Some time ago, while making my way home from college, I was
listening to an episode of a podcast called Science Friday. It was dark and cold outside and I was keen to get home, eat
and be warm, so it's safe to say my mind was not on the material at hand. Even
so, what I heard blew my mind, it seemed too simple to be true, and yet it made
perfect sense. I was immediately annoyed I hadn't come up with the idea myself.
The idea? Universal blood cells. I'm going to keep you in suspense as to what
that means while I explain more about blood in general.
BLOOD
Blood is composed of
red blood cells (also referred to as RBCs, or erythrocytes), white blood cells
(also referred to as WBCs, or leukocytes), and platelets. This varied
composition allows it to perform a myriad of essential functions. For example,
RBCs are responsible for the transport of oxygen, (O2), from the
lungs to every other organ and cell in the body. In addition, they also
exchange this oxygen for a waste product of glucose metabolism, carbon dioxide
(CO2), and return it to the lungs to be expelled. Therefore there is
a constant exchange of CO2 for O2 going
on in the lungs, with blood being continually circulated around the
body to remove/supply both. WBCs are a much more complicated group. There are
five different subtypes of WBCs, but together they form part of our immune
system, allowing us to fend of biological attacks from viruses and bacteria.
Finally, plateletes are responsible for some housekeeping activities such as
blood clotting, maintenance of blood vessel lining, and digestion of harmful
bacteria.
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| The composition of blood, image taken at http://www.myvmc.com/anatomy/blood-function-and-composition/ |
In addition to all
of the above functions blood also has the added function of regulating pH,
regulating body temperature, and generally acting as a delivery mechanism
throughout the body for the supply of nutrients such as glucose, amino acids,
lipids, vitamins and salts. So, blood is important, and it's able to perform
all of these functions because of the complex composition of RBCs, WBCs, and
platelets.
CELL BIOLOGY 101
Cells are the basic
unit of life, by which I mean all living things are composed of either a single
cell, or many cells, termed unicellular or multicellular respectively. But
cells themselves are incredibly complex, and in the same way that we have
specific organs performing specific tasks for us, our cells have tiny
structures inside them called organelles performing specific tasks for them.
This could be the production of protein, or conversion of sugars into cellular
energy. But the complexity of a cell continues to the outside. All animal cells
have a fatty cell membrane called a phospholipid bilayer separating it from the
outside world. This lipid bilayer is protective, but also functional. It's
scattered with protein and sugar molecules that allow it to regulate transport
in/out of the cell, and generally communicate with other cells around it. Sugar
and proteins sometimes combine to form what is called a glycoproteins, and
these are important molecules.
Most people will have heard the term blood group. But what does it actually
mean? Well, there are four different blood groups called group A, B, AB, and O.
It turns out that the glycoprotein surface is what determines your blood group.
There are many different types of sugars, and they have names like glucose,
fructose, and sucrose. In the image below "GAL", "GAL-Nac"
and "FUC" are sugars called galactose, N-acetylgalatosamine and
Fucose respectively. You can see how all three blood groups share the same
GAL-GALNAc-GAL backbone, but differ in their terminal regions. For example,
blood group A has an additional Gal-Nac sugar molecule compared to blood group
O, while blood group B has an additional GAL sugar molecule compared to blood
group O. Blood group A is further subdivided with blood group A1 and A2 being
the most common.
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| Schematic of the A, B, and O blood groups. The red oval represents the red blood cell. The coloured hexagons represent the different sugars that attach to the blood cells to make its blood group. |
Blood groups are extremely important for blood transfusions. The blood
groups between donor and recipient must match otherwise the body will reject
it. All blood groups can donate to themselves, so group A is compatible with A
etc. However, blood group A is not compatible with blood group B, and the
reverse is also true, group B is not compatible with group A. Group AB
cannot be given to group A, or B. Group O however can be given to every blood
type, which makes it extremely valuable. The compatibility of different blood
groups can be summarised in the diagram below. Red arrows indicate blood groups
which cannot be exchanged.
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| The compatibility of each of the blood groups, red arrows indicate blood groups which cannot be exchanged. |
Invasive surgery requires blood transfusion while the operation is being performed, so if there's no time to test the blood type of your patient then you know it's safe transfuse blood O without resulting in any additional complications. Transfusing the wrong blood will lead to activation of the immune system, destruction of the new blood cells which ultimately can result in death.
UNIVERSAL BLOOD CELLS
The idea I ended up stumbling across that night while listening to my podcast was to generate blood group O cells from bags of group A, B, and AB blood. That is, the ability to take any blood type and modify it so that it's capable of being transfused without any immune response in the patient. This idea, if feasible, would make all of the current blood stocks capable of being donated to any patient! That's pretty amazing!
