528
14 Glycolysis
Table 14.3. Maximal activities of various aldolases on
fructose-1,6-bisphosphate compared with fructose-1phosphate [22, 163, 239]
Aldolase
Rabbit
Type A (muscle)
Type B (liver)
Type C (brain)
Rat muscle
Trypanosoma sp.
Eimeria stiedai
Helix pomatia
Drosophila melanogaster pupae
Schistocerca gregaria flight muscle
Ratio
50
1
10
100
100
21
20
10-15
37
in vivo for the switch in energy-yielding metabolism from carbohydrates to fatty acid oxidation
[239].
The similarity in functional properties between
these enzymes is based on a surprisingly high
degree of structural similarity. The aldolase subunits are comprised of 360-363 amino acids. The
sequences are known for all three isoenzymes of
the rat and for aldolases of man and other mammals, chicken, Drosophila melanogaster, Trypanosoma brucei and maize [148, 172, 267]. The
aldolases of mammalian muscle, with only 2 %
substitutions per 100 million years, are amongst
the most conservative known enzymes (see Table 4.12, p.161); the invertebrate aldolases, with
4 % substitutions per 100 million years, have
about the same rate of evolution as have the other
glycolysis enzymes [62]. The sequence similarity
between the aldolases of Drosophila melanogaster and the rabbit is 71 %, and between the aldolase of Trypanosoma brucei and the vertebrate
and insect enzymes about 45-48 % [148, 267].
The close structural relationship between all aldolases is confirmed in a most impressive way by the
combination of aldolase subunits from very different origins into functioning hybrid tetramers, e.g.
type-C subunits from chicken brain will combine
with subunits from lobster muscle or wheatgerm,
but not with those from Ascaris or Loligo. The B
subunits from rat liver do not form functional
hybrid molecules with subunits from lobster or
wheatgerm [249].
Sequence comparisons between A and B subunits of aldolase indicate that the separation of
the isoenzymes is phylogenetically very old; the
human B sequence agrees with that of the rabbit
at 94 % of positions, with the chicken subunit at
81 % of positions, but with rabbit A at only 76 %
of positions [23, 185]. Aldolase C is closer to A
than to B [186]. Spatial structures, albeit with low
resolution, have been published for the enzymes
from human and rat muscle and from Drosophila
[62]. The secondary structure derived from the
amino acid sequence is quite complicated with 12
helical regions and 19 folded-sheet regions [258].
Only one gene for chicken B is present per haploid genome; this consists of eight exons, seven of
which are in the coding region, and one very long
(4.6-kb) intron in the 5'-NT (non-translated)
region [23]. It has been demonstrated for some
aldolase genes that several mRNAs with the same
coding sequence, but with different lengths of the
NT sequence, are produced from the same gene
by alternative splicing. The mRNA for rat A is
longer in brain than in muscle, and in human liver
there are at least four different B mRNAs
[171,213,259].
Two classes of aldolases can be distinguished
from among all organisms. The class-I aldolases,
typical of animals and higher plants, produce a
Schiff base from the substrate; the inhibitor
sodium borohydride, which reacts with all such
compounds, is effective on all enzymes of this
class. The aldolases of class II differ from those of
class I in size (70 compared with 160 kDa); they
are metalloenzymes and are inhibited by ethylenediaminetetraacetic acid (EDTA). Most bacteria, lower fungi and algae contain only enzymes
of class II, although some bacteria and algae possess both classes. The distribution of the two
classes amongst the protozoans appears to follow
no particular rule; Tetrahymena, Leishmania, Trypanosoma and Eimeria have class-I aldolases,
whereas Trichomonas and Entamoeba have
class II [163].
14.6 Glyceraldehyde Phosphate
Dehydrogenases
The glyceraldehyde phosphate dehydrogenases
(GAPDHs) catalyse a complex reaction in which
glyceraldehyde-3-phosphate is oxidized to 3phosphoglycerate, NAD+ is reduced, and ATP is
formed from Pi and ADP. This enzyme makes up
5 to 10 % of the total protein in muscle. The
GAPDHs from bacteria to man are all homotetramers. The subunits of eukaryotic GAPDH usually consist of 328-334 amino acids, corresponding to a mass of 37-38 kDa; the GAPDH of the
nematode Caenorhabditis elegans is somewhat
longer (340 amino acids). The polypeptide chain
consists of an NAD-binding domain, which is
14 Glycolysis
Table 14.3. Maximal activities of various aldolases on
fructose-1,6-bisphosphate compared with fructose-1phosphate [22, 163, 239]
Aldolase
Rabbit
Type A (muscle)
Type B (liver)
Type C (brain)
Rat muscle
Trypanosoma sp.
