TOPIC 3.6 AND 7.6 Enzymes

Proteins Biological catalysts May break a substrate molecule down into simpler molecules, or join two or more substrate molecules chemically together. Enzyme itself is unchanged in reaction; its used to speed up the reaction. Animation- hill.com/sites/ /student_view0/chapter 2/animation__how_enzymes_work.html

Activation energy Required to enable substrate to change into the product 50% chance that the reaction will proceed, otherwise it goes back to a stable form of the reactant again.

Active site  Part of the enzyme’s surface into which the substrate is bound and undergoes and reaction.  Made of different parts of the polypeptide chain folded in a specific shape.  Specificity – the complexity of the binding site is such that only one type of substrate will bind.

How enzymes work Lock and Key model  Model proposed earlier this century that suggested that the substrate was simply drawn into a closely matching cleft on the enzyme molecule Induced Fit Model  more recent studies have revealed that the process more likely involves an induced fit, where the enzyme or reactants change their shape slightly.  reactants bond to enzymes via weak chemical bonds which weaken bonds within the reactant, allowing the reaction to proceed more readily

Induced Fit Model An enzyme fits to its substrate somewhat like a lock and key. The shape of the enzyme changes when the substrate fits into the cleft:  (1) The substrate molecules are drawn into the cleft of the enzyme  (2) The enzyme changes shape, forcing the substrate molecules to combine  (3) the resulting end product is released by the enzyme which returns to its normal shape, ready to receive more.

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Catalysts Catalysts speed up reactions by influencing the stability of bonds in the reactants May also provide an alternative reaction pathway, lowering the activation energy. The presence of an enzyme simply makes it easier for a reaction to take place.

Catabolic Reactions Some enzymes can cause a single substrate molecule to be drawn into the active site. Chemical bonds are broken, causing the substrate molecule to break apart to become two separate molecules. Example: digestion, cellular respiration

Anabolic Reactions Some enzymes can cause two substrate molecules to be drawn into the active site. Chemical bonds are formed, causing the two substrate molecules to form bonds and become a single molecule Examples: protein synthesis, photosynthesis

Enzyme Reaction Rates Enzymes are sensitive molecules. Often have narrow range of conditions under which they operate properly. Temperature:  At low temperatures, there is little activity. As temperature increases until the point is reached when the temperature is so high it damages the protein (denaturation). This causes the enzyme to stop working.

Enzyme Reaction Rates Poisons can cause enzymes to cease functioning Cofactors such as vitamins and trace elements are required for many enzyme to function pH:  Extremes in acidity (pH) can also cause the protein structure of enzymes to denature.

Enzyme Cofactors Cofactors  Enhance enzyme activity  Nonprotein component of an enzyme and may be organic molecules (coenzymes) or inorganic ions (Ca 2+, Zn 2+ )

Enzyme Inhibitors May also be deactivated, temporarily or permanently, via inhibitors. Reversible inhibitors  Used to control enzyme activity  Often an interaction between the substrate or end product and the enzymes controlling the reaction.  Build up of the end product or a lack of substrate may serve to deactivate the enzyme.  May be competitive, noncompetitive, or allosteric inhibitor

Enzyme Inhibitors Competitive inhibition  Enzyme deactivation may be result of competitive inhibitor blocking the active site. Noncompetitive inhibition  Enzyme deactivation may be results of noncompetitive inhibitor binding to another site on the enzyme.  Substrate can still bind to active site, but slows the speed of reaction

Enzyme inhibitors Allosteric inhibitors  Noncompetitive inhibitor binds to enzyme in another site other than active site, which causes the active site to be distorted. The substrate cannot bind to the active site.

Enzyme Inhibitors Irreversible Inhibitors  Poisons  Certain heavy metals bind tightly and permanently to the active sites of enzymes, destroying their catalytic properties.  Examples: Cadmium (Cd), lead (Pb), mercury (Hg), and arsenic (As)  Generally non-competitive inhibitors, except Hg  Heavy metals are retained in the body, and lost slowly.