Chapter 4 - Enzymes and Energy Most enzymes are proteins with diverse structure. Enzymes are chemical catalysts that: –Increase the rate of a reaction. –Are not permanently altered or used up by the reaction. –Do not change the nature of the reaction. enzyme animation

–Reduce the required amount of activation energy required to initiate the reaction. Many enzymes work by orienting molecules so that they can better contact each other. Each type of enzyme has has a highly-ordered, characteristic 3-dimensional shape (conformation). Ridges, grooves, and pockets lined with specific amino acids. Pockets active in catalyzing a reaction are called the active sites of the enzyme.

The molecule affected by the enzyme is the substrate. Substrates have specific shapes to fit into the active sites (lock-and-key model): Better fit may be induced (induced-fit model): Enzyme undergoes structural change.

Enzyme names end with suffix “-ase.” –Enzymes may be named according to their mode of action, e.g. kinase. May specify both the substrate of the enzyme and job category. –Enzymes may be named for the substrate, e.g. lipase, maltase. Different organs may make different enzymes (isoenzymes) that have the same activity. –Differences in structure do not affect the active sites.

Enzyme activity can be affected by several factors: –Changes in pH and/or temperature may denature the enzyme. These changes may break the H bonds stabilizing the molecule.

–Small molecules or ions called cofactors alter the shape of the active site and/or is needed for the substrate to bind with the active site, e.g. Ca ++. –Certain organic molecules called coenzymes are needed to transport small molecules or ions (especially H+) from one enzyme to another.

–Increasing the substrate concentration will increase the reaction rate. Law of Mass Action – the principle that reversible reactions will be forced in the direction where the concentration is lowest. H C0 2 H 2 C0 3 A metabolic pathway is a sequence of enzymatic reactions that results in a final product.

–Metabolic pathways are frequently regulated by inhibition. End-product inhibition occurs when one product binds with the enzyme and prevents it from binding with the substrate. The product may bind with the enzyme at the allosteric site. This is negative feedback. inhibition animation

Competitive inhibition occurs when some other molecule (similar to the substrate) binds with the enzyme. This prevents the substrate from participating in any metabolic pathway. This is typically how poisons work.

Living organisms require the constant expenditure of energy to maintain their complex structures and processes. Central to life processes are chemical reactions that are coupled, so that the energy released by on reaction is incorporated into the products of another reaction = Bioenergetics

Exergonic reactions – give off energy Endergonic reactions – take up energy Reduction – gain e- or H + Oxidation – lose e- or H + Reducing agent – donates e- or H + to another molecule Oxidizing agent – accepts e- or H + from another molecule