Understanding Enzymes: Catalysis and Specificity

Enzymes

• Enzymes are biomolecules (proteins or RNA) which catalyze specific chemical reactions.
• They enhance reaction rates by lowering the activation energy.

Fundamental Properties of Enzymes

• Enzymes speed up reactions by 10^8 to 10^20 fold and are not used up in the process
• Enzymes exhibit specificity for substrate and catalyzed reactions, ranging from absolute to relative.
• Some enzymes are regulated and can sense metabolic signals.

Catalytic Power

• Catalytic Power refers to the ratio of the enzyme-catalyzed rate of a reaction to the uncatalyzed rate
• For example, Urease has a rate constant of 3 X 10^4/sec at 20°C, while the uncatalyzed hydrolysis of urea has a rate constant of 3 X 10^-10/sec
• The ratio of the catalyzed rate to the uncatalyzed rate of the reaction is 10^14

Specificity

• Specificity is defined as the selectivity of enzymes for the reactants they act upon
• In an enzyme-catalyzed reaction, the substrate is not diverted into nonproductive side reactions, preventing wasteful by-products

Enzyme Terminology:

• Substrates are substances upon which an enzyme acts.
• Specificity refers to the selective qualities of an enzyme.
• The active site is the specific site on the enzyme where the substrate binds and catalysis occurs.

Enzyme Structures

• Monomeric enzymes consist of a single polypeptide (e.g., ribonuclease, trypsin).
• Oligomeric enzymes consist of more than one polypeptide (e.g., LDH, aspartate carbamoylase).
• Multienzyme complexes have specific sites to catalyze different reactions in sequence, and only the native conformation is active (e.g., pyruvate dehydrogenase).

Multienzyme Complexes

• In metabolic pathways, several enzymes can catalyze different stages of a process.
• These enzymes may associate noncovalently to form a multienzyme complex.
• Examples of multienzyme complexes include the pyruvate dehydrogenase complex and the electron respiratory chain.
• In other cases, different activities may be found on a single multifunctional polypeptide chain as a result of a gene fusion event.

History of Enzymes

• In 1878, Wilhelm Kühne first used the term "enzyme.".
• The word "enzyme" later referred to nonliving substances like pepsin.
• The word "ferment" was used to refer to chemical activity produced by living organisms.
• In 1897, Eduard Buchner studied the ability of yeast extracts without living yeast cells to ferment sugar
• He found that sugar was fermented even without living yeast cells.
• Buchner named the enzyme that brought about the fermentation of sucrose "zymase" and received the Nobel Prize in Chemistry in 1907 for his discovery of cell-free fermentation.

Enzymes as Proteins

• Enzymes are proteins that function as catalysts
• They speed up chemical reactions by lowering the activation energy.
• Enzymes are very specific and catalyze only one specific chemical reaction.
• The enzyme is recycled and remains unchanged after the reaction takes place.

Enzyme Structure

• Enzymes match with a substrate at the enzyme's active site in a lock and key manner.
• After the enzyme and substrate bind, it is called an enzyme-substrate complex
• The substrate may break apart or bond together to form a product
• Disaccharides turn to monosaccharides in an example

Enzyme Models

• The substrate joins the enzyme at the active site
• The 'Lock and Key Model' describes the nature of active sites of an enzyme
• The active site of an enzyme exhibits a perfect match to a specific substrate.
• In the 'Induced-Fit Model', the active site changes shape slightly when the enzyme-substrate complex joins together, creating a tighter fit.

Factors Affecting Enzyme Action: Temperature

• Temperature: Enzyme rate increases with temperature
• High temperatures can denature the enzyme, destroying its shape
• Reaction rates increase with temperature and peak around 37 - 40°C, then drop rapidly
• Example of temperature's effects include egg frying.

Factors Affecting Enzyme Action: pH

• pH: Enzymes work efficiently at a specific pH.
• Enzymes in blood work best at a pH of 7 (neutral).
• Enzymes in the stomach work best at a pH of 2 (acid).
• Enzymes in the intestine work best at a pH of 8 (base).

Factors Affecting Enzyme Action: Substrate Concentration

• Reaction rate increases as the substrate concentration increases up to a point.
• The limiting factor in the reaction may be the amount of substrate or the amount of enzyme available.

Factors Affecting Enzyme Action: Inhibitors

• Chemical messengers called hormones can signal a cell to start or stop an enzyme from working.
• An inhibitor may prevent or slow the enzyme rate by blocking the active site where the substrate sits down
• There are two types of inhibitors: Competitive and non-competitive.

Competitive Inhibitors

• Competitive inhibitors attach to the enzyme's active site.
• They have a shape similar to the substrate and compete with it.
• Competitive inhibitors are often the end product of the reaction.
• Examples of competitive inhibitors include drugs and poisons such as CO and cyanide.

Non-Competitive Inhibitors

• Non-competitive inhibitors attach elsewhere on the enzyme (not the active site)
• Attachment changes the 3D shape of the enzyme.
• The reaction still occurs, but is inhibited.

Examples of biological enzymes

• Lipase breaks down fats (lipids).
• Protease breaks down proteins.
• Cellulase breaks down fiber (cellulose).
• Amylase breaks down starch (amylose).
• Lactase breaks down dairy products (lactose).
• Sucrase breaks down sugar (sucrose).
• Maltase breaks down grains (maltose).

Regulation

• Regulation of Enzyme Activity ensures that the rate of metabolic reactions is appropriate to cellular requirements
• Regulation is essential to the integration and regulation of metabolism and can be achieved through inhibitors, activators, hormonal control, and the rate of synthesis.

Relation to Other Components of Cell

• All enzymes are proteins except for some RNAs and not all proteins are enzymes

Enzymes and Clinical Diagnosis

• An enzyme test, using blood or urine, measures levels of certain enzymes to assess how well the body's systems are functioning and whether there has been any tissue damage
• Common enzymes used for clinical diagnosis include alanine aminotransferase (ALT), alkaline phosphatase, amylase, aspartate aminotransferase, creatine kinase, and lactate dehydrogenase.