From: winalski@adserv.enet.dec.com (Paul S. Winalski)
Subject: Re: Left-handed sugar
Reply-To: winalski@adserv.enet.dec.com (Paul S. Winalski)
Organization: Digital Equipment Corporation, Nashua NH
Date: Sun, 27 Dec 1992 21:10:04 GMT


In article <72463@cup.portal.com>,
mmm@cup.portal.com (Mark Robert Thorson) writes:
|>
|>Please excuse the cross-posting into the sci newsgroups, but we've been
|>having a discussion in rec.food.cooking about the supposed existence
|>of left-handed sugars which are indigestible hence calorie-free.
|>I contend that if L-sugars were calorie-free, they probably wouldn't
|>be sweet, either.  Can someone in the sci groups clarify this?
|>

Sugar biochemistry 101:

Sugars are compounds of the general form (CH2O)n where n>=3.  They are
poly-alcohols with unbranched carbon skeletons and a -OH group on all but one
of the carbons.  That carbon has a C=O double bond.  The double-bonded carbon
is either the terminal carbon of the chain or the penultimate carbon.  If it
is the terminal carbon, the sugar is also an aldehyde and is referred to as
an aldose.  If it is the penultimate carbon, the sugar is also a ketone and is
referred to as a ketose.  Sugars are also classed based on the number of
carbons in the molecole as trioses (3 carbons), tetroses (4 carbons), pentoses
(5 carbons), hexoses (6 carbons), etc.  For example, glucose is an aldohexose
(6 carbons, double bond on a terminal carbon).  Fructose is a ketohexose
(6 carbons, double bond on the penultimate carbon).

In aqueous solution, the aldehyde or ketone group can react with the hydroxide
at the other terminus to form a ring with an ether (C-O-C) linkage.  Such rings
are 6-membered for aldoses and 5-membered for ketoses.  This is in fact the
normal configuration for such sugars in solution and in the body.  Sugars can
also form ether linkages with other sugar molecules to form polymers known as
polysaccharides.  The simplest polysaccharides contain two sugar molecules
linked together and are called disaccharides.  Examples are sucrose [table
sugar] (glucose + fructose), maltose [barley sugar] (glucose + glucose),
and lactose [milk sugar] (glucose + galactose).  Long-chain polysaccharides
include the starches, glycogens, and celluloses.

Now we come to stereochemistry.  The three-carbon aldose, glyceraldehyde, has
one asymmetric carbon.  It therefore exists in two isomeric forms, its
crystals are asymmetric, and therefore they will rotate polarized light.  The
two forms are named D-glyceraldehyde and L-glyceraldehyde.  The D (dextro-)
form rotates light to the right and the L (levo-) form rotates polarized light
to the left.  The four-carbon aldoses have two asymmetric carbon atoms, and
so they exist in four isomeric forms, two of which are mirror images of
the other two.  Likewise there are a total of eight aldopentoses (in four pairs
of mirror images) and 16 aldohexoses (in eight pairs of mirror images).
Each isomer that differs at a carbon other than the penultimate one has a
unique name.  The penultimate carbon is used to give the compound a D- or L-
prefix, based on how that carbon matches up with the asymmetric carbon of
D- or L-glyceraldehyde.  Note that for sugars other than glyceraldehyde, there
is no relationship between the D- or L- prefix and how the sugar rotates
polarized light.  D- and L- refer to the configuration about the penultimate
carbon atom.  Thus, we have D-glucose and L-glucose, which are mirror images
of each other, likewise D-galactose and L-galactose, etc.  Glucose and galactose
are isomers that differ from each other by the configuration about one of the
asymmetric carbons (but not the penultimate one).

The situation is similar for the ketoses.  Dihydroxyacetone, the 3-carbon
ketone, is a symmetric molecule and so exists in only one isomeric form.
The ketotetroses have one asymmetric center, and thus there are two isomers,
designated D- and L-, once again based on the configuration of the first
asymmetric carbon atom in relationship to glyceraldehyde.  There are four
isomers of the ketopentoses, in two mirror-image pairs, and eight isomers of
the ketohexoses, in four mirror-image pairs.  Once again, the D- or L- prefix
has nothing to do with how polarized light is rotated.  D-fructose rotates
polarized light to the left, which is how it gets its common name, levulose.

It turns out that nearly all of the simple sugars commonly found in nature are
D-isomers.  Levulose is D-fructose.

Cells contain a wide variety of enzymes that can convert one isomeric form
of a sugar to another (isomerases), split a sugar into two sugars, or merge two
simple sugars into a longer sugar (transaldolase, transketolase).  The end
result of this is that all of the simple sugars, regardless of isomeric form,
can be converted into D-glucose, the form that is burned in the main catabolic
pathway for sugars to produce energy.

So the L-sugars *are* metabolizable, albeit more slowly than D-glucose, which
is the isomer that most organisms prefer.

Now there is one more wrinkle to this.  When a hexose forms a ring, that
causes one more carbon atom to be asymmetric.  In their ring forms, these
sugars thus have twice as many isomers.  These are classified as a- (alpha)
or b- (beta), based on their configuration about this new center of asymmetry.
The alpha forms are the ones that are metabolizable.  The alpha and beta forms
readily interconvert in solution, so they are not an issue in metabolism of
simple sugars.  However, when sugars are linked to form polysaccharides, whether
the linkage is in alpha- or beta- configuration is critical, since most enzymes
will accept only one of the configurations. For example, maltose is a
glucose-glucose polysaccharide with the linkage in alpha configuration.  If the
linkage is beta, you have cellobiose, not maltose.  Likewise, long-chain
glucose polymers with the linkages in alpha configuration are starches, which
are readily digestible.  If the linkages are in beta configuration, you have
cellulose, not starch, which is not digestible.  Only a few kinds of bacteria
and protozoans have the proper enzymes for breaking beta linkages in
polysaccharides.

Regarding sweetness, all (or nearly all, anyway) of the simple sugars taste
sweet.  Among the polysaccharides, the long-chain compounds (starch, glycogen,
cellulose) do not have a sweet taste.  Certainly some of the disaccharides do
(sucrose, maltose, galactose), although I'm not sure whether this is due to
their being intrinsically sweet or if it's because they are readily broken down
to simple sugars by enzymes in saliva.


So the bottom line is, L-sugars are digestible, albeit not as readily as their
D- counterparts, and they do taste sweet.  On the other hand, disaccharides
with a beta linkage are not digestible, which is why humans cannot survive on
a diet of leaves or wood.  Cellobiose, the beta-1-6-D-glucose disaccharide,
is not digestible, but I don't think it tastes sweet, either.

In any event, since the L-isomer sugars are fairly rare in nature, dietary
sweeteners based on them would likely be prohibitively expensive.

--PSW
Newsgroups: rec.food.cooking