Biomedical Engineering Reference
In-Depth Information
4000 DP. During secondary wall deposition, the distribution is more narrow, averaging
m of chain length.
The degree of polymerization of cellulose within crystallites is positively correlated with
cotton fiber strength (51), and similar effects are likely to occur for individual cellu-
lose nanofibrils within plant cell walls. It is possible that the different CesA isoforms
employed for primary vs. secondary wall synthesis help to control the degree of poly-
merization. For example, differences in the CSR region occur between the primary and
secondary cell wall CesA proteins (52), and these are likely to affect aspects of protein
function that are as yet unknown. Control of glucan chain length could occur directly
via limitation of the average time of continuous polymerization by a single CesA protein
or be controlled indirectly via different CesA lifetime in the plasma membrane (7); see
also details about CesA lifetime (above).
10,000 DP (50), with each 2000 glucose units contributing to
1
µ
2.4.7
Control Cellulose Nanofibril Diameter
Nanofibril size can be regulated by the number of CesA proteins aggregated together
in the plasma membrane, which in turn determines the number of ß-1,4-glucan chains
that are likely to coalesce without interference from other molecules. Nanofibril cross-
sectional dimensions range from 2 to 25 nm and have been correlated with CSC geom-
etry or higher order aggregation in Gluconoacetobacter xylinum (53), Dictyostelium
discoideum (54), various lineages of algae (55), and tunicates (56). In eukaryotic
D. discoideum , the ability of only one CesA protein (57) to assume different aggrega-
tion states and produce cellulose fibrils with different sizes has been demonstrated (54).
The number of CesA proteins in a rosette CSC is still unknown, but current data sug-
gest that it may be less than the 36 subunits commonly modeled (29) and diagrammed
in Figure 2.3. Recent analysis of extracted, hydrated small cellulose nanofibrils with
cellulose IV crystallinity (indicative of good longitudinal but poor lateral chain order)
indicated that they were
2 . 4-3.2 nm in diameter, which is consistent with 15-25 total
chains (58). Especially given the possibility of fibril coalescence during sample prepa-
ration, such data argue against a fundamental unit of plant cellulose synthesis with 36
chains. Therefore, it may be more likely that 3-4 CesA proteins exist within each
subunit of the rosette CSC, for a total of 18-24 CesAs. This possibility is consistent
with rough estimates of space filled by TMH in the plasma membrane, which predicted
4 CesAs within each of six subunits of the rosette CSC (59).
The minimum crystal-
lite size for cellulose I was estimated to be 2 . 8
×
2 . 8 nm, which would be contained
within a
3 . 5 nm fibril, including less ordered surface chains that may be present in
the cellulose nanofibrils of vascular plants (60). Therefore, one rosette CSC may not
by itself produce cellulose I fibrils. Instead, fibril coalescence could occur before final
crystallization, although rosette CSCs have not been seen in geometric arrays or tightly
packed aggregates even during secondary wall synthesis in plants.
Prior to crystallization, other matrix polymers with ß-1,4-glycan backbones such as
xyloglucan and xylan are able to interact with cellulose and limit the extent of its higher
order coalescence and crystallization. The degree to which this occurs would vary in
parallel with the type and amount of matrix polymers in the cell wall space; for example
larger cellulose nanofibrils and crystallites are produced in tension wood that are severely
depleted in matrix components (61).
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