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there actually are, in the cells, uerw large molecules, mụuch larger than the ones |
whose changes we have been describing, which in some complicated way hold |
the smaller molecules just right, so that the reaction can occur easily. 'Phese |
very large and complicated things are called enzymes. (They were first called |
ferments, because they were originally discovered in the fermentation oŸ sugar. |
In fact, some of the first reactions in the cycle were discovered there.) In the |
presence of an enzyme the reaction will go. |
An enzyme is made of another substance called protein. Enzymes are very |
big and complicated, and each one is diferent, each being built to control a |
certain special reaction. 'Phe names of the enzymes are written in Fig. 3-1 at each |
reaction. (Sometimes the same enzyme may control ©wo reactions.) We emphasize |
that the enzymes themselves are not involved in the reaction directly. Thhey do |
not change; they merely let an atom go from one place to another. Having done |
so, the enzyme is ready to do it to the next molecule, like a machine in a factory. |
Of course, there must be a supply oŸ certain atoms and a way of disposing of |
other atoms. Take hydrogen, for example: there are enzymes which have special |
units on them which carry the hydrogen for all chemical reactions. For example, |
there are three or four hydrogen-reducing enzymes which are used all over our |
cycle in difÑferent places. It is interesting that the machinery which liberates some |
hydrogen at one place will take that hydrogen and use it somewhere else. |
The most important feature of the cycle of Fig. 3-1 is the transformation |
from GDP to GTTP (guanosine-di-phosphate to guanosine-tri-phosphate) because |
the one substance has much more energy in ¡i% than the other. Just as there |
is a “box” in certain enzymes Íor carrying hydrogen atoms around, there are |
special energu-carrying “boxes” which involve the triphosphate group. So, TP |
has more energy than GDP and ïf the cycle is goỉng one way, we are producing |
molecules which have extra energy and which can go drive some other cycle |
which reqguzres energy, for example the contraction of muscle. 'Phe muscle wïll |
not contract unless there is GP. We can take musecle fiber, put it in water, and |
--- Trang 75 --- |
add GTEVP, and the fñbers contract, changing TP to GDP ïf the right enzymes |
are present. So the real system is in the GDP-GTTP transformation; in the dark |
the GTP which has been stored up during the day is used to run the whole cycle |
around the other way. Ấn enzyme, you see, does not care in which direction the |
reaction goes, for ïf it did it would violate one of the laws of physics. |
Physics is of great importance in biology and other sciences for still another |
reason, that has to do with ezperimental techniques. In fact, 1f it were not for |
the great development of experimental physies, these biochemistry charts would |
not be known today. The reason is that the most useful tool of all for analyzing |
this fantastically complex system 1s to lœbel the atoms which are used in the |
reactions. 'Thus, iŸ we could introduee into the cycle some carbon dioxide which |
has a “green mark” on it, and then measure after three seconds where the green |
mark is, and again measure after ten seconds, etc., we could trace out the course |
of the reactions. What are the “green marks”? They are different ¡sotopes. We |
recall that the chemical properties of atoms are determined by the number of |
clectrons, not by the mass of the nucleus. But there can be, for example in |
carbon, six neutrons or seven neutrons, together with the six protons which all |
carbon nuelei have. Chemically, the two atoms C†!2 and C1 are the same, but |
they diÑer in weight and they have different nuclear properties, and so they are |
distinguishable. By using these isotopes of diferent weights, or even radioactive |
isotopes like C!, which provide a more sensitive means for tracing very small |
quantities, it is possible to trace the reactions. |
NÑow, we return to the description of enzymes and proteins. All proteins are |
not enzymes, but all enzymes are proteins. There are many proteins, such as the |
proteins in muscle, the structural proteins which are, for example, in cartilage |
and haïr, skin, etc., that are not themselves enzymes. However, proteins are a |
very characteristic substance of life: first of all they make up all the enzymes, and |
second, they make up much of the rest of living material. Proteins have a very |
Interesting and simple structure. 'Phey are a series, or chain, of diferent ønino |
acids. Thhere are twenty different amino acids, and they all can combine with each |
other to form chains in which the backbone is CO-NH, etc. Proteins are nothing |
but chains of various ones of these twenty amino acids. Each of the amino acids |
probably serves some special purpose. Some, for example, have a sulfur atom |
at a certain place; when two sulfur atoms are in the same protein, they form a |
bond, that is, they tie the chain together at two points and form a loop. Another |
has extra oxygen atoms which make it an acidic substance, another has a basic |
characteristic. 5ome of them have big groups hanging out to one side, so that |
--- Trang 76 --- |
they take up a lot of space. One of the amino acids, called proline, is not really an |
amino acid, but imino acid. 'There is a slight diference, with the result that when |
proline is in the chain, there is a kink in the chain. If we wished to manufacture |
a particular protein, we would give these instructions: put one of those sulfur |
hooks here; next, add something to take up space; then attach something to |
put a kink in the chain. In this way, we will get a complicated-looking chaïin, |
hooked together and having some complex structure; this is presumably just the |
mamner in which all the various enzymes are made. One of the great triumphs |
in recent tỉmes (since 1960), was at last to discover the exacb spatial atomic |
arrangement of certain proteins, which involve some fifty-six or sixty amino acids |
in a row. Over a thousand atoms (more nearly two thousand, iŸ we count the |
hydrogen atoms) have been located in a complex pattern in wo proteins. The |
first was hemoglobin. Ône of the sad aspects of this discovery is that we cannot |
see anything from the pattern; we do not understand why it works the way 1 |
does. Of course, that is the next problem to be attacked. |
Another problem is how do the enzymes know what to be? A red-eyed ly |
makes a red-eyed fly baby, and so the information for the whole pattern of |
enzymes to make red pigment must be passed from one fy to the next. 'This is |
done by a substance in the nucleus oŸ the cell, not a protein, called DNA (short |
for des-oxyribose nucleic acid). 'This is the key substance which is passed from one |
cell to another (for instance sperm cells consist mostly of DNA) and carries the |
information as to how to make the enzymes. DNA ¡is the “blueprint” What does |
the blueprint look like and how does it work? First, the blueprint must be able |
to reproduce itself. Secondly, it must be able to instruct the protein. Concerning |
the reproduction, we might think that this proceeds like cell reproduction. Cells |
simply grow bigger and then divide in half. Must it be thus with DNÑA molecules, |
then, that they too grow bigger and divide in half? Every a‡omn certainly does |
not grow bigger and divide in halfl No, it is impossible to reproduce a molecule |
except by some more clever way. |
The structure of the substance DNÑA was studied for a long time, first chemi- |
cally to fnd the composition, and then with x-rays to fñnd the pattern in space. |
The result was the following remarkable discovery: The DNA molecule is a pair |
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