Enzymes are nature's powerhouses, speeding up chemical reactions in our bodies. They're like tiny molecular machines, each designed to do a specific job, making life's processes possible.

These biological catalysts come in six main types, each with a unique role. From breaking down food to building DNA, enzymes often need helper molecules called cofactors and coenzymes to get the job done.

Enzyme Structure and Function

Enzyme function as catalysts

• Enzymes are biological catalysts that accelerate chemical reactions in living organisms without being consumed or permanently altered
• Speed up reactions by factors of $10^3$ to $10^{17}$ compared to uncatalyzed reactions allowing them to occur at physiologically relevant rates
• Highly specific catalyzing only one or a few similar reactions due to the unique three-dimensional structure of the enzyme's active site
• Lower the activation energy ($E_a$) of a reaction the minimum energy required for reactants to form products
  • Allows more reactant molecules to have sufficient energy to overcome the energy barrier and form products
• Do not alter the equilibrium of the reaction only the rate at which equilibrium is reached
• Exhibit enzyme specificity, recognizing and binding to specific substrates

Classification system for enzymes

• The International Union of Biochemistry and Molecular Biology (IUBMB) classifies enzymes into six main categories based on the type of reaction they catalyze:
  1. Oxidoreductases: catalyze oxidation-reduction reactions
     • Alcohol dehydrogenase oxidizes ethanol to acetaldehyde
  2. Transferases: transfer functional groups from one molecule to another
     • Aminotransferases transfer amino groups
  3. Hydrolases: catalyze hydrolysis reactions breaking bonds using water
     • Lipases hydrolyze triglycerides into fatty acids and glycerol
  4. Lyases: catalyze non-hydrolytic addition or removal of groups from substrates
     • Decarboxylases remove carboxyl groups from amino acids
  5. Isomerases: catalyze isomerization reactions intramolecular rearrangements
     • Glucose isomerase converts glucose to fructose
  6. Ligases: catalyze the formation of covalent bonds coupling the hydrolysis of ATP
     • DNA ligase joins DNA fragments during replication and repair

Enzyme Kinetics and Regulation

• Michaelis-Menten kinetics describes the relationship between substrate concentration and reaction rate
• Inhibition occurs when molecules bind to enzymes and decrease their activity
• Allosteric regulation involves binding of molecules to sites other than the active site, affecting enzyme activity

Cofactors and Coenzymes

Cofactors vs coenzymes in enzyme function

• Cofactors are non-protein molecules required for enzyme activity
  • Can be inorganic metal ions ($Fe^{2+}$, $Mg^{2+}$, $Zn^{2+}$) or organic coenzymes
  • Assist in enzyme function by participating in the catalytic mechanism or maintaining the enzyme's structure
• Coenzymes are organic non-protein molecules that participate in enzymatic reactions
  • Often derived from vitamins (NAD$^+$ from niacin, FAD from riboflavin)
  • Serve as electron carriers (NAD$^+$, FAD), group transfer molecules (coenzyme A, S-adenosylmethionine), or activation energy lowering agents (thiamine pyrophosphate)
  • Not permanently bound to the enzyme and can dissociate after the reaction
  • Can be regenerated and recycled to participate in multiple enzymatic reactions
• Enzymes that require cofactors or coenzymes are called holoenzymes
  • The protein portion of the enzyme without the cofactor is called the apoenzyme