Question

Lactose is a disaccharide sugar present in milk. Lactose is composed of two monosaccharides, galactose and glucose, that are joined together by a $\beta$ -glycosidic bond. The $\beta$ -glycosidic bond forms between the hydroxyl group on $C-1$ of galactose and C-4 of glucose. When lactose is digested, the glycosidic bond between galactose and glucose is hydrolyzed. The enzyme responsible for hydrolyzing lactose is lactase, which is found in the small intestine. Millions of people lack sufficient levels of lactase, and as a result, they experience lactose intolerance. If lactose is not hydrolyzed, it remains in the intestines. Bacteria in the gut ferment the lactose, causing many unpleasant symptoms such as indigestion, cramping, and gas. Fortunately, commercial preparations can enable lactoseintolerant individuals to consume and digest milk products without discomfort. Use the information provided to answer the following questions: a. Draw the structure of lactose. (The structures of galactose and glucose are shown on page 1037 with their carbon numbering scheme.) b. Lactase enzyme activity, like most enzymes, is sensitive to pH. Figure a $\Delta$ illustrates how lactase activity is affected by the pH of a solution. Based on the data in Figure a, what can you conclude about the $\mathrm{pH}$ of the small intestine? c. A glutamic acid in the active site of lactase is suspected to be involved in the catalytic mechanism. Draw the structure of the glutamicacid side chain in the ionization state that likely exists when lactase is most catalytically active.

Lactose is a disaccharide sugar present in milk. Lactose is composed of two monosaccharides, galactose and glucose, that are joined together by a $\beta$ -glycosidic bond. The $\beta$ -glycosidic bond forms between the hydroxyl group on $C-1$ of galactose and C-4 of glucose. When lactose is digested, the glycosidic bond between galactose and glucose is hydrolyzed. The enzyme responsible for hydrolyzing lactose is lactase, which is found in the small intestine. Millions of people lack sufficient levels of lactase, and as a result, they experience lactose intolerance. If lactose is not hydrolyzed, it remains in the intestines. Bacteria in the gut ferment the lactose, causing many unpleasant symptoms such as indigestion, cramping, and gas. Fortunately, commercial preparations can enable lactoseintolerant individuals to consume and digest milk products without discomfort. Use the information provided to answer the following questions: a. Draw the structure of lactose. (The structures of galactose and glucose are shown on page 1037 with their carbon numbering scheme.) b. Lactase enzyme activity, like most enzymes, is sensitive to pH. Figure a $\Delta$ illustrates how lactase activity is affected by the pH of a solution. Based on the data in Figure a, what can you conclude about the $\mathrm{pH}$ of the small intestine? c. A glutamic acid in the active site of lactase is suspected to be involved in the catalytic mechanism. Draw the structure of the glutamicacid side chain in the ionization state that likely exists when lactase is most catalytically active.

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Chemistry A Molecular Approach

Chemistry A Molecular Approach

Nivaldo J. Tro 4th Edition

Chapter 22, Problem 98

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Solved by Expert Kristopher Keyes

07/11/2019

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Lactose is a disaccharide composed of two monosaccharides, galactose and glucose, joined together by a $\beta$ -glycosidic bond. The $\beta$ -glycosidic bond forms between the hydroxyl group on $C-1$ of galactose and $C-4$ of glucose. The structure of galactose Show more…

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Lactose is a disaccharide sugar present in milk. Lactose is composed of two monosaccharides, galactose and glucose, that are joined together by a $\beta$ -glycosidic bond. The $\beta$ -glycosidic bond forms between the hydroxyl group on $C-1$ of galactose and C-4 of glucose. When lactose is digested, the glycosidic bond between galactose and glucose is hydrolyzed. The enzyme responsible for hydrolyzing lactose is lactase, which is found in the small intestine. Millions of people lack sufficient levels of lactase, and as a result, they experience lactose intolerance. If lactose is not hydrolyzed, it remains in the intestines. Bacteria in the gut ferment the lactose, causing many unpleasant symptoms such as indigestion, cramping, and gas. Fortunately, commercial preparations can enable lactoseintolerant individuals to consume and digest milk products without discomfort. Use the information provided to answer the following questions: a. Draw the structure of lactose. (The structures of galactose and glucose are shown on page 1037 with their carbon numbering scheme.) b. Lactase enzyme activity, like most enzymes, is sensitive to pH. Figure a $\Delta$ illustrates how lactase activity is affected by the pH of a solution. Based on the data in Figure a, what can you conclude about the $\mathrm{pH}$ of the small intestine? c. A glutamic acid in the active site of lactase is suspected to be involved in the catalytic mechanism. Draw the structure of the glutamicacid side chain in the ionization state that likely exists when lactase is most catalytically active.

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Key Concepts

Carbohydrate Structure and Glycosidic Bonds

This concept covers the structure of carbohydrates, particularly disaccharides, and the type of bonds that link monosaccharide units. Understanding glycosidic bonds—including the distinction between alpha and beta bonds—is crucial because the configuration determines the molecule’s digestibility and interaction with enzymes. In biochemical contexts, these bonds’ formation and cleavage are central to energy metabolism and molecular recognition processes.

Enzyme Catalysis and pH Sensitivity

Enzyme catalysis involves the acceleration of chemical reactions through the lowering of activation energy by the enzyme’s active site. The activity of enzymes is often dependent on environmental factors such as pH, which can alter the ionization state of active site residues and therefore impact enzyme structure and function. Recognizing the relationship between pH optima and enzyme activity is key to understanding how enzymes perform under physiological conditions.

Amino Acid Ionization States in Enzyme Active Sites

Many enzymatic reactions rely on the specific ionization states of amino acid side chains within the active site, which are influenced by the pH of the environment. This concept explains how residues, such as glutamic acid, can participate in catalysis by donating or accepting protons as needed. The ionization state of these residues is critical for maintaining proper active site geometry and for facilitating the chemical transformations that occur during substrate conversion.

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Transcript

00:02 In question 98 has three parts.

00:06 The first part, we have to draw a structure for lactose.

00:14 Now, lactose has a disaccharide, so we've got two sugar molecules connected.

00:22 Those are going to be galactose and glucose.

00:41 Galactose has the following structure.

00:46 It's a six -membered ring.

00:47 We have our oxygen here, we'll draw our substituents.

01:26 We've got h and oh on most of our carbons.

01:50 Now, normally, the oh would actually be on the bottom here, but as we'll see pretty soon here, that's going to change as we construct the entire molecule.

02:08 So we're going to number these carbons.

02:11 We've got one, two, three, four, five, and six.

02:24 Now, according to the problem, galactose is bonded through c1, bonded to glucose, that is.

02:45 Glucose is bonded through c4, which we'll get too soon, and the type of linkage is a beta glycosidic linkage.

02:55 Because it's a beta glycosidic linkage, the oh on galactose goes up, and then glucose connects through that.

03:23 We're going to draw glucose.

03:27 Now the figure given in the book for glucose is aligned in the same way as galactose.

03:36 The problem is we got to draw c4 on this size.

03:40 So what we're going to do is flip the molecule over.

03:46 So it's going to look like this.

03:56 Oxygen down here.

04:03 Then we'll add a substituence.

04:09 This is flipped.

04:11 The o's and the ohs and hs are going to be.

04:15 Flipped as well.

04:45 And this carbon is carbon four, four, five, six, three, two, one.

04:57 And it's going to be bound to galactose through here...

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