pathways.

Inflammatory Bowel Disease is characterised by what?

Chronic inflammation

• Has both a genetic and dietary component

Inflammatory Bowel Disease is associated with what kind of diet?

High fat, low fibre diet.

Why could lower SCFAs potentially contribute to IBD?

Lower concentrations of SCFA in the lower colon compared to the proximal colon.

A lot of IBDs affect the lower colon meaning the lower SCFA could explain why there is less inflammation dampening.

Gut mucus is composed of what complex glycoproteins?

Mucins which are produced constantly to replenish the mucus layer

How can the mucus layer be degraded if you don’t have enough dietary fibre?

Less dietary fibre means less nutrients for the gut microbiota

Mucins can instead be used as a nutrient source by some members of the microbiota

This causes the mucus layer to be degraded and become much thinner and vulnerable.

Which are the main bacterial phyla which break down glycans in the gut?

Gram negative: Bacteroidetes

Gram positive: Firmicutes and Actinobacteria

Bacteroides. spp are very specialised to breakdown specific glycans in the gut.

True or False.

False.

Bacteroides. spp are generalists which can breakdown a wide range of glycans

• Firmicutes and Actinobacteria are the much more specialised bacteria

Which two Bacteroides species use most of the glycans found in nature between them?

B. Thuringiensis and B. Ovatus 

Bacteroidetes machinery for glycan degradation is encoded in what loci?

Polysaccharide utilization loci (PULs)

Polysaccharide utilization loci (PULs) encode what?

Sus-like systems

What does Sus mean in the context of glycan metabolism?

Starch utilisation system (Sus)

• Each Sus-like system degrades a specific glycan

What are PULs defined by?

Homologues of SusC/D as well as the presence of glycoside hydrolase (GH) genes

• Normally these are linked to sensor-regulators

What is the function of SusD?

SusD binds target glycan with low affinity and is required for oligosaccharides larger than 5 sugars

• Role of SusD is to deliver large oligosaccharides to SusC transporter

What is the role of SusC?

To transport glycans through the bacterial cell membrane.

What is the structure of SusC/D?

SusD sits on top of SusC

• Like a barrel and a lid

SusD opens to ‘capture’ ligand and closes once captured

What is the role of Surface Glycan Binding Protein (SGBP)?

Most PULs will also have a SGBP

• Part of the SusC/D complex and binds glycan target with high affinity

Role is to capture glycans and help provide a competitive advantage in the densely populated gut

What is the utilisome of the gut microbiota?

All the components involved in glycan metabolism.

• SusC/D, Glycoside hydrolases, SGBP, etc

What is the process of glycan metabolism by Bacteroidetes?

SGBP binds a glycan and glycan is cleaved by Glycoside hydrolase enzyme.

These smaller glycans are then taken up by the transporter complex, SusC/D to take them into the cell