why Glyoxilate cycle doesn't operate in vertebrates?

Answer 1

The glyoxylate cycle is a metabolic pathway occurring in plants and several microorganisms.

The glyoxylate cycle allows these organisms to use fats for the synthesis of carbohydrates, a task which vertebrates, including humans, cannot perform.

When fatty acids are consumed by vertebrates they are degraded to many copies of a small 2-carbon compound: acetate. Acetate binds to coenzyme A and enters the citric acid cycle, where it is fully oxidized to carbon dioxide, which is released into the environment. This pathway allows the cell to obtain energy from fat.

Pyruvate is the initial compound in gluconeogenesis. It is converted to oxaloacetate, which is in turn converted to phosphoenolpyruvate (PEP). Seven further reactions bring about the production of glucose. Oxaloacetate is also the initial and at the same time end product of the citric acid cycle. Since acetate groups can enter the citric acid cycle and eventually be converted to oxaloacetate (which can continue to produce glucose in gluconeogenesis), it may seem that the production of glucose from fatty acids is possible.

However, this does not happen in vertebrates. Acetate groups which enter the citric acid cycle are, as mentioned above, fully oxidized to form carbon dioxide. The acetate is therefore lost and cannot be converted to oxaloacetate, and later on to glucose.

In plants the glyoxylate cycle occurs in special peroxisomes which are called glyoxysomes. Vertebrates cannot perform the cycle because they lack its two key enzymes: isocitrate lyase and malate synthase.

Answer 2

Vertebrates cannot perform the Glyoxylate Cycle because they lack two key enzymes (isocitrate lyase and malate synthase). Vertebrates utilize the Citric Acid Cycle.