Highlights Glycolysis II
1. 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.
2. In the next step of glycolysis, the six carbon F1,6BP is split into two three carbon piece (DHAP and G3P) s in a reaction catalyzed by aldolase. This reaction is very unfavorable when there are equal concentrations of products and reactants. To make the reaction go forward in the cell, cells "push" (increase amounts of reactants) and "pull" (decrease concentrations of products) to make the process favorable.
3. In reaction 5, DHAP is converted to G3P, so from this step forward, there are two of each molecule. The enzyme catalyzing this reaction is triose phosphate isomerase, one of the most efficient enzymes known.
4. Reaction 6 of glycolysis involves the only oxidation. The enzyme responsible is glyceraldehyde-3-phosphate dehydrogenase (G3PDH). 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.
5. In reaction 7, ATP is generated in a reaction catalyzed by phosphoglycerate kinase. In this reaction, 1,3BPG + ADP <=> 3-PG + ATP. 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. The others are oxidative phosphorylation (in mitochondria) and photophosphorylation (in the chloroplasts of plants).
8. Reaction 8 is an isomerization catalyzed by phosphoglycerate mutase. This enzyme starts with 3-phosphopglycerate (3-PG) and converts it to 2-phosphoglycerate (2-PG). In between, an intermediate known as 2,3 BPG is made. It is stable and can diffuse from the enzyme and interact with hemoglobin.
9. Reaction nine involves removal of a water molecule from each three carbon intermediate to form the high energy molecule called phosphoenolpyruvate (PEP).
10. 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. It also contributes a fair amount of heat.
11. Glycolysis is regulated by three enzymes - hexokinase (inhibited by G6P), phosphofructokinase (inhibited by ATP), and pyruvate kinase (inhibited by ATP). I will say more about regulation later.
12. Pyruvate is the ending point for glycolysis. Which pathway is taken from that point forward depends on the needs of the cell. Since cells have a VERY strong interest in keeping glycolysis going, the primary consideration is keeping NAD+ levels high. Under aerobic conditions (plenty of oxygen), NAD+ is readily made from NADH without problems. Thus under aerobic conditions, cells (animal and microbial cells) convert pyruvate to acetyl-CoA, CO2, and NADH, since the NADH can readily be converted back to NAD+.
13. Under anaerobic conditions, animals convert pyruvate to lactate using the enzyme lactate dehydrogenase (and producing NAD+ from NADH). Under anaerobic conditions, microbial cells undergo NON-OXIDATIVE (no electrons lost) decarboxylation (formation of CO2) to produce acetaldehyde followed by reduction of acetaldehyde by NADH to form ethanol and NAD+.
14. Addition of electrons and protons to pyruvate (from NADH) creates lactate and regenerates NAD+. This is important in muscles when they run low on oxygen.
15. Metabolism of glucose by anaerobic pathways does not release nearly as much energy as when glucose is metabolism by the aerobic pathway. Note that conversion of pyruvate to ethanol by microorganisms is a two step process. The last step in the process is catalyzed by alcohol dehydrogenase. In microorganisms, the direction of the reaction is towards producing ethanol. Animals also have an alcohol dehydrogenase, but they use it for the reverse direction to break down ethanol. The product of the reverse reaction is acetaldehyde and may be responsible for hangovers.