Enzymes

Living organisms perform a multitude of chemical reactions very rapidly because of the participation of enzymes. Enzymes are biological catalysts, compounds that speed up chemical reactions without being used up or altered in the reaction. The material with which the catalyst reacts, the substrate, is modified during the reaction to form a new product. Because the enzyme itself emerges from the reaction unchanged, a small amount of enzyme can alter a relatively large amount of substrate. The active site of an enzyme will bind with the substrate, forming an enzyme-substrate complex. It is here that the reaction takes place, and when it is complete, the complex dissociates into the enzyme and a product or products. Enzymes are, in part or in whole, proteins that are highly specific in function. Because enzymes lower the energy of activation needed for reactions to take place, they accelerate the rates of reactions. They do not, however, determine the direction in which a reaction will go or its final equilibrium.

Enzyme activity is affected by many factors. Varying environmental conditions such as temperature and pH may alter the three-dimensional shape of an enzyme, thereby affecting its rate of activity. Similarly, the amount of enzyme relative to the amount of substrate can have an effect on the rate of enzyme activity.

Sugars are a vital source of food for all living organisms but must be broken down to be used. Glycoside hydrolase is a class of enzymes involved in the hydrolysis of the glycosidic linkage/bond of a disaccharide. A hydrolysis reaction is a chemical reaction that utilizes water to break apart molecules. This class of enzyme includes sucrase, maltase, and lactase. The disaccharides sucrose, maltose, and lactose have glycosidic bonds that undergo a hydrolysis reaction with the aid of their respective hydrolase enzyme and water.

Saccharomyces cerevisiae (Domain Eukarya, Kingdom Fungi) is the species name of the yeast used as a leavening agent in baking bread where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. Brewer’s yeast is another strain of S. cerevisiae commonly used in alcoholic fermentation (beer and wine). It is a single-cell microorganism found on and around the human body and was domesticated by humans long ago. Yeast cells are unable to utilize all of the sugars equally well but they do synthesize a range of enzymes with some more effective than others. Yeast metabolizes sugars aerobically (in the presence of oxygen) the process is known as respiration; when yeast metabolizes sugars anaerobically (in the absence of oxygen) the process is known as fermentation.

Saccharomyces cerevisiae is glucophilic, meaning glucose will be used at a faster rate than fructose. For example, sucrose is a disaccharide, and yeast cells use the enzyme sucrase/invertase to break it into glucose and fructose. Finally, it uses glucose in respiration or fermentation. Depending on the strain of S. cerevisiae or the conditions of yeast incubation, the yeast may not produce enough maltase enzyme to split maltose in the time frame we are running the experiments. We generally see (but not always), (1 being the sugar that yeast utilize at the greatest rate):

1. Sucrose - disaccharide = glucose + fructose; “table sugar” (sucrase/invertase)
2. Glucose-hexose monosaccharide is found in all living cells and is often referred to as “blood sugar”
3. Maltose - disaccharide = α-D-glucose + α-D-glucose “malt sugar” (maltase)
4. Fructose - hexose monosaccharide “fruit sugar” (isomerase to convert to glucose)
5. Galactose - hexose monosaccharide (five different types of enzymes)
6. Lactose - disaccharide = β-D galactose + β-D glucose “milk sugar” (lactase)