Denaturing Proteins: Four Experiments with Albumin
Four dishes are displayed on an overhead projector, each containing egg white. The dishes are labeled according to the method that will be used to denature the albumin contained within the egg white. The egg whites take on an opaque appearance when the albumin is denatured, and dark spots will appear on the projected images.
Fill the bottom of each Petri dish and the recrystallizing dish with an egg white. The egg whites can be separated from the yolks by careful pouring from the broken egg shell. The yolk should remain inside of the shell. Put the four dishes onto the overhead projector and label each dish on the transparency beneath. The recrystallizing dish should be labeled “Heat.” One Petri dish should be labeled “Metal Ion,” one labeled “Acid” and the other labeled “Alcohol.”
Using a beral pipet, deliver the 1.0 M silver nitrate, dropwise, to the dish labeled “Metal Ion.” Each drop should cause albumin to instantly crash out of solution. The result will be a dark spot on the projected image. The acid should be delivered in the same manner, and a similar result will occur. The Ethanol should be delivered one pipet-full at a time. The albumin will take a little longer to crash out of solution, and the result will be a gradual cloudy appearance rather than discreet spots of darkness on the projected image. Eventually, the whole dish will become opaque. The recrystallizing dish should be taken off of the overhead and set on to a hot plate that has had time to warm up. The albumin should crash out of solution rather quickly. The result is the same as frying an egg in a skillet. The opaque results should be displayed on the overhead projector. The glass top of the projector should be able to handle the heat, but the transparency could melt if the temperature is too high. The dish should be relatively safe to handle, but a thermal glove will be provide just in case.
Denaturation is the disruption of secondary and tertiary protein structure resulting in the insolubility of that particular protein. In their water-soluble state, proteins exist in a globular form, with hydrophilic regions on the outside and hydrophobic regions contained within the globule. When proteins become denatured, some outside force acts to make the globule more linear, exposing the hydrophobic regions to the water.
Denaturing with Heat:
When the egg white is heated, molecular motion within the albumin increases.
The relatively weak hydrophobic interactions and hydrogen bonds holding the protein in globular form are disrupted. The protein begins to unwind, and unwound ends of various molecules entangle, forming a continuous mass rather than discreet globules.
Denaturing by Adding a Metal Ion:
Heavy metals, because of their high affinity and attraction to sulphur, will disrupt disulfide bonds between two cysteines.
Denaturing by Adding an Acid:
Adding acid acts to disrupt ionic bonds within the protein globule. The same thing happens to proteins once they reach our stomachs.
Denaturing by Adding and Alcohol:
The hydroxyl group on the alcohol acts to disrupt hydrogen bonding among the amino acids in the protein globule.