4. Cell Wall

• Primary wall, middle lamellae, pectin-rich cell corner

• Forms a scaffold, regulates cell division, cell shape, stability

• Primary structure: cellulose, interconnected by hemicellulose, glycanes and pectins, in between areproteins

• Glucose to galacturonic acid: epimerization (à galactose), oxidation at C6

• Glucose to arabinose: oxidation at C6 (à glucuronic acid), decarboxylation (à xylose), epimerization

• Glucose to mannuronic acid: epimerization at C2 (à mannose), oxidation at C6

Reducing ends of sugar polymers are semi-acetals (at the anomeric carbon!) and their Haworth ring can reopen into the Fischer chain. Here, they have an aldehyde or keto group that has reducing capacities in the Fehling or Tollens test:

On the other hand, non-reducing sugars are “full” acetals, their ring cannot reopen, and they cannot react in the Fehling or Tollens test.

Cellulose (central metabolite)

• Chain made of glucose molecules, connected via β-1,4-glycosidic bonds

• Different chains interact with each other via hydrogen bonds (both within one strand and between strands) to form microfibrils

• Synthesis occurs within the plasmamembrane: first, saccharose synthase produces UDP-glucose, then, cellulose synthase produces cellulose outside of the cell. Complexes of one saccharose synthase and several cellulose synthases lead to the formation of microfibrils.

Callose (specialized metabolite)

• Helix made of glucose molecules, connected via β-1,3-glycosidic bonds

• Only occurs in special circumstances, for example in dividing (pollen tubes), wounded or infected cells, forming a mechanical barrier.

• “Callose is like concrete”.

Xyloglucans (XyGs)

• Cellulose backbone, additional C6-sugars are attached regularly

• Present in dicots and about half of the monocots

• Example: xyloglucan (D-xylulose is attached every 4 backbone-glucose monomers, additionally L-arabinose and a short chain of D-xylulose, D-galactose and D-fucose.)

• Biochemical properties can be adjusted by modifying mesh size etc. by adding more cross-links

• Xyloglucans are species-specific

Glucuronoarabinoxylans (GAXs)

• Xylulose backbone (β-1,4-glycosidic bond), modified with arabinose and glucuronic acid and sometimes phenolic compounds such as ferulic acid (commelinoid GAXs, increased hydrophobicity)

• Less abundant than xyloglucans

• Due to the glucuronic acid with its negative charge, mechanical stability is increased (two strands can be “glued” together with a calcium or magnesium atom

• Example:

(Two examples: Two examples in total. Two examples each: Four examples in total)

• D-Galacturonic acid (α-1,4-glycosidic bond) with sugars attached to it

  • Potential methylation of the acid residues lead to increased hydrophobicity

• Tighter packing of the cell wall (negative charges in combination with calcium or magnesium ions in between lead to the formation of a tight mesh)

  • Connection between strands can also occur via borate

• Borate anchors pectin polymers to the plasma membrane (binding lipid head groups)

• Functions

  • Adjustment of cell porosity

  • Providing charged surfaces (à crosslinking)

  • Modulation of cell wall pH and ion balance

  • Regulation of cell-cell adhesion

• Major feature: induces ionic bonds, connects callose fibers

Classes of structural cell wall proteins

• Hydroxyproline-rich glycoproteins (HRGPs, extensins)

• Proline-rich proteins (PRPs)

• Glycine-rich proteins (GRPs)

• Arabinogalactan-rich proteins (AGPs)

Examples for general cell wall proteins

• Expansins: initiate cell wall growth

• HRGPs: stabilize the cell wall, responsible for structural integrity (cross-linking)

• Basically every protein that is involved in cell wall degradation, production, etc.

• Glucanase acting together with the extensins(?), transport proteins (through the polymer), defensive enzymes that degrade intruders’ cell walls (chitinases)

Generally, we do not know much about the function of most cell wall proteins.

Type 1:

Type 2: