Learning Objectives

· Explain the role of enzymes in digestion of macronutrients
· Diagram the reactants and products of a chemical reaction in nutrition
· Distinguish between enzymes, substrates and reaction products
· List factors that impact the rate of a reaction
· Test how conditions (pH, temperature, enzyme concentration) impact enzyme activity (enzyme rate)
· Explain the unit measurement parts per thousand (ppt)

Enzymes, Digestion, and Nutrition

Why do we eat? Answers to ‘Why’ questions can be two types: ultimate and proximate. If the answer to “Why do we eat” is “Because I’m hungry” or “Because I enjoy eating,” then the answer is a proximate type. On the other hand, if the answer is “Because metabolism of food is necessary to supply energy to the body and provide basic building material for DNA, proteins, and other biomolecules to keep us alive as a species” then the answer is an ultimate type. An answer of the ultimate type supplies an overarching explanation for why eating is necessary. That is, we eat to take in food stuff that will be broken down (catabolized) into smaller substances that can (1) be used as energy to power internal functions and engage in survival activities and (2) provide biomolecules for growth (anabolism) and health maintenance. The processes of catabolism and anabolism are complementary components of metabolism and are dependent on the function of naturally occurring proteins called enzymes. In fact, production of protein enzymes is also dependent on enzymes in these same processes of digestion, catabolism, and anabolism. In addition to digestion, enzymes participate in many other biological processes related to nutrition. For example, the liver is responsible for many life-sustaining enzymatic reactions that ensure that the body operates within normal homeostatic internal conditions. The liver is a clearinghouse for toxic breakdown products of digestion such as excess amino acids. Another example is the role of many vitamins as coenzymes in various biochemical reactions throughout the body. Essential vitamins are consumed as part of the diet and participate in reactions that support the nervous system, enhance the immune system, prevent anemia, and much more.

Chemical Reactions in Nutrition

Chemical reactions are important to biological functions. Consider natural functions such as the breakdown of food in the digestive system, conversion of sunlight into oxygen and cellulose in plant leaves, or conversion of light energy into a biological signal that can be interpreted by the nervous system. Chemical reactions require two or more chemicals to interact in a way that modifies chemical bonds within each of the chemicals and result in the formation of new chemicals (reaction products). The chemicals that interact as inputs of a reaction are called reactants.
Chemical reactions occur best under conditions that are optimized for each reaction. Optimization of the reaction can be influenced by several factors: (1) the interaction of a specific amount of each of the reactants (concentration); (2) the presence and concentration of an enzyme (a naturally-occurring protein that alters the rate of the reaction); (3) the temperature of the environment in which the reaction takes place; (4) salinity (saltiness due to dissolved ions) of the solution in which the reaction takes place; and (5) the pH of the solution in which the reaction takes place.
These five factors are commonly at play in the digestive system. We drink strong coffee (factor 1), consume whole, nutrient-dense foods as well as isolated extractions such as vitamins (factor 2), consume hot and cold foods (factor 3), enjoy a variety of seasonings including salt (factor 4), and use acidic condiments like vinegar and catsup (factor 5). Our digestive system must process macronutrients taken in under a variety of conditions and in a variety of forms. Protein enzymes have the task to digest foods and drinks to convert them into a form that the body can use. Specific digestive enzymes are needed to carry out catabolism which starts in the mouth and continues throughout the alimentary tract.
Distinct categories of digestive enzymes are specific to each type of macronutrient. Carbohydrates such as whole vegetables, whole fruits, and grains are processed (catabolized) by carbohydrases. (The ‘-ase’ at the end of the term signifies an enzyme.) Meat, nuts, beans, and other protein sources are catabolized with assistance from enzymes called proteinases. Examples of proteinases are pepsin, trypsin, and peptidase. Proteins can be further broken down by nucleases to yield the basic building blocks for DNA and RNA. Fatty foods like bacon, butter, cashew nuts, coconut, or mackerel are digested through activity of enzymes called lipases. Enzymatic activity is, therefore, the key to survival of all living organisms through the life-giving mechanism of nutrient metabolism.
Reactants(enzyme )−→Reaction Products

Enzymes are proteins that fold into very specific shapes in order to interact with macromolecules like carbohydrates, proteins, and lipids (fats). Enzymes catalyze biochemical reactions which means that they assist in assuring that the reaction takes place in an energy-efficient manner.
Another term for reactants of a reaction in the life sciences is substrates. This term is specifically used for biological reactions that involve enzymes as catalysts, as shown in the image below. Recall that enzymes are proteins and, therefore, can be much larger than substrates which can be simple molecules.

Substrates could be complex macromolecules or simple molecules like oxygen. For example, the substrate disaccharide sucrose can be broken down enzymatically into the monosaccharides fructose and glucose.

