Enzymes: Classification, Mechanism, Kinetics, and Clinical Significance

A complete set of 50 vocabulary flashcards covering the definitions, classifications, catalytic mechanisms, kinetics, inhibition, and clinical diagnostic significance of enzymes.

Enzyme

A compound, usually a protein, that acts as a biological catalyst for biochemical reactions, speeding them up millions of times faster without being consumed or altering the final products.

Ribozymes

RNA molecules that act as biological catalysts rather than proteins.

Activation energy

The energy barrier of a reaction, which enzymes lower to accelerate the rate of chemical conversion.

Simple enzyme

An enzyme composed solely of protein (amino acid chains), such as pepsin, trypsin, chymotrypsin, urease, and ribonuclease.

Conjugated enzyme

An enzyme that has a nonprotein component in addition to its protein part.

Apoenzyme

The protein portion of a conjugated enzyme molecule that is catalytically inactive and requires a cofactor to become active.

Cofactors

Nonprotein components required by some enzymes to perform their catalytic functions, including metal ions and organic molecules.

Activators

Metal ion cofactors such as $\text{Mg}^{2+}$, $\text{Fe}^{2+}$, $\text{Zn}^{2+}$, $\text{Mn}^{2+}$, and $\text{Cu}^{2+}$ that are required for catalytic activity.

Coenzymes

Organic, typically vitamin-derived cofactor molecules required by enzymes, such as NAD, FAD, CoQ, PP, and TPP.

Holoenzyme

A catalytically active enzyme formed by combining an apoenzyme with its cofactor ($\text{Holoenzyme} = \text{apoenzyme} + \text{cofactor}$).

Substrate

The specific chemical substance upon which an enzyme acts during a catalyzed reaction.

Active site

The specific area or region of an enzyme molecule where the substrate binds during a reaction.

Metalloenzyme

An enzyme that requires metal ions to perform its catalytic function.

Antienzyme

A substance that inhibits the action of an enzyme on a substrate.

Proenzyme (Zymogen)

The inactive precursor form of an enzyme that must be activated to carry out catalytic functions.

Pepsinogen

An inactive proenzyme produced by the gastric mucosa that is converted into active pepsin by $\text{HCl}$ in gastric juice.

Trypsinogen

An inactive proenzyme produced by the pancreas that is converted into active trypsin by enterokinase in the duodenum.

Enterokinase

An enzyme in the duodenum that converts trypsinogen into active trypsin.

Chymotrypsinogen

An inactive proenzyme produced by the pancreas that is converted into active chymotrypsin upon contact with trypsin in the small intestine.

Oxidoreductases

The major class of enzymes that catalyze oxidation-reduction reactions.

Transferases

The major class of enzymes that catalyze the transfer of a functional group from one molecule to another.

Hydrolases

The major class of enzymes that catalyze hydrolysis reactions.

Lyases

The major class of enzymes that catalyze the addition of a group to a double bond or removal of a group to form a double bond without hydrolysis or oxidation.

Isomerases

The major class of enzymes that catalyze the isomerization (rearrangement of atoms) of a substrate to convert it into an isomer.

Ligases

The major class of enzymes that catalyze the joining together of two molecules into one with the participation of ATP.

Lock-and-Key Model

The rigid model of enzyme action proposed by Fischer, which posits that the substrate fits exactly into the active site.

Induced-Fit Model

The flexible, more widely accepted model of enzyme action proposed by Koshland, in which the active site adjusts its shape slightly to better accommodate the substrate.

Absolute specificity

The highest degree of enzyme specificity, wherein an enzyme will catalyze only one single chemical reaction.

Catalase

An enzyme with absolute specificity that exclusively catalyzes the decomposition of hydrogen peroxide ($\text{H}_2\text{O}_2$) into oxygen and water.

Group specificity

The degree of enzyme specificity where an enzyme acts only on molecules possessing a particular functional group, such as hydroxyl, amino, or phosphate groups.

Carboxypeptidase

A group-specific enzyme that cleaves amino acids one at a time sequentially from the carboxyl end of a peptide chain.

Linkage specificity

The most general form of enzyme specificity, wherein an enzyme acts on a specific type of chemical bond regardless of the rest of the molecular structure.

Phosphatases

Enzymes exhibiting linkage specificity that hydrolyze phosphate-ester bonds across all varieties of phosphate esters.

Stereochemical specificity

The property of an enzyme acting exclusively on a specific stereoisomer due to the inherent chirality of the active site.

L-amino acid oxidase

An enzyme with stereochemical specificity that catalyzes the oxidation of the L-form of an amino acid but does not act on the D-form.

Enzyme activity

A quantitative measure of the rate at which an enzyme converts substrate molecules into products.

Optimum temperature

The temperature at which human enzymes demonstrate maximum catalytic activity, which is $37\,^{\circ}\text{C}$.

Optimum pH

The specific pH at which an enzyme exhibits maximum catalytic activity, such as $1.5 - 2.0$ for pepsin and $8.0$ for trypsin.

Competitive enzyme inhibitor

A molecule that sufficiently resembles a substrate in shape and charge distribution to compete for the active site, an effect that can be overcome by increasing substrate concentration.

Noncompetitive enzyme inhibitor

A molecule that decreases enzyme activity by binding to a site other than the active site (such as $\text{Pb}^{2+}$, $\text{Ag}^{+}$, and $\text{Hg}^{2+}$ binding to sulfhydryl groups), which cannot be overcome by adding more substrate.

Allosteric site

A distinct site on an enzyme other than the active site where noncompetitive inhibitors bind, disrupting secondary and tertiary structures.

Irreversible enzyme inhibitor

A molecule that permanently deactivates an enzyme by forming a strong covalent bond to an amino acid side-chain group at the active site (such as nerve gases and organophosphate insecticides).

Denaturation

The loss of an enzyme's native structural organization and function caused by agents such as high temperature, strong acids, strong bases, organic solvents, or heavy metals.

ALT / SGPT

Alanine aminotransferase (serum glutamic-pyruvic transaminase), a clinical diagnostic enzyme whose increased serum concentration indicates liver damage.

AST / SGOT

Aspartate aminotransferase (serum glutamic-oxaloacetic transaminase), a clinical diagnostic enzyme whose elevated serum concentration indicates liver and heart damage.

ALP (Alkaline phosphatase)

A clinical enzyme whose serum levels become elevated in bone disorders, bone injury, and liver or biliary tract diseases.

Amylase

A pancreatic enzyme whose elevated concentration in the blood is clinically significant for diagnosing pancreatitis.

Lipase

A pancreatic enzyme whose elevated levels in the bloodstream serve as a clinical marker for pancreatitis.

CK (Creatine kinase)

A clinical diagnostic enzyme whose elevated serum level indicates skeletal muscle injury.

LDH (Lactate dehydrogenase)

A diagnostic enzyme whose elevated levels reflect tissue damage across many organs.