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The Roles of Protein in Living Organisms

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The Roles of Protein in Living Organisms

Before we look at the roles of protein we must first look briefly at
the structure of protein to understand how this affects their role in
living organisms. Proteins contain the elements Hydrogen (H), Carbon
(C), Oxygen (O) and Nitrogen (N) and sometimes phosphorus and sulphur
which can be found in the R group. The R group is important in amino
acids because it can form the active site of enzymes and forms bonds
with different molecules or within the molecule itself. Large protein
molecules are made up of monomers called amino acids, each one is
different and so when joined together, with a peptide bond, give each
protein different properties and uses. These amino acids can be joined
in four ways: primary structure, secondary structure, tertiary
structure and quaternary structure, which affect the roles and bonds
that they have. The structure of a basic amino acid is as follows:

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R could be CH3, CH2 or simply H



C
=


OH

H

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H


The first use of proteins is in the cell membrane where they are
channel or carrier proteins and they help with facilitated diffusion
and active transport. They are often globular proteins that are
tightly packed with polar side groups on the outside to enhance their
solubility in water and non-polar folded on the inside to keep water
from getting in and unfolding them. In facilitated diffusion the
protein, acting as a transporter, binds with the glucose molecule and
changes shape so that the glucose molecule is transported through the
membrane onto the other side. The glucose then detaches itself from
the protein which returns to its original shape. Overall proteins role
is to form channels through the membrane and facilitate transfer of
the molecules in accordance with their electrical and chemical
gradients.

The second and probably most widely recognised role of proteins in
living structures is enzymes; they act as organic catalysts and
catalyse many biochemical reactions in the body and in the world
around us. This involves the tertiary structure of proteins where
there are bonds held together by hydrogen and ionicly as well as
sulphur bridges. This encourages and helps with the highly specific
nature of enzymes and their ability to take energy from one reaction
to assist their own. Enzymes are found in nearly every living
organism, for example even in fungi such as Rhizopus where enzymes are
secreted on to the substrate, brown bread, to break it down so it can
be absorbed and used as food for the Rhizopus to grow. Some examples
of enzymes are :

Y Lactase to rid milk of lactose

Y Lipase to break down fats into fatty acids and glycerol

Y Pepsin breaks down protein into polypeptides

Another use of protein is in the immune system of the human body,
these are called immuno-proteins. They are a blood protein whose
activities affect or play a role in the immune system. Some soluble
proteins bind with antigen to induce or affect antigen specific
functions in immuno-regulation and/or hypersensitivity; these then
attack microbes to rid the body of infections etc, another way in
which protein helps protect the body is fibrinogen. When the body
experiences a cut in the skin then the platelets form a "plug" while
the fibrinogen in the blood plasma, which on contact with the air,
hardens to form fibrin which seals the cut and prevents anything
entering the body through the cut.

The next use of proteins in living organisms is as hormones, although
many of the hormones have protein structure not all hormones are
proteins, the hormone protein is generally quite small compared to
most. Hormones are passed through the blood and trigger reactions in
other parts of the body i.e. regulating homeostasis. Insulin is one
such hormone and is a globular protein, like other proteins its shape
is important as it binds to specific receptors on target cells to
regulate the amount of sugar in blood. Glucagon is the growth hormone
which is also a globular protein and encourages growth in humans.

Proteins can also act as transporters and are commonly found as
globular proteins when in this role. Haemoglobin is an example of a
transport protein; it carries oxygen from the lungs to the tissue
whilst myoglobin performs a similar function by taking oxygen from the
haemoglobin and storing it or carrying it around until the muscles
need it. Haemoglobin is well adapted to its role, as all protein
molecules are, because it can easily bond with oxygen under certain
conditions but also release oxygen under other conditions. Haemoglobin
can carry four oxygen molecules and changes its shape slightly when
oxygen binds to it so that more oxygen molecules can bind with it.
Another transport protein is transferrin which carries iron, so as you
can see protein has a very important role in the human body to
transport many vital molecules.

Storage is yet another role which protein plays, it is a prominent
part of seed germination where it is stored in the seed and used as
nitrogen sources for the developing embryo during germination. It can
also be found in the storage organs of plants such as roots and shoot
tubers i.e. potatoes; in these types of situation the normal
properties of the protein are:

Y The proteins have no enzymatic activities

Y They occur normally in an aggregated state within a
membrane-surrounded vesicle (protein bodies, aleuron grains).

Y They are often built from a number of different polypeptide chains

These plant proteins are also very important for human nutrition, as
we need the amino acids to replace the ones we can't make ourselves.
This is also very important in pregnant women as casein is a valuable
supply of protein to the baby, which needs it to develop muscles and
bones. It is also vital for humans to store iron in the form of
ferritin as it can be very toxic if we do not have the ability to
store and release iron in a controlled fashion, the ferratins are
protein bound as a ferric oxide-phosphate mineral.

Structural proteins are of a fibrous nature, and the most familiar of
the fibrous proteins are the keratins. They form the protective
coating of all land vertebrates: skin, fur, hair, wool, claws, nails,
hooves, horns, scales, beaks and feathers. If we look at our hair it
grows at about half an inch per month which means we need to grow 36
amino acids a second so we need a big intake of protein each day.
Collagen is another fibrous structural protein found in humans, with
its insoluble fibres it has very high tensile strength and is found in
bone, tendons, cartilage and skin when it needs extra support.

Contractile proteins are proteins which allow contraction in muscles
and therefore movement. Actin and myosin form sarcomeres, which are
the basic units of contraction, actin is a thin filament contractile
protein and myosin is a thick filament. Myosin filaments slide along
and pull the actin filaments toward the centre of a sarcomere during
contraction and so movement happens.

Buffer proteins help maintain balance in the human body. Blood
proteins have a charge which helps maintain the pH of plasma, in this
way it is a very important protein because the blood transports lots
of vital substances all over the body so without the right pH we could
not function properly. From reading this essay you can see the
importance of proteins in the world around us and how we would be lost
without them, for example we would not have any hair, nails or
skin!!!!

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