Making Things Happen
Enzymes are a class of proteins made via translation. These molecules assist naturally occurring chemical reactions by making it easier to cut, modify, process, or further manipulate material into a final product. In other words, enzymes cause things to happen.
Activation Energy
Activation energy can be considered the threshold a chemical reaction must overcome in order to form a product. An enzyme lowers this threshold. Therefore less energy is required to create the product, the rate of the reaction increases, and the entire process becomes more efficient. Enzymes save energy and make the metabolic job of a cell much easier and faster.
The limits of enzymes
No enzyme can catalyze a chemical reaction that would not occur in nature. Enzymes only enable the reaction to occur faster.
Induced Fit Model
The induced fit model explains how only specific substrates can bind and be modified by specific enzymes. The model suggests that the connection between an enzyme and a substrate changes as they interact.
When a substrate molecule binds to the enzyme, the enzyme changes shape, and since the substrate molecule is bound by the enzyme, its shape is altered, too. This moves the substrate molecule into a better position to transform into the final product, thereby reducing the energy and time required for the chemical reaction to occur. You could compare this to cutting snowflakes out of a sheet of paper. If someone had to cut every side of the snowflake by hand, it would take much longer, require more energy, and likely not look very symmetrical in its final form. This would be like a chemical reaction occurring naturally. However, if the paper is first folded and then cut, fewer cuts are required, the snowflake is produced faster, and all sides will be the same. Enzymes fold and process substrates in much the same fashion.
Glycolysis
Enzymes are essential for processing organic molecules and releasing the energy stored within chemical bonds so cells can use it.
Anatomy of a Word
catabolism
Catabolism, or the catabolic process, is a process by which complex molecules are broken down into simpler ones, releasing energy as a result.
Although many organic molecules can release energy to cells, the principal molecule used to release energy is glucose. The catabolic process of breaking down glucose is termed glycolysis.
Anatomy of a Word
fermentation
Fermentation is how cells produce energy when oxygen is in short supply. In yeast, a by-product of this process is CO2 and ethanol. In animal cells, the by-product is lactic acid.
During glycolysis, glucose is processed through 10 enzymatic steps from a 6-carbon molecule into 2 separate, 3-carbon molecules called pyruvate. These much smaller molecules can be transported into mitochondria where they support its functions. Energy is also released during glycolysis.
The reaction also causes electrons to be released. Nicotinamide adenine dinucleotide (NAD+), a coenzyme, captures and transports electrons from one chemical reaction to another.
When stored energy in the form of ATP (adenosine triphosphate) is used by a cell, one of the phosphate groups is broken off the molecule, turning the ATP into ADP (adenosine diphosphate). During the process of glycolysis, as chemical bonds are broken and rearranged, some of the bonding energy is released and is used to convert ADP back into ATP.
Net energy gain during glycolysis
The products of glycolysis are 2 pyruvate molecules, 2 NADH (reduced NAD+) coenzymes, and 4 molecules of ATP. In the early stages of glycolysis, 2 ATP molecules are used to prepare intermediate molecules for the next steps of glycolysis. While 4 ATP molecules are produced during glycolysis, the net production of ATP is only 2.
TCA Cycle
The next stage of energy release, also known as the citric acid or the Krebs cycle, consists of 9 enzymatic steps that break the 2-pyruvate molecules down into 6 CO2 molecules and release the remaining energy in chemical bonds.
Each pyruvate (3-carbon molecule) must first be converted into acetyl-CoA (a 2-carbon molecule). This also results in the formation and release of a CO2 molecule as well as the formation of another NADH coenzyme.
The further breakdown of acetyl-CoA continues as a cycle of carbon intermediates is introduced. Intermediates are highly reactive molecules with short lives and lots of energy, sort of like your first crush. The 2-carbon acetyl-CoA is combined with a 4-carbon molecule to generate the first intermediate of the TCA cycle, the 6-carbon molecule citrate.
Through 8 more enzymatic reactions, 2 more carbons will be released as CO2, electrons will be captured in the form of 3 NADH (and 1 FADH2), and an additional molecule of ATP will be generated for each acetyl-CoA molecule that enters the cycle. Thus, by the end of the TCA cycle, all carbons from glucose have been released as 6 CO2 molecules and the energy of those bonds captured within 10 NADH, 2 FADH2, and 6 ATP molecules (net ATP production remains 4 ATP). Clearly, the bulk of the energy from glucose is contained within the coenzymes and has not yet been converted into ATP for the cell. That process of ATP formation also happens in the mitochondria as the electron transport system.
Electron Transport System
The electron transport chain recovers energy contained within the coenzymes NADH and FADH2 (an energy-carrying molecule that is the reduced form of flavin adenine dinucleotide). This system is similar to a hydroelectric reservoir, where the dam is the inner membrane of the mitochondria and the water is the electrons. Just as water builds up in the lake and, because of gravity, has a large potential energy to flow through the turbines of the dam to create electricity, protein complexes in the mitochondrial membrane use the energy of the electrons from the coenzymes to move hydrogen ions into the intermembrane space. This reservoir of ions then flows back into the matrix of the mitochondria through the last protein complex called ATP synthase. Just as the flow of water powers the turbine in the dam, the flow of hydrogen ions enables ATP synthase to generate ATP molecules.
Although only a net of 4 ATP molecules were produced during the stages leading up to the electron transport chain, the energy bound in the coenzymes as electrons from glucose are used to generate an additional 32 molecules of ATP. At the end of this process, 36 net molecules of ATP and 6 molecules of CO2 are produced for each molecule of glucose.