Science News - July 17, 2010

soup,” Kim says. This was the case in Xie’s single-enzyme studies: Thousands of partner molecules floated past a solitary enzyme, and each one had an equal chance of reacting with it. But in real cells, each enzyme usually meets only with the partners that happen to live with it.

And different cells may make different numbers of any particular enzyme, even when the cells are genetically identical. A 2008 study in Science showed that this difference can literally mean life or death for a cell: Tumor cells that survived treatment with chemotherapy were shown to make more molecules of a particular enzyme than cells that succumbed to the drug, hinting that the enzyme might play a role in drug resistance.

This variation could also mean that even if one cell follows the Michaelis-Menten equation, large groups of cells taken all together might not, Kim says. And drugs designed using equations that ignore the differences between cells could therefore be less effective.

“Even in an ideal situation where the Michaelis-Menten equation might be working well inside a single cell, it is still unavoidable to witness its breakdown at a population of cells,” Kim says.

Kim and Price showed mathematically that using the Michaelis-Menten equation to calculate how fast a large group of cells will perform a reaction gives a different answer than averaging the reaction speeds of each individual cell. By comparing the old equation with new data on single cells, the researchers found that the standard predictions for how fast enzymes work can be off by about 25 percent.

“ When we first started this we thought, oh, this looks interesting, but maybe it’s negligible. Turns out they have pretty huge effects,” Price says. “For any scenario where we know protein copy number varies between cells, which looks to be common, you’d be off.”

3. 5

Cell-to-cell variation in enzyme level and survival

2

0

Source: cohen et al./science 2008

14 17 20 23 26 29 32 35 38 41 44 47

Time after drug addition (hours)

Life or death Differences in enzyme levels among individual cells were tracked using a red fluorescent marker (right). human tumor cells (at left, each line represents one cell) that withstood a dose of the anticancer drug camptothecin made more of the DDx5 enzyme (as measured by fluorescence level), while enzyme levels dropped in the cells eventually killed by the treatment. Blue lines mark cells that survived treatment. other colors show cells that died, with the darker colors corresponding to earlier cell death.

3

Fluorescence (A.U.) × 105

1 1. 5 2. 5

0.5

predictions and then test them, all at one
go. This would be the killer.”
Such an experiment may be around
the corner. At the moment, there are
limited techniques for getting quantita-
tive data on individual molecules inside
a cell without killing the cell in the pro-
cess. But several groups—including
Xie’s— are developing more. A recent
review paper in Trends in Biotechnol-
ogy heralds single-cell analyses as a new
frontier that will transform differences
between cells “from a source of noise to
a source of new discoveries.”
Even if future observations of the
location and concentration of enzymes
vindicate Michaelis and Menten,
though, many scientists think that the
emerging street-level details of the cel-
lular city will continue to challenge
traditional ideas.

“Any reaction occurring inside the cell will be impacted by these conditions, but we don’t know exactly how,” Grima says. “We’re probably sitting on top of the iceberg.” s

Lisa Grossman is a writer in Seattle.

Enzyme engineering
Understanding how the differences
within and among cells change reaction
rates can also eventually let scientists
engineer better enzymes. Most of Price’s
research focuses on building computa-
tional models of metabolic networks in
cells, which means that he is concerned
with what the cell eats and excretes. Ulti-
mately, he says, better models will mean
better control.