Question

Mention the function of leg-hemoglobin.

Answer 1

Hint: Leg-hemoglobin is a very necessary compound for the leguminous plants in which nitrogen-fixing occurs. The plants with colonies of nitrogen fixers in their root nodule can produce this hemoprotein.

Complete answer:
If more than necessary oxygen is present in the root nodules of the leguminous plants, it may reduce the activity of Nitrogenase as it is oxygen sensitive. Leg-hemoglobin plays an important role in it. It buffers the oxygen concentration in the cytoplasm to ensure a good function of the nitrogen-fixing system.
On the other hand, nitrogen fixation is actually a very energy-consuming process. Hence, a good supply of oxygen is needed in order to carry out enough aerobic respiration.
Therefore, this hemoprotein called leg-hemoglobin has the capability of maintaining the oxygen concentration in the nodules such that it is low enough not to interfere in the nitrogen fixation and high enough to carry out aerobic respiration.

Additional Information: Only those leguminous plants whose roots are colonized by the nitrogen-fixing bacteria can produce leghemoglobin which is a hemoprotein oxygen carrier. It is a part of the symbiotic interaction of Rhizobia and the leguminous plants, without this synthesis of leg-hemoglobin is impossible. This hemoprotein is very similar to hemoglobin and also has a red color, therefore it was believed that the bacterial symbiont in the symbiotic root nodules provided it. But later on, studies showed that the host i.e. the plant expresses heme biosynthesis genes within nodules and the activation of these genes brings out the expression of hemoglobin genes in the root nodules.
The structure of the leg-hemoglobin:
-Monomeric proteins
-Mass around 16kDa
-One heme with iron and one polypeptide chain i.e. the globin
-Similar to animal hemoglobin
-The protein sequence is different from animal hemoglobin at about 80% of the position.

Note: The function of leg-hemoglobin in root nodules of leguminous plants has been a topic of debate among different scientists. The study in 1995 showed that the main function of these oxygen carriers is to scavenge and deliver the limited free oxygen in the cell to the mitochondria for respiration. But later in 2005, scientists believed that the main function of leg-hemoglobin is both buffering of oxygen concentration and delivery of oxygen to mitochondria. The studies also showed that the leg-hemoglobin decreases the free oxygen concentration in the root nodules.