8.1 Energy, matter, and enzymes  (Page 5/10)

Enzyme inhibitors

Enzymes can be regulated in ways that either promote or reduce their activity. There are many different kinds of molecules that inhibit or promote enzyme function, and various mechanisms exist for doing so ( [link] ). A competitive inhibitor is a molecule similar enough to a substrate that it can compete with the substrate for binding to the active site by simply blocking the substrate from binding. For a competitive inhibitor to be effective, the inhibitor concentration needs to be approximately equal to the substrate concentration. Sulfa drugs provide a good example of competitive competition. They are used to treat bacterial infections because they bind to the active site of an enzyme within the bacterial folic acid synthesis pathway. When present in a sufficient dose, a sulfa drug prevents folic acid synthesis, and bacteria are unable to grow because they cannot synthesize DNA, RNA, and proteins. Humans are unaffected because we obtain folic acid from our diets.

On the other hand, a noncompetitive (allosteric) inhibitor binds to the enzyme at an allosteric site , a location other than the active site, and still manages to block substrate binding to the active site by inducing a conformational change that reduces the affinity of the enzyme for its substrate ( [link] ). Because only one inhibitor molecule is needed per enzyme for effective inhibition, the concentration of inhibitors needed for noncompetitive inhibition is typically much lower than the substrate concentration.

In addition to allosteric inhibitors, there are allosteric activator s that bind to locations on an enzyme away from the active site, inducing a conformational change that increases the affinity of the enzyme’s active site(s) for its substrate(s).

Allosteric control is an important mechanism of regulation of metabolic pathways involved in both catabolism and anabolism. In a most efficient and elegant way, cells have evolved also to use the products of their own metabolic reactions for feedback inhibition of enzyme activity. Feedback inhibition involves the use of a pathway product to regulate its own further production. The cell responds to the abundance of specific products by slowing production during anabolic or catabolic reactions ( [link] ).

Key concepts and summary

• Metabolism includes chemical reactions that break down complex molecules ( catabolism ) and those that build complex molecules ( anabolism ).
• Organisms may be classified according to their source of carbon. Autotrophs convert inorganic carbon dioxide into organic carbon; heterotrophs use fixed organic carbon compounds.
• Organisms may also be classified according to their energy source. Phototrophs obtain their energy from light. Chemotrophs get their energy from chemical compounds. Organotrophs use organic molecules, and lithotrophs use inorganic chemicals.
• Cellular electron carriers accept high-energy electrons from foods and later serve as electron donors in subsequent redox reactions . FAD/FADH ₂ , NAD ⁺ /NADH , and NADP ⁺ /NADPH are important electron carriers.
• Adenosine triphosphate (ATP) serves as the energy currency of the cell, safely storing chemical energy in its two high-energy phosphate bonds for later use to drive processes requiring energy.
• Enzymes are biological catalysts that increase the rate of chemical reactions inside cells by lowering the activation energy required for the reaction to proceed.
• In nature, exergonic reactions do not require energy beyond activation energy to proceed, and they release energy. They may proceed without enzymes, but at a slow rate. Conversely, endergonic reactions require energy beyond activation energy to occur. In cells, endergonic reactions are coupled to exergonic reactions, making the combination energetically favorable.
• Substrates bind to the enzyme’s active site . This process typically alters the structures of both the active site and the substrate, favoring transition-state formation; this is known as induced fit .
• Cofactors are inorganic ions that stabilize enzyme conformation and function. Coenzymes are organic molecules required for proper enzyme function and are often derived from vitamins. An enzyme lacking a cofactor or coenzyme is an apoenzyme; an enzyme with a bound cofactor or coenzyme is a holoenzyme .
• Competitive inhibitors regulate enzymes by binding to an enzyme’s active site, preventing substrate binding. Noncompetitive (allosteric) inhibitors bind to allosteric sites , inducing a conformational change in the enzyme that prevents it from functioning. Feedback inhibition occurs when the product of a metabolic pathway noncompetitively binds to an enzyme early on in the pathway, ultimately preventing the synthesis of the product.

Fill in the blank

True/false

Short answer