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What are 'Enzymes'?

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Introduction Enzymes are biological catalysts that carry out thousands of chemical reactions that occur in living cells. They are a class of proteins that have a unique three dimensional structure that allows it to bind with a specific substrate to facilitate a reaction. Many biological reactions will not occur spontaneously in the cell; there is simply not enough energy for the reaction to take place. Enzymes make these reactions possible by lowering the reaction's activation energy. Each cell has tens of thousands of different enzymes that collectively allow both the break down and synthesis of molecules to drive all cellular processes. This investigation will explore the effect of pH on the three-dimensional structure of a protein. Much of the three-dimensional structure of an enzyme is held together by weak interactions including H-bonds, ionic bonds, and hydrophobic interactions. These interactions can be easily disrupted by changes in temperature, salt concentration, and pH. pH levels out of the normal intracellular range would denature enzymes, slowing the enzyme's reaction rates. Hydrogen peroxide (H2O2) is a toxic chemical that is continually being formed as By product of reactions in peroxisomes of living cells. Since it is poisonous, the cells must either get rid of it or change it to something nonpoisonous. ...read more.


Prokaryotes, organisms like bacteria that lack a nuclear membrane, also lack membrane bound organelles such as peroxisomes. Antioxidant enzymes like catalase and superoxide dismutase are located in the periplasmic space which is the space between the inner and outer membranes of the cell wall. There are numerous enzymes located here that would be toxic if they were found inside the cell. The catalase found here can act on toxic molecules that are transported to the periplasm or the enzyme can be released outside the bacterial wall where it can act on toxic molecules in the environment. Catalase that is released by the bacteria plays a role in protecting the bacteria from being destroyed by white blood cells during an infection. What does Catalase look like? Each molecule of catalase is a tetramer of four polypeptide chains. Each chain is composed of more than 500 amino acids. Located within this tetramer are four porphyrin heme groups that are very much like the familiar hemoglobins, cytochromes, chlorophylls and nitrogen-fixing enzymes in legumes. The heme group is responsible for catalase's enzymatic activity. Catalase has one of the highest turnover rates for all enzymes: one molecule of catalase can convert 6 million molecules of hydrogen peroxide to water and oxygen each minute. ...read more.


ENZYMES affects the three-dimensional structure of all enzymes. Enzymes are made up of amino acids. Each amino acid has a -NH2 group and a -COOH group, not to mention certain amino acids that have an extra -COOH group (e.g. aspartate) or an extra -NH3+group (e.g. asparagine). pH is all about concentration of H+ ions. At low pH and high H+ concentration the predominant forms of these groups will be -COOH and -NH3+ or the "protonated forms". At neutral pH the predominant forms will be -COO and -NH3+. At high pH the predominant forms will be -COO- and -NH2. However the actual pH at which each group becomes ionised depends on the particular amino acid and also the environment in which the enzyme is found. The usual way of expressing this is the pK value: this pK is the pH at which half of the groups are ionised. Interactions between these positive and negative charges are a very important part of what holds the structure together in an enzyme. These links are known as salt links, salt bridges or electrostatic interactions and involve a + to - attraction. Changing the pH therefore alters the properties of these salt bridges. Even a small shift away from optimum pH might mean one of these salt bridges is affected and therefore the shape and activity and stability of the protein will also be affected. ...read more.

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