## General Biology 1 Study Guide: Enzymes & Transport Mechanisms ### 1. Enzymes: Biological Catalysts Enzymes are biological catalysts that accelerate chemical reactions without being consumed. They are highly specific, interacting only with particular substrates. Think of enzymes as tiny, specialized machines that speed up specific chemical reactions within cells. #### 1.1 Enzyme Structure Enzymes are primarily proteins (though some RNA molecules, called ribozymes, also function as enzymes). Key structural components include: * **a. Active Site:** * **Definition:** The specific region on the enzyme where the substrate binds. It possesses a unique shape complementary to the substrate. * **Function:** Lowers the activation energy of the reaction, making it easier to proceed. * Sucrase's active site binds sucrose, catalyzing its breakdown into glucose and fructose. * **b. Substrate:** * **Definition:** The molecule(s) undergoing transformation during the reaction. * **Function:** Binds to the enzyme's active site, forming an enzyme-substrate complex. * In lactase, lactose is the substrate, broken down into glucose and galactose. * **c. Enzyme-Substrate Complex:** * **Definition:** The temporary structure formed when the enzyme and substrate bind. * **Function:** Optimally orients the substrate for reaction, sometimes distorting its structure to facilitate the process. * Carbonic anhydrase forms a complex with CO₂ and H₂O, converting CO₂ to bicarbonate ions. * **d. Coenzymes and Cofactors:** * **Definition:** Coenzymes are organic molecules (often derived from vitamins); cofactors are inorganic ions (like metal ions) assisting enzyme function. * **Function:** Often essential for enzyme activity; may aid in electron transfer, stabilize the enzyme-substrate complex, or participate directly in the reaction. * NAD+ (coenzyme) is involved in redox reactions; Mg²⁺ (cofactor) is crucial for DNA polymerase. #### 1.2 Mechanism of Enzyme Catalysis Enzymes lower activation energy through several mechanisms: * **a. Orientation:** Enzymes bring substrates into close proximity and orient them correctly. * DNA ligase orients DNA fragments for phosphodiester bond formation. * **b. Strain:** Enzymes induce stress on the substrate, distorting it towards the transition state. * Lysozyme strains bonds in bacterial cell walls, facilitating breakage. * **c. Microenvironment:** The active site provides a unique environment (e.g., acidic or basic) promoting the reaction. * Pepsin's acidic active site aids protein breakdown in the stomach. * **d. Direct Participation:** Some enzymes form temporary covalent bonds with the substrate. * Chymotrypsin forms a covalent bond with its protein substrate. #### 1.3 Factors Affecting Enzyme Activity Enzyme activity is influenced by: * **a. Temperature:** Enzymes have optimal temperatures; extremes cause denaturation (loss of shape). * Thermophilic enzymes like Taq polymerase function best at high temperatures. * **b. pH:** Each enzyme has an optimal pH range. * Pepsin (stomach, pH ~2); Trypsin (small intestine, slightly basic). * **c. Substrate Concentration:** Increased substrate concentration increases reaction rate until saturation (all active sites occupied). * **d. Enzyme Concentration:** Increased enzyme concentration generally increases reaction rate (if substrate is abundant). * **e. Inhibitors:** * **Competitive inhibitors:** Bind to the active site, competing with the substrate. * **Non-competitive inhibitors:** Bind to an allosteric site, altering enzyme shape and reducing activity. * Penicillin is a competitive inhibitor of bacterial enzymes. #### 1.4 Specific Enzyme Examples | Enzyme | Role | Substrate | Location | |----------------|------------------------------------|----------------------|----------------------| | Amylase | Breaks down starch to maltose | Starch | Saliva, pancreas | | DNA Polymerase | Synthesizes DNA | Nucleotides (A,T,C,G) | Nucleus | | ATP Synthase | Catalyzes ATP production | ADP, inorganic phosphate | Mitochondria, Chloroplast | | Lactase | Breaks down lactose to glucose/galactose | Lactose | Small intestine | This table summarizes key enzymes, their functions, substrates, and locations within the body. ### 2. Cellular Transport Mechanisms: Facilitated Diffusion & Active Transport Cells utilize various mechanisms to transport substances across their membranes. Two key methods are facilitated diffusion and active transport. #### 2.1 Facilitated Diffusion Facilitated diffusion is passive transport (no ATP required), moving substances down their concentration gradient using membrane proteins. * **Efficiency:** Energy-free, relatively fast but limited by protein availability. Works only down the concentration gradient. * **Specificity:** Highly selective; transport proteins recognize specific molecules. Channel proteins form pores for ions; carrier proteins change shape to transport larger molecules. * Glucose transport via GLUT proteins; ion channels for Na+, K+, Cl-. #### 2.2 Active Transport Active transport moves substances *against* their concentration gradient, requiring ATP and specific transport proteins (often pumps). * **Efficiency:** Energy-consuming, but allows concentration of molecules against their gradient. Rate is regulated by ATP, protein availability, and cellular signaling. * **Specificity:** Highly specific; transport proteins recognize and bind specific molecules. Often unidirectional. * Na+/K+ pump; proton pumps in plants, fungi, and bacteria; endocytosis/exocytosis. #### 2.3 Comparison of Facilitated Diffusion and Active Transport | Feature | Facilitated Diffusion | Active Transport | |-----------------|-----------------------|----------------------| | Energy | Passive | Active (ATP required) | | Concentration Gradient | Down | Against | | Speed | Relatively fast | Slower | | Specificity | High | High | Confusing facilitated diffusion with active transport. Remember that facilitated diffusion does not require energy, while active transport does. Consider the direction of movement relative to the concentration gradient when determining the type of transport.