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FUNGI | The Cell Wall |
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Terms defined on this page: |
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anomer enantiomer furanose glucan glucose glycoside Haworth diagram |
hemiacetal
hydroxyl group ligand mannose polysaccharide pyranose stereochemistry |
These would be on the test, |
Since we haven't done this elsewhere, it's time we provided the rudiments of sugar (saccharide) chemistry, so that we can make useful noises about polysaccharides (sugar polymers) -- easily the most common class of biopolymers on the planet. A more extensive and far better introduction may be found at Natural Products.
All sugar monomers of biological importance have structural formulas which looks something like this: CH2OH-(CHOH)n-CHO. In other words, they consist of a chain of carbon atoms, in which each carbon atom has a hydroxyl (-OH) group attached to it, except for C1 (sometimes C2) which has an aldehyde or keto (=O) group. In living organisms, the chain is generally 3-7 carbons long. In biologically important polysaccharides, the monomers are almost always 5- or 6-carbon sugars.
We have only reluctantly provided a reference graphic of a sugar monomer in linear form because, in life, 5- and 6- carbon sugars rarely occur as straight chains. The carbon atoms with the aldehyde (or keto) group reversibly bonds to one of the other carbons by "sharing" a hydroxyl oxygen, forming a C-O-C linkage. This is known as an hemiacetal linkage. Typically, the result is a 5- or 6-member ring -- four or five carbon atoms plus the linking oxygen. A five-member form (e.g. a C1→C4 linkage) form is called a furanose. A six-member ring (e.g. C1→C5 linkage) is a pyranose. A simple example, and perhaps the most common sugar monomer, is glucose. Its usual (pyranose) ring form is shown in the image. It can also occur as a furanose. In fact, the two forms are in equilibrium. Under biologically relevant conditions, the equilibrium so strongly favors the pyranose form of glucose that we can ignore the furanose. However, this is not necessarily the case for all sugars.
This is also the last time we will show the ring carbons. By the universal convention of biochemists, carbon atoms forming part of a ring structure are not shown with a 'C' symbol. They are simply indicated by the intersection of the bonds from the various groups (ligands) to which the carbon atom is attached. Very frequently, hydrogen ligands (H-) are not shown either. A line with nothing at the end means a hydrogen ligand, and an unlabelled intersection of bonds means a carbon atom. See examples below.
Sugar monomers are not always quite this simple. Each of the hydroxyl ligands is moderately chemically active, and all kinds of variants exist. An example, of particular relevance to fungi, is chitin. Chitin is a polymer of N-acetly-2-glucosamine, i.e., a glucose derivative in which the ligand CH3-CH2-NH- substitutes for the OH-group on C2. See the chitin glossary entry for an image.
In most of these examples, we have shown the structure of sugars using a Haworth Diagram. These are easy to draw and to understand, but they are rather crude tools because the bond angles are grossly distorted. Carbon normally forms tetrahedral structures, with the bonds about 108° apart. However, Haworth diagrams will do for our purposes, so long as we don't take them too seriously.
The figure above is labeled "D"-glucose for an important reason: it gives us an excuse to discuss three quick points about stereochemistry. Stereochemistry relates to the properties of compounds which are chemically identical, except that they are asymmetrical, and differ in the arrangement of ligands about one or more asymmetrical backbone atoms.
(1) Note that carbons 1 through 5 are asymmetrical in glucose. Each of these carbons is attached to four different ligands. Thus, the relative positions of the groups attached to the carbon atoms makes a difference. If, for example, we flipped the hydroxyl group on C2 so that it was above the ring, this would no longer be glucose. It would be mannose, a sugar with rather different chemical properties.
