Biological Chemistry I

🔬Biological Chemistry I Unit 5 – Enzymes – Kinetics and Regulation

Enzymes are protein catalysts that speed up chemical reactions in living organisms. They're essential for life processes, operating under mild conditions and catalyzing reactions up to 10^17 times faster than uncatalyzed reactions. Enzyme structure determines function, with the active site binding specific substrates. Enzyme kinetics studies reaction rates, using models like Michaelis-Menten. Factors like temperature, pH, and inhibitors affect enzyme activity, which is tightly regulated in cells.

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What Are Enzymes?

  • Enzymes are biological catalysts that speed up chemical reactions in living organisms
  • Composed of proteins folded into specific three-dimensional structures
  • Catalyze reactions by lowering the activation energy required for the reaction to occur
  • Highly specific, each enzyme catalyzes a particular reaction or set of related reactions
  • Essential for life, involved in processes such as digestion, metabolism, and DNA replication
  • Operate under mild conditions (physiological temperature and pH) unlike many industrial catalysts
  • Capable of catalyzing reactions at rates up to 10^17 times faster than uncatalyzed reactions
  • Produced by living cells and can be isolated and used in various applications (food processing, medicine, biotechnology)

Enzyme Structure and Function

  • Enzymes are primarily composed of amino acids linked together by peptide bonds
  • Amino acid sequence determines the unique three-dimensional structure of each enzyme
  • Active site is the region where the substrate binds and the catalytic reaction occurs
    • Shaped to fit the specific substrate, often described as a "lock and key" or "induced fit" model
    • Contains amino acid residues that interact with the substrate and facilitate the reaction
  • Cofactors are non-protein molecules that some enzymes require for catalytic activity
    • Can be inorganic (metal ions like Fe^2+, Mg^2+, Zn^2+) or organic (coenzymes such as vitamins)
  • Enzymes are not consumed during the reaction and can catalyze multiple rounds of the same reaction
  • Enzyme activity can be affected by factors such as temperature, pH, and the presence of inhibitors or activators
  • Some enzymes have regulatory sites distinct from the active site that modulate their activity

Enzyme Kinetics Basics

  • Enzyme kinetics is the study of the rates of enzyme-catalyzed reactions
  • Reaction rate depends on the concentration of enzyme, substrate, and any cofactors or inhibitors
  • Initial reaction rate (v0v_0) is measured when the substrate concentration is much higher than the enzyme concentration
  • Michaelis-Menten kinetics describes the relationship between substrate concentration and reaction rate for many enzymes
    • Assumes a simple two-step reaction: E + S ⇌ ES → E + P
    • KmK_m (Michaelis constant) is the substrate concentration at which the reaction rate is half of the maximum rate (VmaxV_max)
  • Lineweaver-Burk plot (double reciprocal plot) is used to determine VmaxV_max and KmK_m from experimental data
  • Turnover number (kcatk_cat) represents the maximum number of substrate molecules converted to product per enzyme molecule per unit time
  • Catalytic efficiency (kcat/Kmk_cat/K_m) is a measure of how efficiently an enzyme converts substrate to product

Michaelis-Menten Equation

  • The Michaelis-Menten equation describes the kinetics of many enzyme-catalyzed reactions:
    • v=Vmax[S]Km+[S]v = \frac{V_max[S]}{K_m + [S]}
    • vv is the reaction rate, VmaxV_max is the maximum rate, [S][S] is the substrate concentration, and KmK_m is the Michaelis constant
  • Assumes a simple two-step reaction mechanism: E + S ⇌ ES → E + P
    • E is the enzyme, S is the substrate, ES is the enzyme-substrate complex, and P is the product
  • At low substrate concentrations, the reaction rate is approximately proportional to the substrate concentration (first-order kinetics)
  • At high substrate concentrations, the reaction rate approaches VmaxV_max and is independent of substrate concentration (zero-order kinetics)
  • The Michaelis constant KmK_m is equal to the substrate concentration at which the reaction rate is half of VmaxV_max
    • A lower KmK_m indicates a higher affinity of the enzyme for the substrate
  • The turnover number kcatk_cat is related to VmaxV_max by the equation: Vmax=kcat[E]TV_max = k_cat[E]_T, where [E]T[E]_T is the total enzyme concentration

