ATP Immediate source of energy that drives cellular work Adenosine triphosphate Nucleotide with unstable phosphate bonds Phosphate bonds easily hydrolyzed Nucleoside: adenine joined to ribose 3 phosphates attached to ribose

nucleoside

Hydrolysis of unstable bonds between phosphates Terminal phosphate bonds unstable Products of hydrolysis more stable Exergonic (spontaneous) Produces ADP + P  G = -7.3kcal/mole in lab In living cell –13kcal/mol

ATP performs work requires enzymes Exergonic hydrolysis coupled with endergonic phosphorylation Phosphorylation – transfer of P to another molecule Molecule receiving P becomes more active Page 95

Regeneration of ATP Continual rapid process 10 7 molecules used and made/sec/cell ADP + P  ATP Requires energy --- how much? Endergonic

Enzymes Biological catalysts: Most are proteins Some are ribozymes-RNA Speed up rxns by lowering energy barriers

Even if a reaction is spontaneous, it may take a really long time to get enough energy to start. (example digestion) Enzymes LOWER the amount of energy, helping spontaneous reactions to occur faster.

Free energy of activation E A activation energy Energy required to start a reaction (heat) Needed to get molecules to their transition state, unstable condition to break bonds Spontaneous reactions can be slow Heat can catalyze reactions, but heat is not good for all parts of the cell/body SO, we have enzymes to catalyze reactions (Exergonic, spontaneous reactions)

Activation energy change

Activation energy: With and without enzymes

Example of a spontaneous reaction Urea + H 2 O  CO 2 + NH 3 (Ammonia) Bacteria in air breakdown urea At room temperature and pH 8 Time required 3 million years With enzyme urease – molecules/s

Review of enzymes Composition Lower E A Do not change Very selective

Specificity of enzymes Determined by protein conformation Specific to a substrate Substrate: Substance an enzyme acts on Active site – restricted region of an enzyme which binds to substrate Pocket or groove

Enzyme, substrates, and active site

Changes shape in response to substrate Induced fit – change in shape of active site Occurs as enzyme joins to substrate Specific to a substrate Induced fit animation

Steps in Catalytic Cycle Formation of enzyme-substrate complex Induced fit (like clasping handshake) Side chains of a few amino acids catalyze conversion of substrate to product Product departs Enzyme emerges in its original form

Mechanisms that lower E A (Activation energy) Hold two or more reactants in proper position to react Induced fit may distort substrate’s bond Active site might provide a micro- environment for reaction Side chains may participate directly in the reaction

Rate of Reaction Higher the substrate concentration the faster the reaction Up to a limit Enzyme can become saturated with substrate molecules If saturated – rate depends upon how fast the active sites can convert substrates If saturated – to speed up, add enzyme

Conditions that favor enzyme activity Optimal temperature Optimal pH Cofactors Small non-protein molecules May bind to active site and substrate Inorganic (zinc, iron, copper) Organic (vitamins), called coenzymes

Enzyme Inhibitors Competitive inhibitors Resemble substrate Compete for active site and block active site from substrate If reversible (weak bonds) – overcome by increased concentration of substrate CO binds to hemoglobin Sarin, a nerve gas (binds to an enzyme in the nervous system)

Competitive inhibitor

Noncompetitive inhibitors Bind to another part of enzyme Causes change in shape Substrate can not bind to active site Metabolic poisons DDT, many antibiotics Selective inhibition is necessary in cell to regulate metabolism

Competitive or noncompetitive inhibitor?

Metabolic control: Allosteric regulation Allosteric site Specific receptor site on some part of the enzyme Two receptor sites: active and allosteric Enzymes with these have two or more polypeptide chains Each chain has active site Allosteric site where subunits join

Allosteric enzymes have 2 conformations One conformation is active The other is inactive Activator binds to allosteric to stabilize active site conformation Inhibitor (noncompetitive) binds to allosteric site to stabilize the inactive conformation

Cooperativity Substrate binds to active site of one subunit Induces a conformational change in other subunits Stabilizes active site in subunits More substrate can bind to other active sites EX: Hemoglobin (4 active sites)

Feedback Inhibition Regulation of metabolic pathway End product inhibits enzyme within the pathway Prevents cell from wasting chemical resources

True or False. Enzymes change the direction of the reaction.

Describe the relationship between activation energy and enzymes.

Describe the relationship between active sites and substrates.

What is the difference between competitive inhibitors and noncompetitive inhibitors?

True or False. All protein enzymes work at the same optimal pH.

If an enzyme is added to a solution where its substrate and product are in equilibrium, what will occur? A. additional product will form B. additional substrate will form C. the reaction will change from endergonic to exergonic D. Nothing, the reaction will stay at equilibrium

Describe what allosteric regulation is. Allosteric activation would stabilize the _____________ form of the enzyme. Which means……?

True or False. The types of inhibition with enzyme-catalyzed reactions always have negative impacts on the organism.