HOW DOES IT WORK?
The basic science
behind this is involves the application of a well established biochemical
technique called enzymatic hydrolysis. Enzymes are protein molecules which have
the ability to add or remove chemical components to specific molecules, I've
mentioned them before in a previous blogpost so I'll
skip over a detailed explanation here. There are pre-existing enzymes in
nature which will selectively remove the N-acetylgalatosamine and Frucose
sugars from blood cells. Interestingly, the first attempts were with an enzyme
found in coffee beans, but subsequent attempts using different versions of the
same enzyme resulted in much better efficiency. The image below shows how these
sugars actually look and demonstrates how the conversion of group A or B to
group O involves the removal of just one molecule of sugar. The sugar removed
in each case is highlighted by a coloured star.
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| Conversion of blood groups A and B to group O. Image adapted and modified from the original research paper on this topic, "Bacterial Glycosidases for the production of universal blood cells" |
There are two enzymes used for this,
once called α-N-acetylgalactosaminidase for removing the sugars associated with
blood group A, and a second called α-galactosidase for removal of sugars
associated with blood group B. The actions of enzymes result in blood group O
cells. The structure of α-N-acetylgalactosaminidase enzyme bound to a single
sugar molecule is shown below. On the left is an image of the whole
enzyme/sugar complex (shown in grey, red and blue), with the sugar shown in
purple. The image on the right is the same enzyme/sugar complex zoomed in to
show the the complex in more detail. Enzyme often have small clefts or holes in
their structure into which their substrates fit. All the chemistry associated
with the removal of this sugar molecule happens in and around this small cleft
area, called the active site.
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Biological material
like blood is sensitive to large changes in pH, or temperature. In addition,
it's important to keep it free from any external contamination. Biochemists are
familiar with this so we add reagents to control the pH, we keep solutions
cold, because it helps preserve the integrity, and slow bacterial degradation
of the components and we often work in sterile conditions to prevent any
contamination. This is time consuming and expensive. But one of the cool things
about the procedure used to make this blood is that it is relatively easy.
There are no difficult or expensive conditions required. The enzymes required
were simply added to a 200mL volume of blood which had been washed with buffer
solution, the solution was then mixed gently for 1 hour at room temperature,
after this the blood was washed with a salty solution to remove the enzymes. In
addition, even though there are different enzymes responsible for converting
blood groups A and B it was possible to simply add both enzymes to a single
unit of AB blood and let both of them operate together to produce a unit of
blood group O.
THE DISADVANTAGES
There are some disadvantages with the enzymatic hydrolysis of blood cells.
The research group responsible for demonstrating this modification of blood
cells has shown it works on a small scale, but for this to be really useful it
would need to be able to convert thousands of units of blood every week. Such
large scale conversion would require large quantities of the enzymes used.
Specifically, for the conversion of 1 unit of blood group A1 to blood group O
the researchers mention they used 60mg of enzyme. Recently the Irish Blood
Transfusion Service (IBTS) advertised they needed 1,500 units of group O blood
each week. To convert 1,500 units of blood would therefore require 90,000mg
(90g) of enzyme every week.
It's a little difficult to put this in perspective, but I used to make
enzymes as part of my Ph.D work. This is done via the genetic modification of
microorganisms, bacteria or yeast cells for example, which are then grown in
what is called a bioreactor, or fermentation vessel. Typical yields of protein
are pretty low, in the order of 100mg/L of cell culture material. So the
ability to make 90g of enzyme every week is demanding, and expensive, but by no
means impossible. Biotechnology companies are used to this problem, and utilise
large scale bioreactors, in the order of 20,000L, to mass produce therapeutic
proteins for other medical reasons. This is currently being done for diseases
like diabetes, where insulin is required in large quantities for worldwide
supply, so there's no technological reason why this could not be done for these
enzymes.
SUMMARY
Blood donations are always needed, and in particular blood group O is in
high demand because it is accepted by any blood group. Supply is always going
to be restricted due to legitimate medical reasons such as disease, so anything
that can be done to ease the strain on supply is definitely worth considering.
The research presented here is simple, but clever, and uses well established
biochemistry techniques. It might not be feasible to produce quantities of
enzyme that are required, but there's a few more tricks that biochemists can
perform the make it better. We can modify the protein using genetic
engineering, to make it behave more efficiently, removing the sugar molecules
more quickly. We can also try different ways to manufacture the enzyme which
might improve the quantity we obtain. In short, this is a fantastic idea, and
worth pursuing further, and I'm still annoyed I didn't think of it first!
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