UNIT #3: Enzymes What is an enzyme? How does an enzyme work? How is an enzyme structured? What are some factors that affect enzyma8c reac8ons? How are enzymes controlled in the body? What is an enzyme? How do enzymes work?
In any chemical reac-on, bonds are broken and formed. All reac-ons result in either the absorp>on or release of energy due to the rearrangements of atoms from one substance to another. Analogy for Ac-va-on Energy
Enzymes are organic catalysts. Enzymes are a special group of proteins. Enzymes speed up the cell s chemical reac-ons by lowering the ac>va>on energy. An enzyme s func-on is determined by its complex three-dimensional structure. A protein s structure is created on various levels: Primary structure polypep>de Amino Acids Covalent pep8de bonds Secondary structure Hydrogen bonding β-pleated sheets α-helix Tertiary structure Hydrogen bonding Disulfide bridges Hydrophobic interac8ons Ionic bonding Quaternary structure
Enzymes are specific. The reac-on takes place in the enzyme s ac>ve site. The amino acids around the ac-ve site aaach to the substrate and hold it in posi-on while the reac-ons takes place. Enzymes are specific because other molecules will not fit into the ac-ve site their shape is wrong. The substrate and the ac-ve site both change shape when the enzymesubstrate complex is formed, bending (thus weakening) the target bonds in order to lower the ac>va>on energy needed start the reac-on. Enzymes can be used repeatedly. How are enzymes like a lock-and-key? What do you know about keys?... What are the characteris-cs that dis-nguish one key from another? Where are some places that you use keys? Can a key from one place open the lock of a different key? Substrate Ac>ve site Enzyme changes shape slightly as substrate enters ac-ve site, making the fit more precise. This is called an induced fit. Products Enzyme
Naming of Enzymes Enzymes are commonly named by adding a suffix -ase to the root name of the substrate molecule it acts on or job it does. What are the following enzymes ac-ng upon? Lipase: lipids Lactase: lactose Sucrase: sucrose RNAase: RNA Hydrolase: water Factors Affec>ng the Rate of Cataly>c Ac>vity
Enzyme Concentra-on As the enzyme concentra-on increases, the rate of the reac-on also increases. There are more enzyme molecules available to catalyze the reac-on. Therefore, more enzyme-substrate complexes form. NOTE: In cells, the substrate is always in excess, so the graph does not level out. In the lab, these condi-ons need not apply and a plateau can be reached. Substrate Concentra-on As the substrate concentra>on increases, the rate increases because more substrate molecule can collide with ac-ve sites, so more enzyme-substrates complexes form. At higher concentra-ons the enzyme molecules become saturated with substrates, and there are few free ac>ve sites, so adding more substrate does not make much difference.
How do we ac>vate enzymes? Enzymes can use cofactors & coenzymes. Cofactors and coenzymes are small non-protein molecules that enhance the enzyme s func-on. They may bind temporarily or permanently to the enzyme. They usually help to form the ac-ve site of the enzyme. Cofactors are inorganic substances such as: magnesium, manganese, iron, copper, zinc, calcium, cobalt. Coenzymes are organic molecules such as: most vitamins (B2, B5, C, etc.)
An example of a cofactor is the heme por-on of the protein hemoglobin. Hemoglobin is the protein found in red blood cells. The red blood cell s job is to deliver oxygen through the body. The iron (Fe 2+ ) in the center of the heme cofactor is what binds oxygen molecules. How do we inhibit enzymes?
Compe--ve Inhibitors A compe--ve inhibitor molecule has a similar structure to the substrate molecule, and so it can fit into the ac>ve site of the enzyme. It competes with the substrate for the ac-ve site, so the reac-on is slower. If reversible, the effect of the inhibitor can be overcome by increased substrate concentra>on. Non-compe--ve (Allosteric) Inhibitors A non-compe--ve inhibitor molecule is quite different in structure from the substrate and does not fit into the ac>ve site. It binds to another part of the enzyme molecule, changing the shape of the whole enzyme, including the ac-ve site, so that it can no longer bind substrate molecules.
What condi>ons do enzymes work best under? Enzymes have an op>mum temperature at which they work the fastest. Up to the op-mum temperature, the rate increases with temperature, because the enzyme and substrate molecules both have more kine>c energy and so collide more owen, and also because more molecules have sufficient energy to overcome the ac>va>on energy. Above the op-mum temperature, the rate decreases as more of the enzymes denature. The thermal energy breaks the hydrogen bonds holding the secondary and ter-ary structures together. The enzyme loses its 3-dimensional shape.
Enzymes have an op-mum ph at which they work fastest. Above or below the op-mum ph, the rate decreases as more of the enzymes are denatured. The ph affects the charge of the amino acids at the ac-ve site, so the proper-es of the ac-ve site change and the substrate can no longer bind. Denatured Protein