Friday, July 8, 2011

Glycolysis (reactions from the preparatory phase)

How many times you were looking to a metabolic map and thougt: “This looks like chinese, for me!”?
In this post I will try to make that this kind of thought will not appear, at least when you look to a metabolic map about glycolysis.

Before I start to describe the 10 reactions of glycolysis, there is na important issue to consider…
All the 9 glycolytic intermediates are phosphorylated, that is, they possess at least 1 phosphate (phosphoryl) group. Those groups are important for 3 important functions:
1. Since they are ionized (possess negative charge) at the intracellular pH, that is about 7 units of pH, the glycolytic intermediates will display such charges. It is important to note that glycolysis is a cytosolic process and, thus, our cells do not want that the glycolytic intermediates diffuse to the outside of the cell. As the plasma membrane is impermeable to charged molecules, the presence of phosphoryl groups causes the cell does not need to spend any extra energy to keep the glycolytic intermediates inside, regardless of their intra-and extracellular concentration.
2. Since the phosphoryl groups are part of the substrate of the glycolytic enzymes, the binding energy resulting from the interactions established between these groups and the active site of the enzyme lowers the activation energy and increases the specificity of enzymatic reactions involved.
3. They are essential components in the conservation of metabolic energy. This is a very important aspect, which will allow for channeling a large part of the energy in biochemical reactions for the synthesis of ATP.

Let's start to "dissect" glycolysis…
Reaction 1:
In this reaction, catalyzed by the enzyme hexokinase, glucose is phosphorylated (receive a phosphate group. The phosphate group donor is ATP, which is converted to ADP, ie, there is an energy expenditure associated with this reaction. The phosphate group is then added to carbon 6 of glucose through a phosphoester linkage, forming glucose-6-phosphate. This reaction requires the presence of Mg 2 + ion and is irreversible in cellular conditions.
At first it might seem a bit odd that we are spending ATP in this reaction ... So it was more advantageous to add the phosphate group is an inorganic phosphate existing in the cytosol? So there we spent energy...
However, as I often tell to my students: "Nothing comes by chance in biochemistry!" There is a very simple explanation for this situation. What is happening is that the addition of a phosphate group to glucose is a thermodynamically unfavorable reaction, ie, requires energy. This energy requirement is essentially a consequence of glucose is very rich in electronegative atoms and phosphate also be very electronegative. That is, to make the addition of the phosphate group is necessary to provide energy to overcome the electrostatic repulsions that arise. If the group was to add an inorganic phosphate from the cytosol, the reaction did not occur, because there was enough power to do so. On the other hand, there is a cellular source of phosphate groups which has a large amount of chemical energy - ATP! Therefore, the cell performs an energy coupling, ie, joins a thermodynamically unfavorable reaction (addition of a phosphate group to glucose) to a thermodynamically favorable reaction (ATP hydrolysis). As the energy released is greater than the energy expended, the reaction occurs in the cellular context.
Finally, one important aspect that will deserve a post soon. This reaction is the first point of regulation of glycolysis. Despite this, there is a reaction unique to glycolysis, is also common to other processes that use glucose. Basically, a reaction that occurs is poorly glucose enters the cell, thus preventing it from getting out of it.

Reaction 2:
This is an isomerization reaction in which glucose-6-phosphate (which is an aldose) is converted to fructose-6-phosphate (which is a ketosis). The enzyme that catalyzes this reaction is fosfohexose isomerase. It is a reversible reaction requires the presence of Mg2+ ion.

Reaction 3:
In this reaction, fructose-6-phosphate is phosphorylated at carbon 1, settling down again causing a phosphoester linkage and fructose-1 ,6-bisphosphate. The enzyme that catalyzes this reaction is phosphofructokinase-1 (PFK-1), which is the second regulatory enzyme of glycolysis. This reaction is the main point of regulation of glycolysis and is therefore irreversible in the cellular conditions! It requires the presence of Mg2+.
Again, the donor of the phosphate group is the ATP for the same reasons described above for the reaction 1. The addition of a second phosphate group will increase the instability of the final product because the amount and proximity of electron-rich regions increases.

Reaction 4:
This reaction, catalyzed by aldolase, is responsible for the suffix "analysis" in the word glycolysis. In fact, will promote the aldolase cleavage of fructose-1 ,6-bisphosphate (which is a hexose, it has 6 carbons) in glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (both are trios, they have 3 carbons). This reaction is possible due to the instability that fructose-1 ,6-bisphosphate has as a consequence of the presence of several regions with a high electronegativity (phosphoryl groups and hydroxyl groups).This reaction is reversible, but the rapid consumption of trioses formed causes the equilibrium is shifted in the forward direction.

Reaction 5:
For the remaining steps of glycolysis, it is only possible to use the glyceraldehyde-3-phosphate molecule. As it was a waste not to use the dihydroxyacetone-phosphate, because this molecule contains half of the carbon atoms of glucose, it is converted into glyceraldehyde-3-phosphate by the action of triose phosphate isomerase. Thus it is possible to use the 6 carbons of glucose in this pathway! Once again we have an isomerization reaction, in which case we have the reverse of that in reaction 2, ie, a ketosis (dihydroxyacetone phosphate) is converted into an aldose (glyceraldehyde-3-phosphate). This reaction can occur in both directions, with the majority of the molecules tends to be in the form of dihydroxyacetone phosphate. However, since the glyceraldehyde-3-phosphate is quickly consumed, the balance shifts in the forward direction.
As of this reaction, all expenses and gains of glycolysis must be multiplied by 2 because we now have two molecules of glyceraldehyde-3-phosphate per molecule of glucose.

Main bibliographic sources:
- Quintas A, Freire AP, Halpern MJ, Bioquímica - Organização Molecular da Vida, Lidel
- Nelson DL, Cox MM, Lehninger - Principles of Biochemistry, WH Freeman Publishers

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