The catalase enzyme is one of the most efficient enzymes known, since each enzyme can perform nearly 800,000 catalytic events per second. The key catalase function is protecting cells from hydrogen peroxide (H2O2) molecules by converting them to oxygen (O2) and water (H2O). H2O2 can damage DNA. Catalase is formed by four individual parts, or monomers, that wrap into a dumbbell-shaped enzyme. Each monomer has a catalytic center that contains a heme molecule, which binds oxygen. Each monomer also binds a molecule of NADPH, which protects the enzyme itself from the damaging effects of H2O2. Catalase works best at a pH of 7, and is highly abundant in peroxisomes, which are the pouches inside a cell that break down toxic molecules.
Catalase Structure: All Four One, And One For All
Catalase is a four-part enzyme, or a tetramer. Four monomers wrap around each other to form a dumbbell-shaped enzyme. Each monomer has four domains, or parts -- like body parts that do different things. The second domain is the one that contains the heme group. The third domain is the known as the wrapping domain, which is where the four monomers wrap around each other to form a tetramer. Many salt bridges, or ionic interactions between positively and negatively charged amino acid side chains, hold the four monomers together. The monomers weave around each other, making the tetramer enzyme very stable.
It Carries Tools
Each monomer of the catalase tetramer contains one heme group. Heme groups are disk-shaped molecules that have an iron atom at the center, which binds oxygen. The heme is buried in the middle of the catalytic domain of each monomer. Each catalase monomer also binds an NADPH molecule, but at its surface. The NADPH is there to protect the enzyme from the H2O2 that it must catalyze. An H2O2 molecule can become a superoxide molecule, which is two oxygen atoms bound to each other, with one of them having an extra electron that is highly reactive -- meaning it can interact with the electrons in chemical bonds on other molecules and break those bonds.
It's That Fast
Oxygen radicals, such as H2O2, are produced by normal cellular processes. Since they are dangerous to the cell, they must be converted into benign molecules. Catalase is one of the fastest enzymes known. Each monomer in the catalase tetramer can perform nearly 200,000 catalytic events per second. Since a tetramer has four monomers, each catalase enzyme can do nearly 800,000 catalytic events per second. Catalase needs this level of efficiency because H2O2 is dangerous to the cell. Catalase enzymes accumulate in pouches called peroxisomes within a cell. Peroxisomes are vesicles that degrade molecules that are toxic to the cell, including oxygen radicals such as H2O2.
Researchers have studied the activity of catalase at a pH of 7.4 and at 25 degrees Celsius (77 degrees Fahrenheit). The optimal pH for the catalase reaction is around 7, so one way that researchers stop catalase activity in a test tube is to change the pH by adding a strong acid or strong base. Within the cell, catalase accumulates in peroxisomes, which have varying pHs when measured in different cells. The journal “IUBMB Life” reported that peroxisomes have been found to have pHs that range from 5.8-6.0, 6.9-7.1, and 8.2. Thus, different peroxisomes may contain different amounts of catalase, or may turn the catalase on or off depending on how they regulate their internal pH level.
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