Science News - July 17, 2010

they could have somehow seen exactly what was happening in their test tubes, Michaelis and Menten would have seen the enzyme embrace a sugar molecule (fitting part of it neatly into a cleft) and then breaking it in two. The resulting simple sugars, fructose and glucose, go on to become energy sources for the cell, and the enzyme sits and waits for a new sucrose molecule to come around.

Michaelis and Menten found that the time it takes to transform a spoonful of sucrose to glucose and fructose depends on how much sucrose there was to begin with. The more sucrose, they showed, the faster the reaction — up to a point. After that, the reaction went at a steady pace.

Biologists explained this phenomenon by picturing enzymes and their partners fitting together like a lock and key. Each type of enzyme generally works with only one type of partner, and the two shape themselves to fit together perfectly. But each enzyme can couple with only one partner at a time. When all the enzymes are busy, new partners have to wait for an enzyme to free up.

Researchers were more or less satisfied with that picture for the next 85 years, and plugged in the Michaelis-Menten formula to determine reaction rates in cells. As far as most lab experiments went, it worked.

Vmax

Reaction speed vs. substrate level

Sweet biochemistry Leonor michaelis and maud menten’s classic work on the speed of enzymatic reactions was based on studies of the conversion of table sugar, or sucrose, into two simpler sugars, glucose and fructose, by the invertase enzyme (left). michaelis and menten found that the reaction rate rises with the concentration of sucrose before leveling off at a maximum, shown in graph (right). these studies led to their long-used equation for determining reaction rates.

HO
O CH2

HO CH2

OH
HO H2O +
CH2

HO

Reaction velocity (V )

Vmax 2

CH2

O
HO
OH

OH OH

Fructose O CH2

HO
OH
CH2
OH
OH
+

But in 1998, Sunney Xie, now of Harvard University, and colleagues used a fluorescent marker to watch a single molecule of the enzyme cholesterol oxidase as it met and morphed its partners one at a time. The researchers noticed something strange: The enzyme didn’t always work at the same speed.

“If you had simple
chemical reactions,
you’d expect these times
[between one reaction
and the next] to be con-
stant,” Blank says of Xie’s
work. “These times are
not constant.”
The speeds didn’t
vary randomly, either.

The enzyme seemed to work quickly for several partner molecules in a row, slow down for the next several molecules, then speed up again. If one reaction took a particularly short time, the next one was more likely to go quickly as well, as if the enzyme could remember how long it spent on the last reaction it performed.

In 1998 in Science, Xie proposed that the enzyme was flip-flopping between many different shapes, each of which did the same job at a different speed.

“For many years we just thought that
the substrate fits in the enzyme with this
lock-and-key mechanism. That’s what
we all learned at school,” Blank says. “It’s
basically not true.”
The leading hypothesis posits that
one shape fits best with the partner mol-
ecule and so works more efficiently, but
takes more energy to maintain, Blank
says. Other shapes may not work as well,
“The enzyme molecule, like us, works
hard for a while and then slows down,”
Xie says.

Microscopes still aren’t sensitive enough to take snapshots of these shape-shifting enzymes in action, but a decade of research backs up Xie’s idea. In one particularly illustrative case, Blank and her colleagues recently found that when they tug on part of the enzyme CalB using an atomic force microscope, the enzyme works faster. Pulling the enzyme may open it up, like pulling on a tab in a pop-up book, changing the enzyme’s shape and ability to catalyze reactions.

The ease with which the body absorbs medication and digests food may depend on how much time an enzyme spends in each shape. Blank suggests that shape-shifting enzymes could even drive evolution, if a genetic mutation were to enable a helpful enzyme to stay in a more efficient shape for a longer time.

These shifting reaction rates should shift the outcome of the Michaelis-Menten equation, too. Initially, the scientific community reacted with confusion: If

Michaelis-Menten was wrong, why had
all the experiments so far worked?
“ When Sunney began these researches
in the late ’90s, people said, ‘Gee, if you
have these fluctuating rates, how come
we almost always see Michaelis-Menten
“We just thought
that the substrate
fits in the
enzyme with this
lock-and-key
mechanism.... It’s
basically not true.”