Higher Plant Respiration and Its Relationships to Photosynthesis
85
Wiskich et al. 1990). Thus, the TCA cycle can continue to operate during
glycine decarboxylation, although perhaps at a reduced rate.
Cytosolic ATP: ADP declines if photo respiration is slowed, indicating
that photo respiration contributes to A TP production (Gardestr6m 1987;
Gardestrom and Wigge 1988). This implicates the mitochondrial respiratory
chain as one mechanism of photo respiratory NADH oxidation. Photorespiratory NADH production may exceed the capacity of the respiratory chain
to oxidize it, however, in which case other means of oxidizing NADH, such
as substrate shuttles, are also required (Dry et al. 1987).
4.4.2 Oxidation of Photorespiratory NADH via Substrate Shuttles
If carbon is conserved in photorespiration - i.e., 3/4 of it - so that glycerate
is formed from hydroxypyruvate in peroxisomes and then transported to
chloroplasts, an amount of NADH equal to the amount formed in mitochondria by glycine decarboxylase is required in peroxisomes. Carbon conservation need not, however, be complete (Grodzinski 1992) as serine and
related compounds may be exported from leaves and this can increase with
low CO2 (high photorespiration). Nonetheless, NADH is required in peroxisomes when glycerate is being formed and it might then be beneficial to
transfer redox equivalents from mitochondria to peroxisomes. The oxidation
of NADH by malate dehydrogenase (MDH; OAA ~ malate) coupled to a
malate/OAA shuttle is one mechanism for this transfer (Fig. 4.6). Dry et al.
(1987) concluded that such a shuttle is "likely to operate in vivo as an
adjunct to the NADH reoxidation capacity of the mitochondrial respiratory
chain." NADH oxidation by MDH requires a supply of OAA whereas
OAA is also required for activity of the TCA cycle and is formed in the
TCA cycle from malate. Plant mitochondria can import OAA via a translocator with a high affinity for cytosolic OAA (Ebbighausen et al. 1985) and
are able to export malate (Douce 1985).
Oxidation of NADH by MDH and activity of a malate/OAA shuttle
might seem impossible if the TeA cycle is engaged because MDH would be
catalyzing OAA ~ malate and malate ~ OAA reactions simultaneously.
One population of mitochondria supporting photo respiration and another
the TCA cycle would allow this, but experimental data do not support this
notion (Wiskich et al. 1990). Another possibility, and one supported by
experimental evidence (Dry and Wi skich 1985; Wiskich et al. 1990), is
that within individual mitochondria, glycine decarboxylase and malate and
OAA transporters are spatially separated from TCA cycle enzymes. This
separation might be facilitated by enzyme or enzyme complex "attachment"
to specific locations on the matrix side of the inner mitochondrial membrane.
To the extent that photo respiratory NADH is oxidized by the respiratory
chain and glycerate is formed from glycine, NADH required in peroxisomes
must come from nonmitochondrial sources. These sources can include the
85
Wiskich et al. 1990). Thus, the TCA cycle can continue to operate during
glycine decarboxylation, although perhaps at a reduced rate.
Cytosolic ATP: ADP declines if photo respiration is slowed, indicating
that photo respiration contributes to A TP production (Gardestr6m 1987;
Gardestrom and Wigge 1988). This implicates the mitochondrial respiratory
chain as one mechanism of photo respiratory NADH oxidation. Photorespiratory NADH production may exceed the capacity of the respiratory chain
to oxidize it, however, in which case other means of oxidizing NADH, such
as substrate shuttles, are also required (Dry et al. 1987).
4.4.2 Oxidation of Photorespiratory NADH via Substrate Shuttles
If carbon is conserved in photorespiration - i.e., 3/4 of it - so that glycerate
is formed from hydroxypyruvate in peroxisomes and then transported to
chloroplasts, an amount of NADH equal to the amount formed in mitochondria by glycine decarboxylase is required in peroxisomes. Carbon conservation need not, however, be complete (Grodzinski 1992) as serine and
related compounds may be exported from leaves and this can increase with
low CO2 (high photorespiration). Nonetheless, NADH is required in peroxisomes when glycerate is being formed and it might then be beneficial to
transfer redox equivalents from mitochondria to peroxisomes. The oxidation
of NADH by malate dehydrogenase (MDH; OAA ~ malate) coupled to a
malate/OAA shuttle is one mechanism for this transfer (Fig. 4.6). Dry et al.
(1987) concluded that such a shuttle is "likely to operate in vivo as an
adjunct to the NADH reoxidation capacity of the mitochondrial respiratory
chain." NADH oxidation by MDH requires a supply of OAA whereas
OAA is also required for activity of the TCA cycle and is formed in the
TCA cycle from malate. Plant mitochondria can import OAA via a translocator with a high affinity for cytosolic OAA (Ebbighausen et al. 1985) and
are able to export malate (Douce 1985).
Oxidation of NADH by MDH and activity of a malate/OAA shuttle
might seem impossible if the TeA cycle is engaged because MDH would be
catalyzing OAA ~ malate and malate ~ OAA reactions simultaneously.
One population of mitochondria supporting photo respiration and another
the TCA cycle would allow this, but experimental data do not support this
notion (Wiskich et al. 1990). Another possibility, and one supported by
experimental evidence (Dry and Wi skich 1985; Wiskich et al. 1990), is
that within individual mitochondria, glycine decarboxylase and malate and
OAA transporters are spatially separated from TCA cycle enzymes. This
separation might be facilitated by enzyme or enzyme complex "attachment"
to specific locations on the matrix side of the inner mitochondrial membrane.
To the extent that photo respiratory NADH is oxidized by the respiratory
chain and glycerate is formed from glycine, NADH required in peroxisomes
must come from nonmitochondrial sources. These sources can include the