Encyclopedia of Biology

formate, methanol, methylamine, carbon monoxide, or
acetate) as electron donors for the reduction of carbon
dioxide to methane.

me-too drug Acompound that is structurally very
similar to already known DRUGs, with only minor
pharmacological differences.

Meyerhof, Otto Fritz(1884–1951) GermanPhysiol-
ogist, Chemist Otto Fritz Meyerhof was born on April
12,1884, in Hannover to Felix Meyerhof, a merchant,
and Bettina May. He went to the Wilhelms Gymnasium
(classical secondary school) in Berlin, leaving at age 14
only to have kidney problems two years later, which
kept him confined for a long period. He eventually stud-
ied medicine at Freiburg, Berlin, Strassburg, and Heidel-
berg and graduated in 1909. From 1912 he worked at
the University of Kiel, becoming a professor in 1918.
Meyerhof conducted experiments on the energy
changes in cellular respiration. For his discovery of the
fixed relationship between the consumption of oxygen
and the metabolism of lactic acid in the muscle, he was
awarded, together with the English physiologist A.V.
HILL, the Nobel Prize in physiology or medicine in

1922. In 1925 Meyerhof successfully extracted the
      enzymes that convert glycogen to lactic acid from the
      muscle. He introduced the term glycolysisto describe
      the anaerobic degradation of glycogen to lactic acid,
      and showed the cyclic nature of energy transformations
      in living cells. This metabolic pathway of glycolysis—
      conversion of glucose to lactic acid—is now known as
      the Embden-Meyerhof pathway, after Meyerhof and
      Gustav George Embden.
      During World War II, he went to the United States
      and became research professor of physiological chem-
      istry, aposition created for him by the University of
      Pennsylvania and the Rockefeller Foundation. He died
      from a heart attack on October 6, 1951.

Michaelis-Menten kinetics The dependence of the
initial rate of conversion of a SUBSTRATE(S) of the
product (P) by an ENZYME or other catalyst (E).
The simplest mechanism:

yields, under initial STEADY STATEconditions, and [P] =

0, the Michaelis-Menten equation,

ν= ——————

where νis the rate of conversion (Ms –1), V=k 2 [E] is

the maximum rate at [S] = ∞ for a particular

enzyme/catalyst concentration, k 2 is the turnover num-

ber (s –1), and Km= (k–1+k 2 )/k 1 is the Michaelis con-

stant under the conditions used. In the case of an

impure enzyme or catalyst, [E] is given as gl –1instead

of M. This equation leads to a hyperbolic dependence

of νupon [S], which is frequently observed in practice

even when [S] is not in great excess over [E].

microevolution The smallest scale of evolution;

changes within a species; a change in allele or genotype

frequencies over time.

See alsoMACROEVOLUTION.

microfilament(actin filament) Aminute solid heli-

cal rod—about 7 nm in diameter, composed of the

protein actin found in most eukaryotic cell cyto-

plasm—that makes up part of the cytoskeleton and

often is found in association with microtubules. Plays

a role in cell mobility, cytokinesis, and, with myosin,

part of the contractile mechanism of skeletal muscle.

One of three protein filaments of the cytoskeleton,

along with microtubules and intermediate filaments.

The cytoskeleton provides structural support for the

cell and movement of organelles, chromosomes, as

well as the cell itself.

micronutrient A compound essential for cellular

growth, being present in concentrations in minute

amounts in the growth medium.

See alsoTRACE ELEMENTS.

microtubule Alengthy hollow cylindrical structure

composed of the protein tubulin. One of three protein

filaments of the cytoskeleton, along with microfila-

ments and intermediate filaments, it is also found in

cilia, flagella, and centrioles. The cytoskeleton provides

structural support for the cell and movement of

220 me-too drug

E + S ES E + P

k 1 k 2

k-1 k-2

V[S]

Km+[S]