Balancing carbohydrates and proteins for optimum rumen microbial yield.

Increasing energy demands are not only exploiting the fossil resources but, also depleting natural environment. Biofuels from lignocellulosic biomass is a renewable, ecofriendly, sustainable and could be a promising alternative to fossil fuels. However, pretreatment is an essential step to disarray the layers of lignocellulose prior to enzymatic hydrolysis. Among various pretreatments of lignocellulose, the biological pretreatment using microorganisms such as bacteria and fungi are gaining popularity due to its financial and environmental benefits. Careful selection of the suitable microbial consortium for efficient pretreatment of biomass is a critical step. The co-culture of bacteria and/or fungi in consolidated bioprocessing (CBP) is highly beneficial in the breakdown of complex biopolymers due to their high enzyme activity. Our selection of highly promising bacterial and/or fungal consortium has the ability to produce various extracellular enzymes including cellulase, hemicellulase, and lignases. It can be used in CBP for efficient biological pretreatment of lignocellulosic biomass following production of biofuels and bioproducts.

Biological Pretreatment of Lignocellulosic Biomass for Biofuels and Bioproducts: An Overview

Carbohydrates are the most abundant class of compounds found in plants. They account for 50-80% of the dry biomass of forage species (Van Soest, 1982). From a plant perspective, carbohydrates play important roles in intermediary metabolism, energy transfer and storage, and plant structure. Photosynthetic energy is fixed in carbohydrates via the Calvin cycle and these carbohydrates serve as initial substrates for nearly all intermediary pathways in plants. Energy is translocated within plants as the disaccharide sucrose (Hawker, 1985), and stored in polymers such as starch and fructans (Manners, 1985; Pontis and Del Campillo, 1985). Carbohydrates constitute most of the plant cell wall and therefore play an important role in the structural integrity of individual cells, tissues, and organs (Hatfield, 1989). Carbohydrates are extremely important from a nutritional perspective, providing the primary source of energy in ruminant diets. In ruminants, nearly all carbohydrate digestion (> 90 %) occurs within the rumen (Armstrong and Smithard, 1979; Sutton, 1979), although under certain circumstances, such as high rates of passage, a significant amount of carbohydrate digestion can occur in the small and large intestines (Hoover, 1978; Nocek and Tamminga, 1991). Simple sugars are rapidly fermented within the rumen to yield volatile fatty acids which are absorbed into the blood through the rumen wall (Morrison, 1979; Baldwin and Allison, 1983). Polysaccharides must be degraded to simple sugars before being utilized. Nonstructural polysaccharides, such as starch and fructans, are rapidly and generally completely degraded within the rumen (Nocek and Tamminga, 1991), while degradability of structural polysaccharides varies considerably. Pectins are rapidly and essentially completely degraded by ruminal microbes (Hatfield, 1989, 1993a). Cellulose and hemicellulosic polysaccharides are more slowly and incompletely degraded. Cellulose degradability of forages varies from 25-90% while hemicellulose digestibility varies from 45-90% (Pigden and Heaney, 1969). Degradation of {3-glucans is intermediate to that of pectins and cellulose (Van Soest et aI., 1991). This ability to degrade and utilize structural carbohydrates confers upon ruminants their unique ecological niche (Chesson and Forsberg, 1988).

Carbohydrates and Forage Quality

Processes of Digestion and Factors Influencing Digestion of Forage-Based Diets by Ruminants

An alpha-l-arabinofuranosidase (EC 3.2.1.55) was purified from the cytoplasm of Butyrivibrio fibrisolvens GS113. The native enzyme had an apparent molecular mass of 240 kDa and was composed of eight polypeptide subunits of 31 kDa. The enzyme displayed an isoelectric point of 6.0, a pH optimum of 6.0 to 6.5, a pH stability of 4.0 to 8.0, and a temperature optimum of 45 degrees C and was stable to 55 degrees C. The K(m) and V(max) for p-nitrophenyl-alpha-l-arabinofuranoside were 0.7 mM and 109 mumol/min/mg of protein, respectively. The enzyme was specific for the furanoside configuration and also readily cleaved methylumbelliferyl-alpha-l-arabinofuranoside but had no activity on a variety of other nitrophenyl- or methylumbelliferyl glycosides. When the enzyme was incubated with cellulose, carboxymethyl cellulose, or arabinogalactan, no release of sugars was found. Arabinose was found as the hydrolysis product of oatspelt xylan, corn endosperm xylan, or beet arabinan. No activity was detected when either coumaric or ferulic acid ester linked to arabinoxylobiose was used as substrates, but arabinoxylobiose was degraded to arabinose and xylobiose. Since B. fibrisolvens GS113 possesses essentially no extracellular arabinofuranosidase activity, the major role of the purified enzyme is apparently in the assimilation of arabinose-containing xylooligosaccharides generated from xylosidase, phenolic esterase, xylanase, and other enzymatic activities on xylans.

/pdf/purification-and-characterization-of-an-a-l-4aszc4k86v.pdf

Purification and Characterization of an α-l-Arabinofuranosidase from Butyrivibrio fibrisolvens GS113

The ruminant animal is dependent upon anaerobic microorganisms for the digestion of plant materials in the rumen. The resultant microbial cells and fermentation acids are primary nutrients for the animal. Polysaccharides, such as hemicelluloses, are only partially degraded because of the chemical complexities of plant materials and biochemical processes involved in degradation.

Microbial digestion of hemicelluloses in the rumen.

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Microbial digestion of hemicelluloses in the rumen.