The Glyoxylate Cycle: A Related
In plants and in some bacteria, but not in animals, acetyl-CoA can serve as the starting material for the biosynthesis of carbohydrates. Animals can convert carbohydrates to fats, but not fats to carbohydrates. (Acetyl-CoA is produced in the catabolism of fatty acids.) Two enzymes are responsible for the ability of plants and bacteria to produce glucose from fatty acids. Isocitrate lyase cleaves isocitrate, producing glyoxylate and succinate. Malate synthase catalyzes the reaction of glyoxylate with acetyl-CoA to produce malate.
These two reactions in succession bypass the two oxidative decarboxylation steps of the citric acid cycle. The net result is an alternative pathway, the gly-oxylate cycle (Figure 19.9). Two molecules of acetyl-CoA enter the glyoxylatecycle; they give rise to one molecule of malate and eventually to one molecule of oxaloacetate. Two two-carbon units (the acetyl groups of acetyl-CoA) give rise to a four-carbon unit (malate), which is then converted to oxaloacetate (also a four-carbon compound). Glucose can then be produced from oxalo-acetate by gluconeogenesis. This is a subtle, yet very important, distinction between the glyoxylate cycle and the citric acid cycle.
The carbon skeletons that enter the citric acid cycle as acetyl-CoA are effectively lost
by the decar-boxylation steps. This means that if oxaloacetate (OAA) is drawn
off to make glucose, there will be no OAA to continue the cycle. For this
reason, fats cannot lead to a net
production of glucose. With the glyoxylate cycle, the bypass reac-tions go
around the decarboxylations, creating an extra
four-carbon compound that can be drawn off to make glucose without depleting
the citric acid cycle of its starting compound.
Specialized organelles in plants, called glyoxysomes, are the sites of the glyoxylate cycle. This pathway is
particularly important in germinating seeds. The fatty acids stored in the
seeds are broken down for energy during germi-nation. First, the fatty acids
give rise to acetyl-CoA, which can enter the citric acid cycle and go on to
release energy in the ways we have already seen. The citric acid cycle and the
glyoxylate cycle can operate simultaneously. Acetyl-CoA also serves as the
starting point for the synthesis of glucose and any other compounds needed by
the growing seedling. (Recall that carbohydrates play an important structural,
as well as energy-producing, role in plants.)
The glyoxylate cycle also occurs in bacteria. This point is far
from surprising because many types of bacteria can live on very limited carbon
sources. They have metabolic pathways that can produce all the biomolecules
they need from quite simple molecules. The glyoxylate cycle is one example of
how bacteria manage this feat.
In plants and bacteria, the glyoxylate cycle is a pathway that bypasses
the two oxidative decarboxylations of the citric acid cycle. As a result of
this pathway, plants can convert acetyl-CoA to carbohydrates, which animals