(2) If we took the mirror image of the entire molecule, all of the bonds would be in the same relative position. Thus we would have a molecule that ought to have exactly the same chemical properties as glucose, which it does -- sort of. The difficulty is that, when this reversed glucose interacts with some other asymmetrical biochemical, the two molecules no longer mesh in the same way. Consequently, we must distinguish between D-glucose and its mirror image (enantiomer), L-glucose. Don't worry about telling the difference. The biologically relevant form for sugars is usually the D-enantiomer. You can assume a figure shows the D-enantiomer unless someone tells you differently.
(3) C1 is a special case. In the linear form, C1 is not
asymmetrical because it has only three ligands. However, when the C1 forms a pyranose linkage to C5, it
becomes asymmetrical. In terms of our diagram, the -OH group
on C1 might point down or up. Free glucose in solution is, once again, in equilibrium
between the two forms, referred to as
Fungal cells maintain a very high turgor pressure, so the integrity of the cell wall is a critical matter. Cabib et al. (2001). The composition of the fungal cell wall is rather variable. The variability appears to have phylogenetic significance, but few, to our knowledge, have followed that trail (but see Grun, 2003). In general, mycology has leapt directly from the ponderous fallacies of classical typological systematics to the facile, but sometimes equally fallacious, paradigms of molecular systematics. Consequently, there is remarkably little honest biology and biochemistry being applied to phylogenetic issues.
The situation is not improved by the usual non-specialist texts which characterize the fungal cell wall as a relatively simple structure made up of "cellulose" and chitin. Consider that the fungal cell wall can make up 30% or more of the dry weight of the fungus, and that the fungi are characterized by external digestion of food followed by selective absorption of the digestion products. Clearly, we can expect that the fungal cell wall will be a complex, specialized system.
It is all that; and, in addition, it is a highly dynamic system, constantly being regenerated and remodeled according to the needs of the moment. Adams (2004). Thus, many of the cell wall-associated proteins are enzymes whose function is to hydrolyze chitin and polysaccharides. The lesson is that this type of cell wall is, from a metabolic point of view, very different from insect exoskeletons or a plant cell walls, which are terminally differentiated structures.
Not unexpectedly, attempts to understand the biosynthesis of cell wall
components have run into a maze of regulatory pathways which are difficult to
sort out. García et
al. (2004) applied brute force genomics methods to analyze gene
responses to several different physical and chemical agents affecting cell wall
integrity. The genetic responses in each case involved on the order of 100
different genes, with a significant different cohort of genes activated by each
agent. Similarly, Lesage
et al. (2004) identified 135 genes involved in the synthesis and
regulation of the
We include two diagrams of the fungal cell wall by Grün (2003) and Cabib et al. (2001). We've also thrown in Joan Miró's (1940) Chiffres et Constellations just because it has somewhat the same feel to it.
While each of these images speaks to us in its own way, we will work primarily
with
Grün's concept. The cell wall is generally constructed of three layers: (1) an
The
The
The bulk material of the cell wall is usually in the form of
In addition to
The outermost layer of the cell wall is composed of diverse proteins bearing
polysaccharide side chains composed of mannose. The usual explanation is
that these are attached through their mannan side chains via a (1→3)
linkage with the
Finally, the fungal cell wall contains variable amounts of chitin. In many systems chitin is a major constituent of the cell wall. In others, it is involved only in cell division or reproductive structures and is virtually absent otherwise. Again. we are reluctant to say much about it, absent more detailed, phylogenetically-grounded studies of the actual ultrastructure in particular cases.
In general, the study of the fungal cell wall tends to be strong on models and somewhat weaker on data. One virtue of the brute force genomic and proteomic studies now being produced is that they clearly confront us with the scope of the problem. Fungal cells probably lack the diversity of metazoan tissues. However, each fungal cell must, for that very reason, be competent to perform a much wider variety of functions than a typical terminally-differentiated metazoan cell. Consequently, their superficial similarity and simplicity are likely to mask a very complex, plastic biochemical repertoire. Perhaps, after all, the Miró is the best representation, given the current state of our knowledge. ATW051113.
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