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A catalyst is a molecule that increases the rate of a reaction without being consumed itself.

Catalysts change the kinetics of a reaction, not the thermodynamics. In other words, they lower the activation energy without altering the equilibrium or free energy change of a reaction.



An enzyme is a biological catalyst that structurally facilitates a chemical reaction. Like all catalysts, enzymes increase the rate of a chemical reaction without being consumed.

Most enzymes are proteins, but RNA molecules called ribozymes also have enzymatic activity.


How will the free energy diagram of a catalyzed reaction differ from that of an uncatalyzed reaction?

The catalyzed reaction will have a lower activation energy.

Note that the overall change in free energy will be the same for the catalyzed and uncatalyzed reactions.


What effect does a reduced activation energy have on reaction kinetics?

A lowered activation energy increases the reaction rate.

Each reaction must overcome an activation energy barrier to progress from reactants to products. When this barrier is lower, reactants are more likely to collide with sufficient energy, speeding up the reaction.


Why are enzymes generally ineffective catalysts over a broad temperature range?

Enzymes must be at a certain optimal temperature to maintain their structure.

The structure of an enzyme, especially in relation to its active site, is essential to its function as a catalyst. Like other proteins, enzymes denature, or lose their original conformations, above a certain temperature. However, reactions generally progress more slowly at low temperatures, further narrowing the range of optimal activity.


Why are enzymes generally ineffective catalysts over a broad pH range?

Enzymes can only maintain their functional structure at a certain optimal pH.

The structure of an enzyme, especially in relation to its active site, is essential to its function as a catalyst. The active site often contains positively or negatively charged groups, which contribute to the specificity of the site. Changes in pH can affect these sites, reducing enzyme activity.


At the molecular level, why are prolonged, high fevers dangerous to human health?

At higher-than-optimal temperatures, most human enzymes lose function and cannot support necessary biological processes.

Most human enzymes function most efficiently at an optimal temperature of 37 ºC. While these enzymes can remain active at slightly higher temperatures, their function is impaired.



A substrate is a specific molecule that an enzyme acts upon, usually via interactions with the enzyme's active site.

For example, starch is the substrate of salivary amylase. This means that amylase, an enzyme, catalyzes a reaction involving starch as a reactant.


What is the functional significance of the active site?

The active site of an enzyme is the structural component where enzyme-substrate interactions take place.

In other words, the active site is the catalytic region of an enzyme. It is structured to facilitate the binding of its substrate, often through the presence of certain amino acid residues.


How does the lock-and-key model explain enzyme-substrate specificity?

The lock-and-key model posits that a substrate will fit perfectly into the active site of its corresponding enzyme, without any conformational changes taking place.

In this model, the active site is the "lock" and its complementary substrate is the "key."


How does the induced fit model explain enzyme-substrate specificity?

The induced fit model posits that active sites are flexible. When a substrate approaches the enzyme, the conformation of the active site will change to better fit the substrate.

The induced fit model is a more recent adaptation of the lock-and-key model.


What major limitation prevents the lock-and-key model from being fully accurate?

It wrongly portrays all enzymes as being rigid and inflexible.

In reality, the active site of an enzyme can change its conformation to facilitate binding to the substrate.


Enzymes are usually limited to acting on a single substrate or class of substrate molecules. What term describes this quality?


The substrate(s) that can be accommodated by an enzyme are determined by the shape of its active site. For example, proteases are specific to protein substrates, while lipases act on lipid-based substrates.


allosteric site

An allosteric site is a region of an enzyme, separate from the active site, where molecules can bind and affect enzyme function.

Allosteric binding can either facilitate or inhibit the binding of substrate to the active site.


How does a competitive inhibitor alter an enzyme-catalyzed reaction?

A competitive inhibitor binds to an enzyme on its active site, inhibiting the reaction.

Specifically, these inhibitors compete with the substrate and block it from binding the active site.


How does a noncompetitive inhibitor alter an enzyme-catalyzed reaction?

A noncompetitive inhibitor binds to an enzyme on a region outside of its active site, inhibiting the reaction.

Specifically, these inhibitors bind an allosteric site, inducing a structural change in the enzyme that decreases its efficiency. Substrates can still enter the enzyme's active site.


What is the difference between cofactors and coenzymes?

Cofactors are a broad group of compounds that are required for the proper functioning of enzymes. Coenzymes are a class of small, organic cofactors.

Cofactors can also be inorganic substances, such as ions.


Many coenzymes are derived from molecules like niacin and riboflavin, which must be consumed as part of the diet. What category describes these molecules?


Vitamins are often used to synthesize coenzymes, which are non-protein organic compounds that are essential for proper enzyme function.


What class of substrate is common to maltase, sucrase and lactase?

The substrates of these enzymes are all disaccharides.

Maltase breaks down maltose into two glucose molecules. Sucrase cleaves sucrose into glucose and fructose, and lactase breaks down lactose into glucose and galactose.


What name is given to the bonds circled in the structure below?

These are phosphoanhydride bonds, which exist between phosphate groups on molecules like ATP (shown) and ADP.

The name of these bonds comes from the reaction that forms them: dehydration (removal of H2O). Predictably, they can be broken by the reverse reaction, hydrolysis (addition of H2O).


What molecule is shown below, and what is its biological role?

This molecule is adenosine triphosphate (ATP), the major form of cellular energy.

ATP consists of three phosphate groups bound to the ribonucleoside adenosine. The cleavage of the third phosphate bond facilitates the release of energy, which the cell harnesses to drive biological processes.


Phosphofructokinase, a glycolytic enzyme, is allosterically inhibited by ATP. What common homeostatic process does this example demonstrate?

This is an example of negative feedback. In such processes, increased concentration of a product decreases the rate of the reaction that forms that product.

Glycolysis involves the breakdown of glucose to form ATP, among other products. If increased amounts of ATP are already present, glycolysis will slow.



Bioenergetics is the study of energy transformations within organisms. Specifically, it concerns the energy released and used during the formation and breaking of chemical bonds.

Bioenergetics is closely related to thermodynamics, especially Gibbs free energy.


What single comparison between reactants and products can be made to determine whether a reaction will proceed?

The change in Gibbs free energy, or ΔG, determines reaction spontaneity.

A negative ΔG means the reaction will be spontaneous, while a positive ΔG denotes a nonspontaneous reaction.


What term can be used to describe a thermodynamically favorable reaction?

Thermodynamically favorable reactions are termed exergonic. In such reactions, ΔG is always negative, meaning that free energy is released.

The opposite type of reactions are endergonic. In these processes, ΔG is positive and the reaction will not proceed spontaneously.



Metabolism collectively refers to the biological processes that occur within cells. Specifically, these processes either generate energy through the breakdown of molecules or use energy to build molecules.

Metabolism is also known as "cellular respiration."



Catabolism is the biological breakdown of molecules into smaller units. Catabolic processes are accompanied by the generation of energy.

The opposite of this class of metabolic reactions is anabolism.



Anabolism is the creation of larger biomolecules from smaller units. Anabolic processes require energy input.

The opposite of this class of metabolic reactions is catabolism.


What broad distinction separates aerobic and anaerobic respiration?

Aerobic respiration requires oxygen, while anaerobic respiration does not require oxygen.

As part of metabolism, both processes involve the breakdown of biological molecules and the eventual release of energy.


What is the chemical formula of glucose?


Broadly, glucose is a carbohydrate; specifically, it is a monosaccharide.