Re: Why can't yeast metabolize galactose and lactose?

Hi Melody,

Yeast are able to utilize galactose quite well, although they prefer glucose.

You are correct that yeast are unable to metabolize lactose.  There are two
reasons for that.  First of all, they do not have the ability to import
lactose from whatever they are growing in or on.  Second of all, they do
not have an enzyme to break lactose into its constituent sugars, galactose
and glucose.  The enzymes that process galactose and glucose are unable to
process lactose directly.

Researchers have been able to genetically engineer yeast in such a way that
they can use lactose.  One way to do this is to engineer the yeast to
secrete the enzyme lactase.  This enzyme breaks lactose in to galactose and
glucose.  The yeast secrete the enzyme into the growth media, then they can
import and use the resulting glucose and galactose.
You can read more about that here: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=195890

Other researchers were able to construct a strain of yeast that can grow on
lactose by giving it the ability to import lactose and express lactase
(beta galactosidase) internally.  You can read about that here: http://www.pnas.org/cgi/reprint/82/23/7909

Alex Brands
Lehigh University