Eimeria stiedai
Helix pomatia
Drosophila melanogaster pupae
Schistocerca gregaria flight muscle
Ratio
50
1
10
100
100
21
20
10-15
37
in vivo for the switch in energy-yielding metabolism from carbohydrates to fatty acid oxidation
[239].
The similarity in functional properties between
these enzymes is based on a surprisingly high
degree of structural similarity. The aldolase subunits are comprised of 360-363 amino acids. The
sequences are known for all three isoenzymes of
the rat and for aldolases of man and other mammals, chicken, Drosophila melanogaster, Trypanosoma brucei and maize [148, 172, 267]. The
aldolases of mammalian muscle, with only 2 %
substitutions per 100 million years, are amongst
the most conservative known enzymes (see Table 4.12, p.161); the invertebrate aldolases, with
4 % substitutions per 100 million years, have
about the same rate of evolution as have the other
glycolysis enzymes [62]. The sequence similarity
between the aldolases of Drosophila melanogaster and the rabbit is 71 %, and between the aldolase of Trypanosoma brucei and the vertebrate
and insect enzymes about 45-48 % [148, 267].
The close structural relationship between all aldolases is confirmed in a most impressive way by the
combination of aldolase subunits from very different origins into functioning hybrid tetramers, e.g.
type-C subunits from chicken brain will combine
with subunits from lobster muscle or wheatgerm,
but not with those from Ascaris or Loligo. The B
subunits from rat liver do not form functional
hybrid molecules with subunits from lobster or
wheatgerm [249].
Sequence comparisons between A and B subunits of aldolase indicate that the separation of
the isoenzymes is phylogenetically very old; the
human B sequence agrees with that of the rabbit
at 94 % of positions, with the chicken subunit at
81 % of positions, but with rabbit A at only 76 %
of positions [23, 185]. Aldolase C is closer to A
than to B [186]. Spatial structures, albeit with low
resolution, have been published for the enzymes
from human and rat muscle and from Drosophila
[62]. The secondary structure derived from the
amino acid sequence is quite complicated with 12
helical regions and 19 folded-sheet regions [258].
Only one gene for chicken B is present per haploid genome; this consists of eight exons, seven of
which are in the coding region, and one very long
(4.6-kb) intron in the 5'-NT (non-translated)
region [23]. It has been demonstrated for some
aldolase genes that several mRNAs with the same
coding sequence, but with different lengths of the
NT sequence, are produced from the same gene
by alternative splicing. The mRNA for rat A is
longer in brain than in muscle, and in human liver
there are at least four different B mRNAs
[171,213,259].
Two classes of aldolases can be distinguished
from among all organisms. The class-I aldolases,
typical of animals and higher plants, produce a
Schiff base from the substrate; the inhibitor
sodium borohydride, which reacts with all such
compounds, is effective on all enzymes of this
class. The aldolases of class II differ from those of
class I in size (70 compared with 160 kDa); they
are metalloenzymes and are inhibited by ethylenediaminetetraacetic acid (EDTA). Most bacteria, lower fungi and algae contain only enzymes
of class II, although some bacteria and algae possess both classes. The distribution of the two
classes amongst the protozoans appears to follow
no particular rule; Tetrahymena, Leishmania, Trypanosoma and Eimeria have class-I aldolases,
whereas Trichomonas and Entamoeba have
class II [163].
14.6 Glyceraldehyde Phosphate
Dehydrogenases
The glyceraldehyde phosphate dehydrogenases
(GAPDHs) catalyse a complex reaction in which
glyceraldehyde-3-phosphate is oxidized to 3phosphoglycerate, NAD+ is reduced, and ATP is
formed from Pi and ADP. This enzyme makes up
5 to 10 % of the total protein in muscle. The
GAPDHs from bacteria to man are all homotetramers. The subunits of eukaryotic GAPDH usually consist of 328-334 amino acids, corresponding to a mass of 37-38 kDa; the GAPDH of the
nematode Caenorhabditis elegans is somewhat
longer (340 amino acids). The polypeptide chain
consists of an NAD-binding domain, which is