Most reactions that occur in organisms require the help of enzymes in order for a reaction to take place. An enzyme is the biological catalyst that increases the rate of a chemical reaction and causes specific biochemical reactions within a metabolic process.
Although a metabolic process generally refers to a specific process within a cell or living organism, a metabolic pathway is a series of intertwined chemical reactions in which the product of one enzyme acts as the substrate for the next.
Endergonic and Exergonic Reactions
Anabolic reactions require energy to proceed. A chemical reaction that requires an input of energy to proceed is known as an endergonic reaction. Therefore, anabolic reactions are endergonic reactions. An example of an anabolic reaction is the synthesis of protein.
Catabolic reactions release energy as they proceed. A chemical reaction that releases energy is known as an exergonic reaction. Therefore, catabolic reactions are exergonic reactions. Cellular respiration is an example of a catabolic reaction.
The enzyme's active site is the place where the chemical reaction occurs, and the kind of molecule that reacts with an enzyme is known as its substrate. The active site consists of a specific sequence of amino acids that help the reaction by forming and breaking bonds with the molecule with which the enzyme interacts. The physical shape of the enzyme's active site aids in this specificity. The amino acids that are found in the active site and their location, order, and orientation give the active site a very specific size, shape, and chemical behavior. Because of these amino acids, an enzyme's active site is entirely unique and will only bind to a specific target—the enzyme's substrate or substrates.
The Calvin Cycle
The Calvin cycle, which takes place after energy from the sun has been captured and converted to ATP, is catalyzed by an enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).
- In the first stage of the cycle, RuBisCO combines three 5-carbon molecules (ribulose bisphosphate, or RuBP) with three carbon dioxide molecules and ATP to produce six molecules of a 3-carbon compound (3-phosphoglyceric acid, or 3-PGA). Because this process converts carbon dioxide into organic compounds used by living things, it is known as carbon fixation.
- In the second stage of the cycle, ATP and NADPH convert the six molecules of 3-PGA into a different 3-carbon compound, glyceraldehyde 3-phosphate (G3P). One of these G3P molecules exits the cycle to be made into a sugar.
- The remaining G3P molecules get recycled into RuBP, and the cycle repeats.
Steps of Glycolysis
Note that the first half of the glycolysis pathway is endergonic because it takes in two ATP molecules. However, the second half of the pathway produces four ATP molecules. The net change in ATP is two ATP produced, making the entire glycolysis pathway energy-producing and thus exergonic.