Question

Several of the enzymes of glycolysis fall into classes that we will see often in metabolism. What reaction types are catalyzed by each of the following: (a) Kinases (b) Isomerases (c) Aldolases (d) Dehydrogenases

Short answer

(a) Phosphorylation, (b) Isomerization, (c) Cleavage, (d) Oxidation

Step-by-step solution

Understanding what kinases do

Kinases are enzymes that catalyze the transfer of a phosphate group from a high-energy molecule, such as ATP, to a specific substrate. This process is called phosphorylation. In glycolysis, kinases are involved in adding phosphate groups to glucose and its derivatives.

Understanding what isomerases do

Isomerases are enzymes that catalyze the conversion of a molecule from one isomer to another. This means they rearrange the molecular structure without adding or removing anything. In glycolysis, isomerases convert glucose-6-phosphate to fructose-6-phosphate and triose phosphate isomerase converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.

Understanding what aldolases do

Aldolases are enzymes that catalyze the breaking of carbon-carbon bonds in aldol reactions. Specifically, in glycolysis, aldolase cleaves fructose-1,6-bisphosphate into two three-carbon molecules: glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Understanding what dehydrogenases do

Dehydrogenases are enzymes that catalyze the removal of hydrogen atoms from a molecule, typically involving the transfer of electrons to a coenzyme like NAD+ or FAD. In glycolysis, glyceraldehyde-3-phosphate dehydrogenase catalyzes the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.

Key concepts

Kinase Function

Kinases are crucial enzymes in the process of glycolysis. They function by transferring a phosphate group from a high-energy donor molecule, typically ATP, to a substrate molecule. This process is known as phosphorylation. Phosphorylation plays a vital role in regulating the activity of proteins and various biological pathways. For instance, in glycolysis, the enzyme hexokinase phosphorylates glucose to form glucose-6-phosphate; subsequently, phosphofructokinase-1 phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate.

During phosphorylation, ATP is often converted to ADP (adenosine diphosphate) as it donates one of its phosphate groups to the substrate. The newly attached phosphate group can introduce a significant structural change to the substrate, which might, in turn, affect the substrate's function and interactions with other molecules.Some kinases in glycolysis include:

• Hexokinase
• Phosphofructokinase-1
• Pyruvate Kinase

Isomerase Role

Isomerases play an essential role in glycolysis by catalyzing the rearrangement of molecules. This means they convert one isomer into another, altering the specific arrangement of atoms without adding or removing anything from the molecule. A great example of this in glycolysis is when glucose-6-phosphate is rearranged into fructose-6-phosphate by the enzyme phosphoglucose isomerase.

Another crucial step is catalyzed by triose phosphate isomerase, which converts dihydroxyacetone phosphate (DHAP) to glyceraldehyde-3-phosphate (G3P). Both of these molecules are three-carbon sugars, but only G3P can continue down the glycolytic pathway.

By changing the molecular structure, isomerases ensure that the sugars are in the correct form for subsequent enzymatic steps in glycolysis, contributing to the overall energy extraction from glucose.

• The primary isomerases in glycolysis are:
• Phosphoglucose Isomerase
• Triose Phosphate Isomerase

Aldolase Reaction

Aldolases are enzymes that catalyze reactions in which carbon-carbon bonds are broken in specific types of reactions known as aldol reactions. In glycolysis, the enzyme aldolase carries out a critical step by splitting the six-carbon sugar fructose-1,6-bisphosphate into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

This split efficiently produces smaller sugars that can be further processed in the glycolytic pathway to generate energy. While G3P continues directly in the glycolytic pathway, DHAP needs to be isomerized to G3P by triose phosphate isomerase before it can enter further reactions.

• In summary, the role of aldolase in glycolysis is to:
• Cleavage fructose-1,6-bisphosphate
• Create two smaller interconvertible sugars (G3P and DHAP)

Dehydrogenase Action

Dehydrogenases are enzymes that remove hydrogen atoms from molecules, typically involving the transfer of electrons to a coenzyme such as NAD+ or FAD. These enzymes are critical in oxidation-reduction (redox) reactions within metabolic pathways like glycolysis. A key reaction involves the enzyme glyceraldehyde-3-phosphate dehydrogenase, which catalyzes the oxidation of glyceraldehyde-3-phosphate (G3P) to 1,3-bisphosphoglycerate (1,3-BPG).

This reaction also includes the reduction of NAD+ to NADH, which is crucial for the cell's ability to generate ATP. The process helps in maintaining the cell's redox balance and provides substrates for the mitochondrial electron transport chain.

• Important points about dehydrogenases in glycolysis:
• They facilitate redox reactions
• Convert NAD+ to NADH
• Ensure continuity of the glycolytic pathway