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Press Release: The 1992 Nobel Prize in Physiology or
Medicine
NOBELFÖRSAMLINGEN KAROLINSKA INSTITUTET
THE NOBEL ASSEMBLY AT
THE KAROLINSKA INSTITUTE12 October 1992
The Nobel Assembly at the
Karolinska Institute has today decided to award the Nobel Prize
in Physiology or Medicine for 1992 jointly to
Edmond H.
Fischer and Edwin G. Krebs
for their discoveries
concerning "reversible protein phosphorylation as a biological
regulatory mechanism".
Summary
Thousands of proteins
participate in a complex interplay in a cell. They are the tools of
the
living organism, regulating its reactions and activities. For
example, proteins maintain the
metabolic flux, dictate growth and
cellular division, release hormones, and mediate
muscular
work.
Protein interactions are strictly
controlled. One of the most important regulatory mechanisms
isreversible protein phosphorylation. This means that enzymes
phosphorylate and dephosphorylate proteins. Both these enzymatic
processes are in turn regulated, often in several steps, allowing
amplification and fine control. The 1992 Nobel Prize in Physiology
or Medicine is awarded to the American biochemists Edmond
Fischer and Edwin Krebs. They purified and characterized
the first enzyme of this type. Their fundamental finding initiated a
research area which today is one of the most active and
wide-ranging.
Reversible protein phosphorylation is
responsible for regulation of processes as diverse as mobilization
of glucose from glycogen, prevention of transplant rejection by
cyclosporin, and development of a cancer form like chronic myeloic
leukemia.
Reversible protein phosphorylation
Thousands of proteins participate in the complex interplay
in a cell. They constitute the tools of the living organism,
regulating all its reactions and activities. For example, proteins
maintain the metabolic flux, dictate growth and cellular division,
release hormones, and mediate muscular work. Proteins are composed
of amino acid residues and have a defined three-dimensional
structure. It is this form that dictates the molecular functions.
The interactions are strictly regulated. One of the most important
mechanisms is phosphorylation of proteins. This means
covalent attachment of one or several phosphate groups to the
protein (Fig. 1).
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Fig. 1. Reversible protein
phosphorylation. A protein kinase moves a phosphate group (P) from
ATP (ADP(P)) to the protein. The biological properties of the
protein is thereby altered. There is also a protein phosphatase that
is able to remove the phosphate group. The amount of phosphate that
is associated with the protein is thus determined by the relative
activities of the kinase and the phosphatase.
The
phosphorylation influences the conformation and charge of the
protein, thereby also its activity. In this manner, the biological
function of a protein can be set at different levels. However, the
phosphate groups can also be removed from the protein in a regulated
fashion dephosphorylation. This fact constitutes the basis
for the designation reversible protein phosphorylation.
The discoveries of Fischer and
Krebs
Edmond Fischer and Edwin Krebs characterized
the first protein which revealed a novel mechanism for enzyme
control through reversible protein phosphorylation. The basic
discoveries were made in the mid 1950's through studies of a special
muscle system.
Muscles are composed of a large number of
cells capable of contraction or relaxation. For a resting muscle to
contract, it has to get energy in the form of sugar, glucose. The
glucose is released from glycogen, which is the body storage form of
sugar. Glycogen is stored in the liver, and also in muscle cells.
When they are told to initiate contractile work, they quickly
mobilize their glycogen deposits, converting them to the glucose
fuel. In order to achieve this, the organism utilizes a specific
glycogen catabolizing protein, termed phosphorylase. This
enzyme was discovered by the biochemists Carl
and Gerti Cori, bestowing them with the Nobel Prize in
Physiology or Medicine in 1947. Enzymes are proteins with the
specific role of making biological reactions possible, in short they
are catalysts.
It was known that the enzyme phosphorylase
can be regulated by small molecules. Edmond Fischer and
Edwin Krebs detected that phosphorylase could be converted
from an inactive to an active form by a principally novel mechanism.
This is carried through by transfer of a phosphate group from the
energy-rich compound ATP to the protein. They also showed that this
process is catalyzed by an enzyme, a protein kinase.
Enzymes do not only catalyze the attachment of phosphate
groups but also their removal. Such enzymes are named
phosphatases. In this manner, the glycogen catabolizing
phosphorylase is regulated by two enzymes working in opposing
directions in a reversible process, one kinase and one phosphatase.
Fischer and Krebs, in their fundamental biochemical
studies, showed how proteins in the muscle cell rapidly make the
energy supply accessible for muscular work.
A
mechanism of biological amplification
Step by step, it
has become evident that protein phosphorylation constitutes a
fundamental mechanism, influencing all cellular functions. For
example, Edwin Krebs showed that the effects of cyclic
AMP are mediated through a specific protein kinase. Cyclic AMP
was discovered by Earl
Sutherland (Nobel laureate 1971). It is formed in response to a
large number of hormones and molecular signals. The stress hormone,
adrenalin (epinephrine), mediates catabolism of glycogen stored in
the liver. This liberates glucose into the blood, giving the muscle
and heart energy to combat stress.
