Deconstruction by Enzymes 2: Hemicellulases

Chun-Hung Lin Ph.D. 林 俊 宏Institute of Biological Chemistry

Academia Sinica

E-mail: chunhung@gate.sinica.edu.tw

Degradation of Hemicellulose

Knowledge offered to you includes

(1) What is hemicellulose?

(2) Why is ligno-cellulose considered to be a renewable alternative to fossil fuels?

(3) What enzymes are involved in hemicellulose degradation?

(4) General aspects, e.g. catalytic mechanism, synergistic effect, how to reduce the enzyme cost, a suitable process for biofuel production

Chemical Compositions of Biomass

1. Heterogeneous compositions

2. The composition varies among species

3. In average, biomass consists of ~25% lignin and

~75% carbohydrate polymers (cellulose and hemicellulose)

4. Structures of cellulose and hemicellulose

The carbohydrate fraction is made of primarily of glucose;however, the pentose fraction is rather significant. Xylose is second only to glucose in natural abundance and it is the most copious sugar in the hemicellulose of hardwoods and crop residues.

Structure of Cellulose

Glucose-1,4-linked polymer

Monosaccharide Compositions andStereochemical Configuration

1. Glucose, Mannose, Galactose, Glucuronic acid, and

Galacturonic acid

2. Xylose, Arabinose

3. - and -linkage, anomeric center, reducing end

Hexose and Pentose

Hexose and Pentose

Glucose-1,4-linked polymer

Structure of Hemicellulose

Difference between Cellulose and Hemicellulose

1. Different compositions

2. Degree of polymerization (DP)

Cellulose: DP varies from 2000 to 15000

Hemicellulose: DP varies from 100 to 200

Hemicellulose: A heterogeneous mixture of different polysaccahrides and compositions

Xyloglucan: the predominant hemicellulose.It coats the surface of the cellulose microfibrils, limiting their aggregation and connecting them via tethers for the mechanical properties.

Hemicellulose: A heterogeneous mixture of different polysaccahrides and compositions

-1,3-1,4-Glucan: associated with cellulose microfibrils in the primary cell wall of grasses

Hemicellulose: A heterogeneous mixture of different polysaccahrides and compositions

O-Acetyl-4-O-methyl-glucurono-xylan: Existing as the primary

walls of grasses and secondary walls of all angiosperms

(hardwoods). It is the most common building block in hardwoods

Hemicellulose: A heterogeneous mixture of different polysaccahrides and compositions

Arabino-4-O-methylglucurono-xylan: a major component of

Softwoods (gymnosperms).

Hemicellulose: A heterogeneous mixture of different polysaccahrides and compositions

Acetyl-galacto-glucomannan: Another major component of softwoods

Enzymes involved in the degradation of hemicellulose

Catalytic Reaction Mechanism

Synergistic Effect