Catalase is a common enzyme found in nearly all forms of life exposed to oxygen, including animals, plants, and microorganisms. It is classified as a hemoprotein because each active unit contains four iron-containing heme groups necessary for its function. The primary biological role of catalase is to provide cellular protection against the harmful byproducts of oxygen metabolism.
The Specific Chemical Reaction
Metabolic processes that utilize oxygen inevitably generate toxic side products, particularly hydrogen peroxide (\(\text{H}_2\text{O}_2\)). This molecule is a Reactive Oxygen Species (ROS) that can inflict significant oxidative damage on cellular components, including DNA, proteins, and lipids, if not quickly neutralized.
Catalase functions by rapidly converting this harmful hydrogen peroxide into two relatively benign substances: water and gaseous oxygen. The chemical equation for this detoxification reaction is \(2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2\).
The efficiency of this process is high, as catalase possesses one of the highest turnover rates recorded for any enzyme. A single molecule of catalase can process and convert millions of hydrogen peroxide molecules every second, with reported turnover frequencies reaching up to \(4 \times 10^7\) reactions per second.
Catalase and Oxygen Tolerance in Microbes
The production of catalase is directly linked to a microbe’s ability to survive in the presence of oxygen, making it a marker for certain metabolic lifestyles. Obligate aerobes, which rely solely on oxygen for growth, and facultative anaerobes, which can grow with or without oxygen, must produce catalase to neutralize the \(\text{H}_2\text{O}_2\) they generate. Many medically relevant bacteria, such as Staphylococcus species and most Gram-negative rods, fall into these catalase-positive categories. The enzyme allows these microbes to detoxify hydrogen peroxide, which is either produced internally or encountered in the environment, such as when targeted by the host immune system.
Conversely, obligate anaerobes, which are poisoned by oxygen, typically do not produce catalase. They often use alternative enzymes, like superoxide reductase or various peroxidases, that degrade toxic oxygen species without generating oxygen as a byproduct. For instance, species within the genera Streptococcus and Clostridium are generally catalase-negative. While some aerotolerant anaerobes can survive in oxygen, they usually rely on other detoxification enzymes, which is why they also test negative for catalase.
The Catalase Test for Microbial Differentiation
The functional difference in catalase production among bacteria forms the basis of a simple and widely used laboratory technique called the catalase test. The procedure involves placing a small sample of a bacterial colony onto a microscope slide and then adding a drop of 3% hydrogen peroxide solution.
If the organism possesses the catalase enzyme, the hydrogen peroxide is instantly broken down into water and oxygen gas. This release of oxygen is visibly apparent as the rapid formation of bubbles or frothing at the site of the reaction, indicating a positive result. If the organism is catalase-negative, no bubbles are produced when the hydrogen peroxide is added.
This quick visual assessment is routinely used to distinguish between two large groups of Gram-positive cocci that are morphologically similar under a microscope. For example, Staphylococcus species, which are catalase-positive, are readily separated from Streptococcus and Enterococcus species, which lack the enzyme and are catalase-negative. The test is also valuable for distinguishing the spore-forming, catalase-positive Bacillus species from the spore-forming, catalase-negative Clostridium species.