Lactase (a protein)
From Proteopedia
Lactase is an enzyme, i.e. a protein that catalyzes a chemical reaction. The function of lactase is to hydrolyze lactose, a disaccharide found in cow milk and milk products. Individuals who lack lactase have trouble digesting milk unless they take a medication like Lactaid, which contains lactase from yeasts or bacteria to supplement the lack of endogenous enzyme.
As you read through this page, click on the green links to update the interactive 3D image. You can manipulate this image with by clicking and dragging in the 3D window (see this tutorial for instructions). This page assumes basic textbook knowledge of protein chemistry. For a refresher, see the relevant section of the first chapters of any college biology textbook, e.g. this one.
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Structure
Lactase is a protein. Like many proteins that function as enzymes (e.g. polymerase, amylase, kinase, peroxidase), its name ends in -ase. Proteins are macromolecules assembled from amino acid building blocks. The amino acids are linked through condensation reactions, forming a linear chain (the primary structure).
You can also click on the +/- spin button under the spinning molecule, and then hover over parts of the structure to get the residue number and other information. The amino acids are connected by peptide bonds. For this reason, proteins may also be called polypeptides. This protein has 1225 amino acids. Can you find the peptide bonds in this segment of three amino acids (showing the amino acids 833 through 835)?
In order to be functional, the polypeptide has to fold into a specific shape by forming secondary and tertiary structure. Secondary structure refers to the pattern of hydrogen bonds formed by the main chain of the amino acid residues.
Tertiary structure refers to the overall shape resulting from the assembly of secondary structure elements through interactions between their sidechains. The details are easier to see if you zoom in and just show part of the structure.  Tertiary interactions include (a) ionic interactions, (b) hydrogen bonds, (c) disulfide bonds and (d) hydrophobic interactions.
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Function
Like all enzymes, lactase first binds to its substrate lactose. While bound to the enzyme, lactose reacts with water, causing its hydrolysis into the carbohydrate building blocks galactose and glucose. The building blocks then leave the enzyme, which is ready for another lactose molecule to bind. In this way, a single enzyme can support millions of consecutive reactions. Lactase belongs to a larger group of enzymes called beta-galactosidases. These enzymes occur in animals, plants, fungi, bacteria and other organisms.