Solutions and Concentration

Living organisms such as bacteria, fungi, squirrels, and humans are composed of water, and most biological reactions occur in the aqueous solution of our cells. As noted above, the concentration of enzymes and substrates can influence the rate and efficiency of a chemical reaction.
Solutions consist of two constituents: Solute and solvent. The solute represents substances present in lesser amount dissolved in a solvent which is present in a greater amount. The ratio of these amounts is directly related to the concentration of a solution. By definition, the concentration of a solution can be regarded as the amount of solute in a solution.

Concentration =# particles of solute

volume of solvent

Concentration can be measured in any units that convey information about the ratio of the solutes to the entire solution. Unit examples are: Molarity, molality, normality, mass percent, volume percent, or parts per thousand/million/billion (ppt/ppm/ppb).
We are most familiar with the unit of percent but the other unit options to be used in this laboratory experiment are straightforward to understand. For example, 1% is 1/100 whereas 1 ppt is 1/1,000. We can also convert from one unit to another. Since 1/100 is ten times more than 1/1,000, then 1% converted to ppt is 10 ppt.

Rate of Reaction in Biology

As noted above, reactions are influenced by several factors. For example, enzyme concentration and substrate concentration both affect the amount of reaction products created in a given amount of time.
What is a Rate?
Reaction rate can be understood as the change in amount over time. Higher rate means greater change per unit time.
Rate(enzyme )= change

time interval
Which solution shown below is likely to result in the most reaction product in a given amount of time? Which solution shown below is likely to result in the highest production rate of reaction product (recall factor 1 mentioned above)?

Solutions with a high concentration of substrate will produce more reaction product in the presence of ample amounts of enzyme and they will produce reaction product at a higher rate.
Likewise, consider the impact of different enzyme concentrations (factor 2) as shown in the solutions below. Which solution shown below is likely to result in the most reaction product in a given amount of time?

Solutions with a high enzyme concentration will produce more reaction product in a given amount of time than solutions with a low enzyme concentration.
Other factors that affect enzyme reaction rate (enzyme activity)
In most chemical reactions, increases in temperature (factor 3) lead to increases in the reaction rate. An increase in temperature raises the speed of the reactants in solution and that increases the likelihood of reactants being close enough to react and also that they will have enough energy for the reaction to occur. Connection: Why do we refrigerate milk?
Reactions involving enzymes behave in a similar manner. However, enzymes (and all proteins) are sensitive to high temperatures and can lose their shape and functionality. This process is called denaturation. Connection: Why do egg whites change when cooked?
Likewise, enzymes are affected by the pH of the solution (factor 5) and by any other ions present in the solution. Just as high temperatures can affect the shape and functionality of an enzyme, so too can the concentration of charged ions in solution (pH of a solution is related to its H+ ion concentration). Most enzymes function properly only within a narrow range of pH. For example, gastric enzymes released by cells in the acidic environment of the stomach (pH = 2.0) do not function optimally in the physiological pH environment (pH = 7.4) of the small intestine (ileum).1

Orientation to the Model Enzyme System

You will gather data from an enzyme-catalyzed reaction that breaks down hydrogen peroxide into water and oxygen gas. In living organisms, the breakdown of hydrogen peroxide is critical because high concentrations of peroxide can be toxic in the body and blood. Catabolism of hydrogen peroxide involves the enzyme catalase and takes place largely in the liver, corroborating the role of the liver as an organ that has an important role in detoxifying the body.
hydrogen peroxide(enzyme )−→water + oxygen
By measuring the changes on oxygen gas concentration over a specific period of time we can determine the rate at which this reaction occurs.
In this set of laboratory exercises, you will study the effect of three different factors on enzyme reaction rate.
Procedure I Overview
Enzyme Reaction Rate ‐ Concentration Dependence (factor 1): You will explore how enzyme concentration affects enzyme reaction rate.
Procedure II Overview
Enzyme Reaction Rate ‐ Temperature Dependence (factor 3): You will explore how temperature affects enzyme reaction rate.
Procedure III Overview
Enzyme Reaction Rate ‐ pH Dependence (factor 5): You will explore how pH affects enzyme reaction rate.

Summary of Formulas Needed for Calculations

Enzyme Reaction: the enzyme decomposes hydrogen peroxide into water and oxygen gas
hydrogen peroxide (enzyme)−→water + oxygen Gas
As a balanced chemical equation this is written as shown below.
Determine the rate of change (or simply the rate) using timed data.
rate=changetime intervalrate=changetime interval

Example: Enzyme reaction rate = O2 rate
Enzyme reaction rate=O2rate=change in oxygen concentrationtime intervalEnzyme reaction rate=O2rate=change in oxygen concentrationtime interval

Sample Calculation: Determine the enzyme reaction rate if the oxygen concentration increases by 13.35 ppt during a 30.0 second time interval. Note: ppt stands for parts-per-thousand and is a unit for measuring concentration.
Enzyme reaction rate=13.35ppt30.0s=0.445ppt/s

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