Glucose

Synthesis and Reactions

As you have learned, glucose can be synthesized from water and carbon dioxide by a process known as photosynthesis. However, that's not the only way in which glucose can be created. It can also be synthesized from pyruvic acid and acetyl CoA by reversing the reactions that we studied earlier in the oxidation of glucose. Consequently, glucose can be made from such things as fats, carboxylic acids, and also amino acids by first converting them into acetyl CoA or pyruvic acid. (This diagram is also shown in Example 13 in your workbook.)

Diagram representing the synthesis of glucose by photosynthesis and other methods (via pyruvic acid). [67033.jpg]

In turn, glucose can be used as a starting material to make fats, carboxylic acids and amino acids, and other chemicals.

Glucose can also be converted into a variety of other sugar molecules by doing such things as reorienting the location of hydroxyl groups, such as when it is converted into galactose, or by oxidizing one carbon and reducing another by shifting the locations of the hydrogen atoms, such as when it is converted into fructose.

D- or L- Designation

The bonding pattern of the hydrogens and hydroxyl groups around each carbon atom is very important to the structure of carbohydrates. You should recall that a carbon atom which is bonded to four different groups is an asymmetric carbon atom.

Glucose, with six carbon atoms, has four asymmetric carbon atoms (marked in this diagram with *). The arrangement of the OH's and H's on these atoms is very important. Structural formulas for sugar molecules are often written in this vertical arrangement with the aldehyde or the ketone group at or near the top. When written in this particular way, the position of the OH on the last asymmetric carbon atom will tell us whether we are dealing with a "D" sugar or an "L" sugar. "D" stands for dextro and "L" stands for levo. If the OH is on the right, then we are dealing with a "D" sugar, in this case D-glucose.

Structure of glucose with asymmetric carbon atoms marked with *. [67037.jpg]

Each of these sugars is a "D" sugar because the OH on the last asymmetric carbon atom is written on the right. Therefore, D-galactose, D-glucose, and D-fructose. Had the OH group attached to the last asymmetric carbon atom been on the left, then these would have been "L" sugars. When you see D's and L's in front of the names of carbohydrates, this is the reason for it.

Structures of D-galactose, D-glucose and D-fructose. [67034.jpg]

Left-Right Orientation of OH Groups

When you look at the model for a particular saccharide and compare it to a structural formula, there may not seem to be much relationship as regards the location of the OH's. In part, this is because the structural formula is written in two dimensions and the models are in three dimensions. The formulas have four simple directions - up, down, left, right. With models you have the directions of the tetrahedral arrangements of the bonds around the carbon atoms.

Structure and model of D-glucose. [67036.jpg]

When deciding whether a hydroxyl group is on the right side or the left side of the carbon atom in a model, you must arrange the model in such a way as to have the lower numbered carbon at the top and the next higher numbered carbon below the one that you're looking at and have this carbon closer to you than either the preceding or following carbon. In this diagram the bonds on the second carbon atom are wedged (wide end toward you) to give the impression that the second carbon is closer to you than the first or third.

Structure of D-glucose with orientation around second carbon atom emphasized. [glucos1.jpg]

You can see the same thing here in the hand held model. The OH on the second carbon is on the right when the molecule is held in this position. Note that in the model the rest of the molecule is not nearly so orderly as in the diagram. That is one of the advantages of such diagrams, they are more orderly than what they are representing. The advantage here is that the diagram systematically represents the locations of the H and OH groups. The disadvantage is that it does not represent the true shape of the molecule. For that reason, different diagrams are used to represent different aspects of sugar molecules.

Mvc-007f.jpg (39470 bytes)

When we go to the third carbon atom, keeping it closer to us than the second and fourth, you can see that the OH is on the left.

Mvc-008f.jpg (43318 bytes)

When we look at the fourth carbon atom, still making sure that it's closer to us than the third or the fifth carbon atoms, you can see that the OH is on the right side.

Mvc-009f.jpg (40334 bytes)

Going on to the fifth carbon, which is the last asymmetric carbon atom, the OH is also on the right. That is what makes this molecule of glucose a "D" molecule, D-glucose.

Mvc-010f.jpg (41090 bytes)

We don't need to worry about the position of the OH on the sixth carbon atom, because it is is not an asymmetric carbon. It really doesn't matter whether it's drawn left or right, bond rotation and having two hydrogens on the carbon atom make that irrelevant.

When you are in the lab, be sure to work with the models to get familiar with these aspects of the structure of glucose molecules. One thing you should be sure to note is that the shapes of the molecules are flexible even though the representation on paper is quite rigid.

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E-mail instructor: Sue Eggling

Clackamas Community College
Š2001, 2003 Clackamas Community College, Hal Bender