HUMAN BIOLOGY

Chemistry of Life 35

Disrupting a protein’s shape prevents it
from functioning normally

When a protein or any other large molecule loses its normal
three-dimensional shape, it is denatured. For example, hydro-
gen bonds are sensitive to increases or decreases in tem-
perature and pH. If the temperature or pH exceeds a protein’s
tolerance, its hydrogen bonds break, polypeptide chains
unwind or change shape, and the protein no longer func-
tions. Heating tools used to straighten hair temporarily break
hydrogen bonds in disulfide bridges in the hair’s keratin
chains (Figure 2.28A). Heat used to cook an egg destroys
weak bonds that contribute to the three- dimensional shape of
the egg white protein albumin. Some denatured proteins can
resume their shapes when normal conditions are restored,
but not albumin. There is no way to uncook a cooked egg

white (Figure 2.28B). In some cultures people enjoy uncooked

dishes made of raw shrimp or other seafood soaked in lemon

or lime juice. The acid in the citrus juice “cooks” the bits of

fish by denaturing the proteins they contain (Figure 2.28C).

Figure 2.27 Animated! A hair is built from multiple

polypeptide chains of the protein keratin. Getty Images

Figure 2.28 Changes in the chemical structure of a protein may show up in changes in the structure or functioning of body parts.
A This woman is using heat to change the shape of the structural protein keratin in her hair. B The heat of cooking denatures the
protein albumin in egg white. C Raw lobster is chemically cooked when the acid in lime juice denatures protein in the seafood.

A

© Alena Ozerova/Shutterstock.com

C

© rj lerich/Shutterstock.com

B

© Matthew Farruggio

hoW does a protein get its final shape?

• After amino acids have been linked to form a protein, the protein
  folds into its secondary structure, a coil or an extended sheet.


• More folding produces the third level of protein structure, which
  dictates how the protein will function.


• Proteins with more than one polypeptide chain have a fourth
  level of organization called quaternary structure.


• If some factor disrupts a protein’s final shape, it becomes
  denatured and cannot function properly.

taKe-home message

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).