Cellobiose and cellodextrin metabolism by the ruminal bacterium Ruminococcus albus

Area of Science:

• Microbiology
• Biochemistry
• Rumen Microbiology

Background:

• Ruminococcus albus is a key fibrolytic bacterium in the rumen.
• Cellobiose metabolism in R. albus occurs via hydrolytic and phosphorylytic pathways.
• The relative importance of these pathways was previously unclear.

Purpose of the Study:

• To elucidate the primary metabolic pathways for cellobiose utilization by R. albus.
• To quantify the contributions of hydrolytic versus phosphorylytic enzymes.
• To characterize the key enzymes involved in glucose phosphorylation.

Main Methods:

• Enzymatic assays to measure cellobiose consumption rates in whole cells and crude extracts.
• Quantification of hydrolytic and phosphorolytic activities.
• Characterization of glucokinase activity using various substrates and nucleotides.
• Determination of kinetic parameters (Km) for glucose and GTP.

Main Results:

• Phosphorolytic cleavage of cellobiose significantly exceeded hydrolytic cleavage, indicating phosphorylases are key.
• Cellodextrin phosphorylase showed activity against cellohexaose.
• Phosphorylase activity was cytoplasmic, while hydrolytic activity was both cytoplasmic and membrane-associated.
• A GTP-dependent glucokinase preferentially phosphorylated glucose using GTP or ITP.

Conclusions:

• Phosphorylation is the dominant initial step in cellobiose metabolism by R. albus.
• GTP-dependent glucokinase plays a significant role in glucose phosphorylation.
• Energy conservation through phosphorolytic cleavage likely enhances R. albus growth efficiency in the rumen.