Highlights Carbohydrates II
1. The 'chair' configuration of glucose is more stable than the 'boat' form. This is due to steric hindrance that is more apparent in the boat form than the chair. Please note that the boat and chair are NOT different anomeric forms of glucose.
2. Sorbitol is a so-called sugar alcohol that is related to D-sorbose by reduction. Sorbitol is used as an artificial sweetener because it tastes sweet, but is not readily metabolized.
3. An alpha1-4 glycosidic bond joins an aldose in the alpha configuration to an oxygen linked to carbon 4 of the next sugar. An alpha 1-6 glycosidic bond joins an aldose in the alpha configuration to an oxygen linked to carbon 6 of the next sugar. Amylose is a polymer of glucose (polysaccharide) that has almost exclusively alpha 1-4 links. Starch is a carbohydrate storage form of plants that contains amylose (and other polysaccharides). Cellulose is a polymer of glucose that has beta 1-4 links. In contrast to starch, non-ruminant animals cannot digest cellulose, but they can digest starch. Ruminant animals, like cattle, contain a bacterium in their rumen that digests the cellulose for them.
4. Glycogen is a polymer of glucose with alpha1-4 bonds and alpha 1-6 branch bonds. It is found primarily in the liver and muscles of animals.
5. Disaccharides are carbohydrates with two sugar (monosaccharide) subunits. The most common disaccharide is sucrose, which contains a subunit of glucose linked to a subunit of fructose via glycosidic bonds between each.
6. Another common disaccharide is lactose (glucose + galactose, linked in an alpha-1,4 bond). Lactose is abundant in milk. Maltose is a disaccharide of glucose. You should know what sugars compose sucrose and lactose, but you do not need to draw structures.
7. Bacterial cell walls contain carbohydrate polymers cross linked with short peptides containing D amino acids.
8. Polymers of anionic carbohydrates create "slippery" solutions in water due to the repulsion of the many negative charges.
1. Glycolysis is a metabolic pathway for the breakdown (catabolism) of glucose and related sugars. The pathway requires two ATPs to start the process and generates 4 ATPS (for a net of two ATPs) per glucose. Also generated during glycolysis are two NADHs.
2. The two NADHs produced in glycolysis are a factor in determining which pathways is taken after pyruvate is produced in glycolysis. As seen in slide 3, three different pathways are possible AFTER glycoysis. Pyruvate is the last molecule made in glycolysis.
3. Both animals, plants, and microorganisms have the same pathway when oxygen is available (middle path in slide 3). In animals, the left pathway is taken when oxygen is missing (anaerobic - such as in muscles during heavy exertion). In microorganisms, the right pathway is taken during anaerobic conditions.
4. Pyruvate can be thought of as the end point for glycolysis. Reactions after pyruvate depend on conditions.
5. Note the difference between anaerobic options for animals (pyruvate + NADH <=> lactate + NAD+) and microorganisms (pyruvate + NADH <=> ethanol + CO2 + NAD+).
6. You do not have to memorize specific structures, enzymes or names in glycolysis, except the ones noted in the text that follows.
7. Hexokinase (need to know) is an enzyme that catalyzes the first step in glycolysis. It uses an ATP to put a phosphate onto glucose, making glucose-6-phosphate (G6P) and ADP.
8. G6P is converted to fructose-6-phosphate (F6P) in the next step of the pathways. Hexokinase is also able to convert fructose to F6P (using ATP) in a reaction very similar to that for glucose. This means that fructose too can readily enter the glycolysis pathway. Other sugars have other ways of entering.
9. Conversion of F6P to fructose-1,6-bisphosphate (F1,6BP - need to know) is catalyzed by the enzyme phoshofructokinase (PFK) (need to know). This reaction also requires ATP and is an irreversible reaction. ATP is an allosteric effector. High levels of ATP inhibit the enzyme. Low levels stimulate the enzyme. This is consistent with the energy needs of the cell - when ATP is low, cells need glycolysis to run, so PFK is turned ON. When ATP is high, cells don't need glycolysis to run, so PFK is turned OFF. PFK is a major control point for glycolysis because it stops the pathway for entry of either glucose or fructose.
10. In the next step of glycolysis, the six carbon F1,6BP is split into two three carbon pieces.
11. The two three carbon pieces are then converted into the same form - glyceraldehyde-3-phosphate (G3P). The enzyme doing the catalysis, triose phosphate isomerase, is one of the most efficient enzymes on earth.
12. Reaction 6 of glycolysis involves the only oxidation. The enzyme responsible is glyceraldehyde-3-phosphate dehydrogenase (G3PDH - need to know). In the reaction, the aldehyde of G3P is converted to an acid group, which is subsequently linked to a phosphate. Note that the energy of the oxidation provides the necessary energy to put the phosphate on. ATP is not required.
13. In reaction 7, ATP is generated. Since there are two three carbon molecules per six carbon glucose, two ATPs are produced per starting glucose. This reaction is referred to as a substrate level phosphorylation (ATP being made directly from ADP by transfer of a phosphate from another molecule with phosphate). Substrate level phosphorylation is one of three types of phosphorylation in cells. We will talk more about the others later.
14. Reaction 8 is a simple isomerization. Reaction nine involves removal of a water molecule from each three carbon intermediate to form the high energy molecule called phosphoenolpyruvate (PEP).
15. Reaction 10 is the "big bang" reaction of glycolysis. It produces another ATP for each PEP (by substrate level phosphorylation) and in turn, each PEP is converted to pyruvate, the end product of glycolysis. The enzyme, pyruvate kinase, is an important one, as it provides yet another control point for glycolysis. Pyruvate kinase is controlled by both allosteric and covalent modifications. This reaction is VERY energetically favorable and helps to "pull" earlier reactions that are not so favorable.