Enzymes: Biological Catalysts Overview

Enzymes 1

• Enzymes are biological catalysts
• Almost all life processes rely on enzyme activity
• Almost all enzymes are proteins, except for ribozymes (RNA modifying catalysts)
• In enzyme-catalyzed reactions, substrate concentrations are typically higher than enzyme concentrations
• Enzyme catalysis involves substrate binding to a specific site on the enzyme (active or catalytic site)
• During enzyme catalysis, the ES complex forms, transitions to ES*, and then to the EP complex before dissociating into product and free enzyme.
• Enzyme reactions follow this series: E + S <=> ES <=> ES* <=> EP <=> E + P
• Initially, enzymes were named arbitrarily by their discoverers
• As knowledge expanded, enzyme classification systems became more complex
• Currently, enzymes are grouped into 6 classes by the International Union of Biochemists (IUB)
• The IUB assigns a unique number to each enzyme and a textual name
• Enzyme classification is based on the type of chemical reaction catalyzed

IUB Enzyme Classifications

• Oxidoreductases: Add or remove hydrogen atoms
• Transferases: Transfer functional groups
• Hydrolases: Add water across a bond, hydrolyzing it
• Lyases: Add or remove water, ammonia, or carbon dioxide across double bonds
• Isomerases: Catalyze isomerizations
• Ligases: Join two chemical groups using energy from ATP

Enzyme Nomenclature

• Each enzyme is given a four-digit number specifying its class, subclass, substrate, and reaction type
• Enzyme names combine the substrate name, product name, and functional class
• In everyday use, enzymes are often called by their common (trivial) names.

Enzyme Composition

• Enzymes are also classified by their composition
• Simple enzymes are composed entirely of protein
• Complex enzymes are composed of protein plus a small organic molecule (cofactor)
• Complex enzymes are also known as holoenzymes

Coenzymes

• Coenzymes act as transporters of chemical groups between reactants.
• Chemical groups carried by coenzymes can vary in complexity
  • e.g., hydride ion (H+ + 2e-), a molecule of hydrogen, or an amine group (-NH2)
• Coenzymes are chemically changed during enzyme action, thus they can be considered "second substrates"
• Coenzymes are recycled to their original form after donating a chemical group to an acceptor molecule.

Enzyme Specificity

• Enzymes are highly specific for the type of reaction they catalyze
• Enzymes are substrate specific
• Some enzymes exhibit broad substrate specificity (e.g., alcohol dehydrogenase and hexokinase).
• Enzymes are generally specific for a particular steric configuration (D or L optical isomers) of a substrate
• Racemases are an exception; they convert D isomers to L isomers and vice-versa.

Isozymes

• Isozymes are multiple forms of an enzyme that work on the same substrate and produce the same product.
• Isozymes result from slight variations in the genes.
• Various isozymes are expressed in different tissues of the body -Example: Lactic dehydrogenase (LDH) is a tetrameric enzyme composed of two types of protein subunits (H and M) in different arrangements
• Different subunit arrangements produce different isozymes, each with a specific tissue distribution -Example: HHHH (Heart-type), HHHM, HHMM, HMMM, MMMM (Muscle-type) - LDH isozymes are widely used clinically as markers of tissue damage.

Enzyme-Substrate Interactions

• Two models explain enzyme-substrate binding:
  • Key and lock model
  • Induced fit model
• The induced fit model proposes that the initial enzyme-substrate interaction is weak and possibly not completely complementary but it induces conformational changes in the enzyme

This quiz covers the essential concepts of enzymes as biological catalysts crucial for various life processes. You'll learn about enzyme classification, mechanisms of action, and the formation of enzyme-substrate complexes. Test your knowledge on how enzymes function and their importance in biochemical reactions.