25. Ahern’s BB 350 at Oregon State University – Sugar Metabolism I

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1. Replacement of the hydrogen on the hydroxyl of an anomeric carbon by any other atom creates a glycoside. The bond is referred to as a glycosidic bond. Many carbohydrates containing multiple sugar units have glycosidic bonds. Sucrose (glucose + fructose) has a glycosidic bond joining the two sugars, as does lactose (glucose + galactose). Carbohydrates with two sugars are known as disaccharides. Sugars that are not glycosides can change readily from alpha to beta, but glycosides are locked in the configuration they were in when the hydrogen was replaced from the hydroxyl group of the anomeric carbon.

2. As we get older, we tend to make less of the enzyme known as lactase. Lactase breaks down lactose into glucose and galactose. If lactase is absent or deficient, lactose makes it into the intestine where bacteria break it down and creat a large amount of gas and cause considerable pain.

3. Polymers of sugars create polysaccharides. Amylose is a polysaccharide of plants that consists of glucose units linked solely in alpha-1,4 linkages. Glycogen is a storage polysaccharride of animals that contains glucose units linked in alpha1,4 linkages and every ten residues or so, there is a 1,6 branch to a new chain of glucose. Glycogen is therefore very branched. Amylopectin is a polysaccharide of plants that also is a glucose polymer with alpha 1,4 linkages and alpha 1,6 branches, but the branches are not nearly as frequent as in glycogen. Starch is a mixture of amylose and amylopectin.

4. Cellulose is a polymer of glucose, but instead of having units linked alpha 1,4, cellulose has the units linked beta 1,4. Most animals cannot digest cellulose and thus cannot derive energy from it. Since cellulose is a component of plant cell walls, much energy is lost as a result.

5. Ruminant animals, such as cows, have bacteria in their rumens (specialized stomachs) that contain the enzyme cellulase. Cellulase can break the beta 1,4 bonds between the glucoses in cellulose and provide energy to cows.

6. Sugars sometime have amino groups attached to them.

7. Proteoglycans are carbohydrates attached to proteins. The sugars in the carbohydrates are chemically altered to have a negative charge and the ngeative charges repel each other and give solutions containing them a slimy feel. They are found in hyaluronan (a compound of synovial fluid of our joints) and in mucus.

8. Glycoproteins are proteins that have small oligosaccharides attached to them. Some of them provide cellular identity for blood types.

Highlights Energy

1. The free energy of a reaction (Delta G) is the energy that is available for (or required for) doing things in cells (catalyzing reactions, doing work, etc.). By examining the free energy change that occurs in a reaction, one can determine if a reaction is favorable (go forward) or not favorable (go backward). Favorable reactions have Delta G values that are negative (also called exergonic reactions). Unfavorable reactions have Delta G values that are positive (also called endergonic reactions). When the Delta G for a reaction is zero, a reaction is said to be at equilibrium. Equilibrium does NOT mean equal concentrations.

2. For a reaction A = B (note that all reactions are theoretically reversible. I use the symbol = to indicate a reversible reaction), if the Delta G is negative, the forward reaction (A – B) is favored. If the Delta G is positive, the reverse reaction (B -A) is favored. If the Delta G is zero, there is no net change in A and B, as the system is at equilibrium.

3. The Delta G for the reaction A= B can be calculated from

Delta G = DeltaGzero + RTln ([B]/[A]). I will simplify this for our class to the following form:

Delta G = DeltaGzero + RTln ([Products]/[Reactants])

4. Note that if [Products] are more than [Reactants], the ln term is POSITIVE. If the [Products] are less than [Reactants], the ln term is NEGATIVE. If the [Products] = [Reactants], the ln term is ZERO.

5. DeltaGzero is a constant that has a specific value for each reaction.

Post time: Jun-21-2017
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