Glycolysis (general ideas) – part 2

The full oxidation of glucose releases about 2840kJ/mol. It is important to note that when I mention full oxidation I mean not only glycolysis but also the following processes (Krebs cycle and cellular respiration), but those will be posted in the future… J

Glycolysis is composed by 10 reactions, occurring in the cytosol (soon I will put a post about the details of all of them). The 4^th reaction is responsible for the suffix ”lysis” in the word glycolysis, because is in that step that the carbon backbone of glucose is cleaved.

The 10 glycolytic reactions are grouped in two distinct phases…

The first 5 comprise the “preparatory phase”, that is characterized, as the name suggests, by a preparation of the initial substrate, glucose. This preparation allows the conversion of glucose into something that then will give us energy. For that, in this phase it will be a consumption of 2 ATP molecules, and glucose (that is a hexose, because it has 6 carbons) will be splitted in two glyceraldehyde-3-phosphate molecules (that is a triose).

The last 5 reactions are grouped in the “payoff phase”. As the name indicates, in this phase the cell will get the energy contained in glucose. In order to become energetically favorable, glycolysis must produce more than 2 ATP, to compensate the amount spent in the preparatory phase. Indeed, it produces 4 ATP in the payoff phase (which renders an outcome of +2 ATP). Besides that, it is also obtained 2 NADH and 2 pyruvate molecules.

Global reaction of glycolysis:

Glucose + 2NAD+ + 2ADP + 2 Pi →2pyruvate + 2NADH + 2H+ + 2ATP + 2H2O

In glycolysis we have 3 types of chemical transformations:

1. Degradation of the carbon backbone of glucose. Glycolysis promotes the conversion of glucose (6 carbons) in 2 pyruvate molecules (3 carbons each).

2. Phosphorylation of ADP, that is, addition of a phosphate group to ADP, originating ATP.

3. NAD⁺ reduction, that is, the transfer of 2 electrons and one H⁺ (thus, it can be said that there is a transfer of a hydride ion H^-) to NAD⁺, yielding NADH.

Thinking on glycolysis occurring in the cytosol, it is easy to understand that diffusion of the metabolic intermediates can be a serious problem to the efficiency of glycolysis. In that context, it was observed that glycolytic enzymes are grouped in complexes, allowing a direct transfer of the intermediates from one enzyme to the following one. Once the product of a reaction is the substrate of the next one, the formation of such complexes strongly minimizes the negative impact of diffusion on overall glycolysis rate.

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