UDP-Glucose is the Immediate Precursor for Cellulose Synthesis
Although cellulose was characterized as an aggregation of glucose units by
Anselme Payen in 1839, it was in 1895 that Tollens proposed that cellulose is a
chain of glucose molecules (French, 2000). While the structure of cellulose was
being determined and debated, studies on its biosynthesis did not truly begin
until the identification of nucleotide sugars, and specifically UDP-glucose as a
glucose donor in biosynthetic reactions (Leloir and Cabib, 1953). The transfer
of glucose from UDP-glucose to cellulose was first described by Glaser in 1958
using particulate fraction from cell-free extracts of the bacterium Acetobacter xylinum
(Glaser, 1958). However, when UDP-glucose was used as the sugar
donor in experiments using digitonin-solubilized fractions from various plants,
the polysaccharide product obtained in vitro
was identified as callose (β-1,3-glucan)
instead of cellulose (Feingold et al.
, 1958). Using particulate extracts from plants,
the synthesis of cellulose was reported by Barber and colleagues in 1964, and from
their experiments these authors concluded that the sugar donor for synthesis of
cellulose was guanosine 5'-diphosphate (GDP)-glucose and not UDP-glucose
(Barber et al.
, 1964). In these experiments, the particulate extracts from plants
also allowed synthesis of an alkali-insoluble polysaccharide from GDP-mannose
and from a mixture of GDP-glucose and GDP-mannose. Recently, a cellulose
synthase-like protein (AtCsl
A9), identified as a β-glucomannan synthase, has
been shown to possess β-mannan synthase, β-glucan synthase, and β-glucomannan
synthase activities (Liepman et al.
, 2005). This β-glucomannan synthase can
catalyze the production of β-mannan when supplied with GDP-mannose, a
β-glucan when supplied with GDP-glucose or β-glucomannan when supplied
with a combination of GDP-glucose and GDP-mannose. It is now clear that in
the earlier experiments where GDP-glucose was used as a sugar donor with plant
extracts, techniques for characterizing the in vitro
products did not allow a clear
distinction to be made between the possible β-glucomannan product and cellulose (Barber et al.
, 1964; Chambers and Elbein, 1970). Moreover, it was felt at the time
that synthesis of the major homopolymers of glucose in plants could be regulated
by using different nucleotide sugars—UDP-glucose for callose synthesis, adenosine
diphosphate (ADP)-glucose for starch synthesis, and GDP-glucose for
cellulose synthesis (Barber et al.
, 1964). We now know that in plants, although
ADP-glucose is the precursor for starch synthesis, the precursor for synthesis of
callose and cellulose is UDP-glucose. Support for the role of UDP-glucose as a
precursor of cellulose in plants came from studies tracing the flow of carbon from
glucose to cellulose in developing cotton fibers (Carpita and Delmer, 1981).
Evidence for the role of UDP-glucose as the precursor for cellulose synthesis in
plants did not come easily, and only a brief historical account is given here to
highlight one of the many difficulties encountered in dissecting the mechanism of
cellulose synthesis in plants. A detailed account of the early years and the progress
that has been made since then is provided by Delmer in a number of excellent
review articles (Delmer, 1983, 1999). Suffice it to say that as late as 1983, in one of
her reviews Delmer summarized that ‘‘convincing in vitro
synthesis of cellulose
from UDP-glucose using plant extracts has never been conclusively demonstrated’’
(Delmer, 1983). In plants, UDP-glucose functions as a glucose donor in
a number of glucosyl transfer reactions. From genome sequencing, it is now
known that plants have the largest number of carbohydrate-modifying enzymes,
and consequently UDP-glucose could participate as a glucose donor in many
different reactions when unpurified plant extracts are used for in vitro
synthesis (Coutinho et al.
, 2003). Furthermore in plants, polysaccharides, such as
xyloglucan, have a backbone similar to cellulose, and it is important to distinguish
the synthesis of these polysaccharides from synthesis of cellulose. Although not
much has changed since the early days in the manner in which in vitro
synthesis reactions were performed, a few modifications in the reaction conditions
and better product characterization (described later) has allowed conclusive demonstration
of in vitro
cellulose synthesis from UDP-glucose using extracts from
a variety of plants (Colombani et al.
, 2004; Kudlicka and Brown, 1997; Kudlicka et al.
, 1995, 1996; Lai-Kee-Him et al.
, 2002; Okuda et al.
, 1993; Peng et al.