Re: What is the chemical equation for the souring of milk?

Hi,

Thanks for submitting your question to the MadSci Network. In simple terms, the balanced equation for the conversion of lactic acid (lactate) from lactose is:

However, this simple reaction isn't really what happens when bacteria convert lactose to lactate, because the production of lactic acid from lactose is part of the biochemical pathway known as anaerobic respiration. Depending on the starting sugars and the end products, this pathway is common to all organisms, and represents the first steps in reclaiming the energy that was stored in glucose during the process of photosynthesis.

Lactose is a disaccharide, which means that it is two sugar molecules (a glucose and a galactose) held together. So, the first thing that has to happen is that the lactose has to be broken into gluctose and galactose. This is a hydrolysis reaction, because a molecule of water is used to split the sugars apart. This hydrolysis reaction is carried out in bacteria by an enzyme called beta-galactosidease, and in humans by lactase.

Once, the cell has freed the glucose from the lactose, the glucose can enter the glycolysis pathway, whereby the cell derives some energy from the oxidation of the glucose. This is a very long pathway, but its length exemplifies how complex it is to convert lactose to lactate, so bear with me.

First, glucose is phosphorylated by the enzyme hexokinase, resulting in the formation of glucose-6-phosphate. This reaction also converts an ATP to ADP and a proton (H⁺).

Second, glucose-6-phosphate is converted to fructose-6-phosphate by the enzyme phosphoglucose isomerase.

Third, fructose-6-phosphate is phosphorylated by phosphofructokinase, resulting in fructose-1,6-bisphosphate. This reaction also includes the conversion of an ATP to ADP and a proton (H⁺).

Fructose is now split into two three-carbon molecules, which are looking more like lactate.

First, the enzyme aldolase splits fructose-1,6-bisphosphate into dihydroxyacetone-phosphate (DHA-phosphate) and glyceraldehyde-3-phosphate.

Second, DHA-phosphate is converted into glyceraldehyde-3-phosphate by the enzyme triose phosphate isomerase. So, at this point, 1 glucose has been converted into 2 glyceraldehyde-3- phosphates.

Next, glyceraldehyde-3-phosphate is converted into 1,3-bisphosphoglycerate by the enzyme glyceraldehyde-3-phosphate dehydrogenase. This reaction takes a phosphate group (Pi) from the solution and adds it to the glyceraldehyde-3-phosphate. In the process, NAD+ is converted to NADH, and an associated H⁺.

Next, the enzyme phosphoglycerate kinase transfers the phosphoryl group that was picked up from solutiom in the previous step to an ADP, forming a new ATP and a 3-phosphoglycerate molecule. This is the first place where energy is captured during the oxidation of glucose.

Next, 3-phosphoglyycerate is converted to pyruvate in three steps.

First, the enzyme phosphoglycerate mutase converts 3-phosphoglycerate to 2- phosphoglycerate.

Second, the enzyme enolase converts 2-phosphoglycerate to phosphoenolpyruvate (PEP) and a molecule of water.

Third, the enzyme pyruvate kinase converts PEP to pyruvate, and also converts an ADP and a H⁺ into ATP, regenerating the ATP that was spent when either glucose or fructose-6- phosphate were phosphorylated (above).

The formation of pyruvate is considered to be the end of glycolysis. What happens after the formation of pyruvate depends on the type of metabolism your organism is using. Since we're concerned with the formation of lactate by bacteria (or by muscle cells), I will focus on that, but pyruvate can be converted to ethanol by yeast, or to acetyl-CoA by the rest of our cells, by other enzymes.

Pyruvate is converted to lactate by the enzyme lactate dehydrogenase, which also consumes an NADH and a free H⁺, forming NAD+. This regenerates the NAD+ that was used when glyceraldehyde-3-phosphate was converted into 1,3-bisphosphoglycerate (again above).

So, thats how you get from glucose to lactate. As you can see, it takes 11 different enzymes to catalyze all of the reactions, as well as a number of different co-factors (ATP, Pi, NAD+, etc.). However, the process is even more complex when we consider galactose.

Before galactose can enter the glycolysis pathway, it has to be converted into glucose-6-phosphate. This is a complicated pathway in and of itself, known as the galactose-glucose interconversion pathway. This pathway has 4 steps.

First, galactose is phosphorylated by the enzyme galactokinase, resulting in galactose-1- phosphate.

Second, galactose-1-phosphate is uridylated by the enzyme galactose-1-phosphate uridyl transferase, forming UDP-galactose. This reaction also consumes a UDP-glucose, and results in the formation of glucose-1-phosphate.

Third, UDP-galactose is converted into another UDP-glucose by the enzyme UDP-galactose-4- epimerase, and galactose-1-phosphate uridyl transferase converts the UDP-glucose into glucose-1-phosphate the next time a galactose-1-phosphate is uridylated.

Finally, glucose-1-phosphate is converted to glucose-6-phosphate by the enzyme phosphoglucomutase.

Then of course, glucose-6-phosphate can enter glycolysis, where it will be converted to pyruvate, and then to lactate. To accomplish this for galactose requires 15 different enzymes, and when you remember that you started with lactose, you can see that the conversion of lactose to lactate requires the activity of 16 different enzymes!!

So now, you can see that the equation for the conversion of lactose to lactate can be written as:

You can find much more detail about glycolysis and anaerobic respiration in a good college-level biochemistry textbook, such as Biochemistry, by L. Stryer. You can also search our archives for the many answers that we already have about anaerobic respiration, glycolysis, lactate, lactose and lactase, and metabolism in general.

Keep asking questions!