Factors Affecting Enzyme Activity

  • Temperature influences enzyme activity by affecting the kinetic energy of molecules and the stability of the enzyme structure
    • Optimal temperature is the temperature at which the enzyme exhibits maximum activity
    • Higher temperatures can denature enzymes, causing a loss of activity
  • pH affects enzyme activity by altering the ionization state of amino acid residues in the active site and the enzyme structure
    • Optimal pH is the pH at which the enzyme exhibits maximum activity
    • Extreme pH values can denature enzymes or alter the ionization of key residues
  • Ionic strength and the presence of salts can affect enzyme activity by influencing the solubility and stability of the enzyme and substrate
  • Substrate concentration affects the reaction rate, as described by the Michaelis-Menten equation
  • Product concentration can influence the reaction rate through product inhibition or by shifting the equilibrium of reversible reactions
  • Enzyme concentration directly affects the reaction rate, with higher concentrations leading to faster rates until the substrate becomes limiting
  • Presence of inhibitors or activators can modulate enzyme activity by binding to the enzyme and altering its structure or function

Enzyme Inhibition and Activation

  • Enzyme inhibitors are molecules that decrease enzyme activity by binding to the enzyme
  • Competitive inhibitors bind to the active site, competing with the substrate
    • Increase the apparent KmK_m without affecting VmaxV_max
    • Can be overcome by increasing the substrate concentration
  • Non-competitive inhibitors bind to a site other than the active site, altering the enzyme's structure and function
    • Decrease VmaxV_max without affecting KmK_m
    • Cannot be overcome by increasing the substrate concentration
  • Uncompetitive inhibitors bind only to the enzyme-substrate complex, not to the free enzyme
    • Decrease both VmaxV_max and KmK_m
  • Mixed inhibitors can bind to both the free enzyme and the enzyme-substrate complex
    • Affect both VmaxV_max and KmK_m, depending on the relative affinities for the free enzyme and the complex
  • Enzyme activators are molecules that increase enzyme activity by binding to the enzyme
    • Allosteric activators bind to a site other than the active site, inducing a conformational change that enhances activity
    • Some activators are required for enzyme function, such as certain metal ions or coenzymes

Enzyme Regulation in Cells

  • Enzyme activity is tightly regulated in cells to maintain homeostasis and respond to changing conditions
  • Allosteric regulation involves the binding of effector molecules to sites other than the active site, modulating enzyme activity
    • Allosteric enzymes often have multiple subunits and exhibit cooperative binding of substrates or effectors
    • Positive cooperativity occurs when the binding of one substrate molecule enhances the binding of subsequent molecules
    • Negative cooperativity occurs when the binding of one substrate molecule reduces the affinity for subsequent molecules
  • Covalent modification, such as phosphorylation or acetylation, can regulate enzyme activity by altering the enzyme's structure or function
  • Feedback inhibition is a common regulatory mechanism in metabolic pathways
    • The end product of a pathway inhibits the activity of an earlier enzyme in the pathway, preventing excessive production
  • Compartmentalization of enzymes within the cell can control their access to substrates and regulate their activity
  • Gene expression can be regulated to control the amount of enzyme produced in response to cellular needs
    • Induction is the increased expression of an enzyme in response to the presence of its substrate or other signals
    • Repression is the decreased expression of an enzyme when its activity is not needed

Real-World Applications

  • Industrial biocatalysis uses enzymes to catalyze reactions in the production of chemicals, pharmaceuticals, and food ingredients
    • Enzymes offer high specificity, mild reaction conditions, and reduced environmental impact compared to traditional chemical processes
    • Examples include the use of lipases in detergents, proteases in leather processing, and glucose isomerase in high-fructose corn syrup production
  • Medical applications of enzymes include diagnostic tests, enzyme replacement therapies, and targeted drug delivery
    • Diagnostic tests measure the activity of specific enzymes to detect diseases (liver function tests, cardiac markers)
    • Enzyme replacement therapies provide functional enzymes to treat genetic disorders (Gaucher disease, Fabry disease)
    • Enzymes can be conjugated to drugs or antibodies for targeted delivery to specific tissues or cells
  • Biotechnology and genetic engineering use enzymes for DNA manipulation, protein production, and metabolic engineering
    • Restriction enzymes are used to cut DNA at specific sequences for cloning and gene manipulation
    • DNA polymerases are used in polymerase chain reaction (PCR) to amplify DNA fragments
    • Engineered enzymes can be produced in recombinant organisms for various applications (biofuels, bioremediation)
  • Enzymes in agriculture and food processing improve crop yields, food quality, and shelf life
    • Cellulases and amylases are used in animal feed to improve nutrient availability and digestion
    • Pectinases and other enzymes are used in fruit juice clarification and wine production
    • Transglutaminases are used to modify the texture and stability of dairy products and processed meats


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.