Download Light and Life in the Universe. Selected Lectures in Physics, Biology and the Origin of Life PDF

TitleLight and Life in the Universe. Selected Lectures in Physics, Biology and the Origin of Life
Author
TagsBiology Physics
LanguageEnglish
File Size10.8 MB
Total Pages326
Document Text Contents
Page 1

This book is sold subject to the con-r
dition that it shall not, by way of trade,
be lent, resold, hired out, or otherwise
disposed of without the publisher's
consent, in any form of binding or cover
other than that in which it is published.

Page 2

LIGHT AND LIFE
IN THE UNIVERSE

Selected Lectures in Physics, Biology
and the Origin of Life

EDITED BY

S. T. BUTLER
M.SC., PH.D., D.SC.

Professor of Theoretical Physics

AND

H. MESSEL
B.A., B.SC, PH.D.

Head of the School of Physics

UNIVERSITY OF SYDNEY

PERGAMON PRESS

OXFORD · LONDON · EDINBURGH · NEW YORK

PARIS · FRANKFURT

Page 163

THE FUNCTIONS OF PROTEINS 171

we would expect diamonds to turn gradually into black graphite.
This is the theoretical expectation. In practice, however, the
reaction is so exceedingly slow that this does not happen in
millions or even billions of years. Diamond is stable not because
it is in the lowest energy state, but because it moves toward such
a state so slowly that for practical purposes we can say it does
not move at all. We say that it is in a metastable state.

Now the role of catalysts, whether biological or not, is to
speed up a reaction. The reaction itself is one that would, in
principle, take place anyhow. It must proceed from the less
probable to the more probable state, and the equilibrium point
that is eventually reached is the same whether a catalyst i$ present
or not. The catalyst just makes everything go faster. Although
the catalyst participates in a reaction, it is not used up, so that
it can increase the rates of reactions of a virtually unlimited
number of molecules.

This property of catalysts was considered very mysterious in
the early days of chemistry, but we now understand, in principle
if not in every individual case, how a catalyst acts.

What is the reason some reactions are fast and some are
slow? The reason is that in general, in going from one form to
another, a molecule has to pass through several intermediate
states. We can imagine that the atoms in a molecule are vibrating,
and only when the molecule is in a certain shape can it react.
In passing from one intermediate state to another, a molecule may
be going either uphill or downhill, although of course the over-all
reaction must be downhill. Now as we saw before, there is no
absolute prohibition against individual molecules going uphill, i.e.,
to a less probable state; but the less probable the state, the fewer
molecules will reach it. If, in dropping from one state to another,
the molecules have to make a climb for part of the way, most of
them are stopped here. They are in a metastable state. If the
climb is low, eventually they will hop over it; but the higher the
hop, the fewer will do it in unit time and the slower will be the
over-all reaction (see Figure 16). We call the intermediate climb
the activation energy of the over-all reaction. This is analogous
to a tank of water some way above the ground. Suppose the tank
is not quite full. The water can flow down if we lift it first a little

Page 164

172 LIGHT AND LIFE IN THE UNIVERSE

II
1

1 ·
1 *

Figure 16.—An analogue of activation energy. To fall to a lower energy
state, the molecules have to jump a barrier. The higher the barrier, the

fewer jump over it in a given time.

way over the edge. The height we have to lift it before it will
flow down spontaneously is the analogue of the activation energy.

What a catalyst does is to lower the initial barrier to a
reaction, or the activation energy. It forms a compound, or
complex, of reactant-catalyst. The energy barrier to form this
complex is low, so that the reaction proceeds rapidly. This reactant-
catalyst compound is itself unstable and breaks down to the final
product. The energy barrier to pass from the catalyst-reactant
complex is low, so this reaction also proceeds rapidly. Overall,
we can visualize the reaction in the following way. Without the
catalyst, the initial compound, in order to drop to a lower energy

Page 325

(a) Electrostatic bond (between
ammonium ion and car boxy I
group of amino acid gly-
cine).

(c) Van der Waa ls ' interactions (between two a l i -
phatic hydrocarbons).

(b) Hydrogen bond (between
tyrosine and uraci l ) .

Figure 3.10.—The important weak chemical bonds in biological systems.

T
H
E
C
O
N
C
E
P
T
O
F
T
E
M
P
L
A
T
E

S
U
R
F
A
C
E
S

Page 326

340 LIGHT AND LIFE IN THE UNIVERSE

shape to a given amino acid. They would then have the capacity
for the lining up of identical amino acids along a polypeptide chain.
No evidence, however, exists for such molecules. Instead, as we
can show, a specific template class does in fact exist.

A chemical argument against the existence of protein templates.
This failure of proteins ever to evolve a template role may

have its origin in the composition of the amino acid side groups.
The argument can be made that no template whose specificity de-
pends upon the side groups of closely related amino acids, like
valine, or alanine, could ever be copied with the accuracy demanded
for efficient cellular existence. This follows from the fact that some
amino acids are chemically very similar. For example, valine and
isoleucine differ only by the presence of an additional methyl group
in isoleucine. Likewise, glycine and alanine also differ by only one
methyl group. This immediately poses the question whether any
copying process can be highly accurate which must distinguish
between such closely related molecules. Our answer depends in part
upon what we mean by highly accurate. It is clear from amino acid
sequence study that an accuracy of at least 99.9% efficiency is
achieved. But it also seems chemically unlikely that a methyl group
difference could ever be the basis of a copying process with errors
less than 1 in a million.

This means that if proteins were the templates for their own
replication, the informational content of the templates would be in
constant flux. At the same time their protein products would also
show great variation. Now, even though it is impossible to give a
good quantitative argument either for the error level or its con-
sequences for the maintenance of co-ordinated cellular metabolism,
there seems no valid reason for the development of such an obviously
borderline cellular system if a more efficient template system is
possible.

Similer Documents