The fact that cyclic AMP
mediates its effects via stimulation of a protein kinase activating
the enzyme phosphorylase explains how a hormone signal can lead to
quick mobilization of sugar. The serial protein phosphorylations
then work as a biological amplifying system (see Fig. 2).
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Fig. 2. Protein phosphorylation
reactions that are coupled in series can act as a biological
amplifier. We are dealing with a controlled chain reaction. When the
level of glucose in blood is lowered the amount of the hormone
adrenaline rises. This elevates the cyclic AMP content in the liver
cell. This activates a cyclic AMP dependent protein kinase, which
phosphorylates a kinase that in turn switches on the glycogen
degrading enzyme phosphorylase. Hence glycogen is converted to
glucose which can enter the blood stream. When the blood glucose
rises the adrenaline level in blood goes down. The stimulation is
turned off and the phosphatase reactions take over turning the
glucose production down. In muscle cells a rise in calcium is the
signal for muscular work. Calcium ions also switch on the
phosphorylation reactions so that the muscle is provided with the
required energy.
Subsequent to these findings of
Fischer and Krebs, novel protein kinases are
continously found. We now estimate that perhaps one percent of the
genes in the entire genome encode protein kinases. These kinases
regulate the function of a large proportion of the thousands of
proteins in a cell. In addition, the system includes a large number
of phosphatases, which in an opposite manner regulate the removal of
the protein phosphate groups from proteins.
Inhibitors and activators
Some of the
innumerable cellular processes regulated by reversible protein
phosphorylation are shown in Fig. 3. They concern almost all
processes important to life. Imbalance between kinases and
phosphatases can cause disease and nondesirable tissue reactions.
Blood pressure, the inflammatory reaction, and brain signal
transduction - just to name a few examples - are being regulated
through different hormonal interactions and these interactions in
turn are mediated through kinases and phosphatases. We therefore
expect the development of drugs which make it possible to influence
imbalances by supplying inhibitors and activators directed against
the phosphorylation/dephosphorylation components.
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Fig. 3. How the cell is affected by
protein phosphorylation. 1. Hormone receptors (e.g. the adrenaline
receptor) are phosphorylated by specific kinases, which prevent
over-stimulation. 2. Phosphorylation can control cell shape and
motility. It can even lead to the outgrowth of long processes 3.
Phosphorylation of ribosomes affect protein synthesis. 4. Proteins
that regulate genes can be reversibly phosphorylated, causing an
adapted expression of the genomic information. 5. Hormones and
neurotransmitters are contained in storage vesicles. Phosphorylation
reactions regulate their release. 6. The proteins that control
muscle contraction can be phosphorylated by kinases. Reversible
protein phosphorylation thereby affects e.g. blood pressure and
respiration. 7. Phosphorylation regulates the enzymes that govern
metabolism.
Phosphorylation stimulates cellular
growth
The wide-ranging importance of reversible protein
phosphorylation makes it difficult to select a single,
representative example when so many could be chosen with equal
right. However, the activation of the immune response constitutes a
suitable model. It illustrates how a series of protein
phosphorylations in a cascade amplifies the strength of the initial
signal. It further shows how phosphorylation and dephosphorylation
intimately interact. The model also gives an insight into work
performed by Fischer and Krebs in recent years. The
example also shows how drugs that influence phosphorylations are
used to save transplants threatened by rejection.
In
infections, our immune system is activated by non-self compounds,
the antigens. They are consumed by macrophages which
transport the antigenic constituents to defined surface structures
(Nobel
Prize 1980 to Benacerraf, Dausset and Snell). The antigens are
then recognized by specialized lymphocytes. The lymphocytes get into
contact with the macrophages via a special surface protein.
Edmond Fischer showed that this protein works as a
phosphatase, removing a phosphate group from an enzyme. This
constitutes the start of a chain reaction where a whole
cascade of novel phosphorylating enzymes (including several
detected by Edwin Krebs) are activated. Their counterparts,
the phosphatases, are equally essential in the extended cascade. In
the end, an elevated number of specific lymphocytes have been
recruited to combat the infection.
However, sometimes the
immune defense causes problems, for example following organ
transplantations. The recipient's immune response then attacks the
transplanted kidney, liver or pancreas, trying to reject it.
Cyclosporin is a drug used with great success in prevention of such
graft rejection. It works by intervention of a phosphorylation
reaction - it inactivates the phosphatase calcineurin. This enzyme
is necessary for development and growth of the specific lymphocytes
that attack the transplant.
Under certain conditions,
protein phosphorylation can also be of importance for development of
cancer. The nuclear DNA of the cell contains a hundred-odd
oncogenes. Normally, they produce proteins participating in the
regulation of cellular growth. However, should alterations in the
oncogenes, mutations, develop, this can lead to formation of
products that give abnormal cellular growth, cancer. In several
instances an erroneously regulated protein kinase activity is
responsible. Chronic myeloic leukemia constitutes such an example.
References
Alberts et al. The Molecular
Biology of the Cell. Garland Press, 1990, 2nd
edition.
Phosphorylation-dephosphorylation cycle of proteins pp.
129-131, 710-712, 736-737, 777-778.
Fredholm, B. Molekylär
farmakologi - vägen till selektiv farmakoterapi. Läkartidningen
1991, 88: 320-421. |
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