In sugarcane plants, the process of carbon dioxide fixation occurs through the C4 pathway. The C4 pathway is an adaptation mechanism in plants that allows them to efficiently fix carbon dioxide in conditions of high temperature and low carbon dioxide levels.

The enzyme that fixes CO2 in malic acid in the sugarcane plant is Phosphoenol pyruvic acid carboxylase (PEP carboxylase).

Here is a detailed explanation:

1. The C4 pathway:
- Sugarcane is a C4 plant, which means it has evolved a specialized mechanism to enhance its photosynthetic efficiency.
- In the C4 pathway, carbon dioxide is initially fixed into a four-carbon compound called oxaloacetate, which is then converted to malic acid or another four-carbon compound.
- This initial fixation of CO2 occurs in the mesophyll cells of the leaf.

2. Role of Phosphoenol pyruvic acid carboxylase (PEP carboxylase):
- PEP carboxylase is an enzyme that catalyzes the fixation of CO2 into oxaloacetate during the C4 pathway.
- This enzyme has a higher affinity for carbon dioxide than the enzyme involved in the Calvin cycle, ribulose biphosphate carboxylase (Rubisco).
- PEP carboxylase has a higher affinity for CO2 because it does not have oxygenase activity, which means it does not fix oxygen instead of carbon dioxide like Rubisco.
- It is located in the mesophyll cells of the leaf, where it fixes CO2 into oxaloacetate.

3. Formation of malic acid:
- After CO2 is fixed into oxaloacetate by PEP carboxylase, it is converted into malic acid or another four-carbon compound.
- This conversion occurs in the mesophyll cells and involves various enzymatic reactions.

4. Role of malic acid:
- Malic acid is a stable organic acid that can be transported to the bundle sheath cells, where the Calvin cycle occurs.
- In the bundle sheath cells, malic acid releases CO2, which is then fixed by Rubisco and enters the Calvin cycle for further carbon fixation and glucose production.

In conclusion, the enzyme that fixes CO2 in malic acid in sugarcane plants is Phosphoenol pyruvic acid carboxylase (PEP carboxylase). It plays a crucial role in the C4 pathway by catalyzing the initial fixation of CO2 into oxaloacetate, which is then converted into malic acid. This mechanism allows sugarcane plants to efficiently fix carbon dioxide and enhance their photosynthetic efficiency.