|
H-C-OH H-C-OH H-C-OH H-C-OH
| | | |
H-C-OH or H-C-OH but not HO-C-H or HO-C-H
| | | |
CHO CH_{2}OH CH_{2}OH CHO

The configuration of the remainder of the molecule is immaterial to action
by these oxidizing bacteria; hence, the enzymes in this case are apparently
concerned only with the configuration arrangement of a portion of the
molecule, instead of with the whole hexose grouping, as in the cases of the
other reactions which have been thus far considered.

It is apparent from these illustrations, and from many more which might be
cited, that there is a very definite relation between the molecular
configuration of a carbohydrate and its biochemical properties, as
represented by the possibilities of the action of enzymes upon it. The
probable nature of this relationship will be better understood after the
general questions involved in the mode of enzyme action have been
considered (see chapter XIV). But for the present, it will be sufficient to
note that it seems to be necessary that the enzyme shall actually fit the
molecular arrangement of the compound at all points, in the same way that a
key fits its appropriate lock; or a still better illustration is that of
the fitting of a glove to the hand. On the basis of the latter
illustration, it is just as impossible for a dextro-enzyme to affect a
levo-sugar, or for [alpha]-glucase to affect a [beta]-glucoside, as it is
to fit a right-hand glove upon a left hand. Further attention will be given
to these matters in later chapters.

POLYSACCHARIDES

The polysaccharides which, like the simpler saccharides, or sugars, which
have thus far been studied, undoubtedly serve as reserve food for plants,
are known under the general name of "starches." They are substances of high
molecular weight, whose constitution is represented by the general formula
(C_{6}H_{10}O_{5})_{n}. It should be noted that an exactly accurate formula
should be (C_{6})_{n}(H_{12}O_{6})_{n-1}; but since the value of _n_ is
very high, the simpler formula is approximately correct. The value of _n_
has not been accurately determined for any of the individual members of the
group, but is probably never less than 30 and may often be