In molecules with a limitless range of potential shapes,
the things that can be accomplished are just as limitless. Here we
will deal with just a few very important but general applications to which
proteins are put in living systems, and please note that these
designations are artificial labels and some proteins can reasonably be
included in more than one group:
Some protein types have relatively stable forms and functions:
Some protein types have relatively stable forms and functions:
STRUCTURAL ELEMENTS, especially on small scales
like
on and within
cells. On a cellular level, proteins take the forms you
might associate with construction materials: cables, struts, and
sheets. Cells with particular forms are held in those shapes by
proteins. Above the cell level, proteins are common in structural
elements, such as cartilage, but there are non-proteins found at that
level as well, like the
starch cellulose in plants or the salt calcium carbonate in bones.
MOVEMENT ELEMENTS. With very few
exceptions, any kind of visible movement in living things, whether the
movement of microscopic cell parts or the movement of your own arm, is
exerted by protein systems. Force for movement is generally exerted
as a pull, which can come from compression of a spring-like protein or the
movement of
two proteins across one another, shortening the length of the
complex.
COMMUNICATION MOLECULES. These may work
several different ways, and this is a function often performed by
non-proteins as well. Communication may be over distances, as is
done by protein-based
hormones, smell-message pheromones, or
neurotransmitters carrying nerve impulse messages across tiny gaps
(synapses) between nerves in a sequence, or communication may be across
barriers, as is accomplished by many proteins embedded in cell membranes
that carry signals in and out. Another type of protein that could be
considered in this group are antibodies, which are
created by the
immune system with two active domains. One end of the molecule has domains specifically
built to attach to antigens, molecules (usually protein-based)
that have gotten into the body but which are different from any of the
body's molecules - they are foreign, and automatically treated as being
dangerous. An important source of antigens is external
molecules on disease-causing invaders like viruses and bacteria, but any
foreign molecule of sufficient size can stimulate an antibody response. The other
antibody domain is the communications domain, activated when
antibody-antigen binding changes the shape of the antibody molecule;
this
marks molecules and the invaders carrying them for various responses
designed to remove them. If your immune system is working properly,
you currently have antibodies in circulation to every disease organism
(among other things) that you have even been exposed to, which will
prevent new individuals carrying an old disease from being able to
"set up shop" in your system. Simply put, the antibodies
will "flag" returning invaders for immediate removal.
Some proteins function through changes and
transitional forms. Most of these are capable of
temporarily binding other molecules, called substrates or ligands, and
doing something with them, as antibodies above were mentioned doing:
CARRIERS / TRANSPORTERS. Proteins may form
temporary complexes with atoms and molecules that increase solubility, to
move materials around in circulation fluids, or may pass materials through
barriers such as cell membranes. A well-known carrier in humans are
our lipoproteins, lipid-carriers implicated in arterial problems.
RECEPTORS. These proteins change in
response to some input, often when another molecule connects to it
temporarily. Living things have many different types of receptors,
for things like communications molecules, nutrients, smells and tastes,
and even wavelengths ("colors"), levels of light, and magnetic
fields.
CONTROL AND REGULATION MOLECULES. These are
directly involved in adjusting the function of metabolic systems, often by
attaching to molecules that are vital to the flow of a particular process
or pathway.
ENZYMES. Important enough that
several later sections will be spent discussing their operations, these
are the
catalysts used in almost every known biological
systems. A catalyst speeds up the rate at which a chemical reaction
takes place by reducing the amount of energy it takes to get it started (activation
energy). Enzymes bind to substrates, bringing potential
pieces together in synthesis reactions (anabolism) so that they bind together,
or grabbing larger molecules and stressing them in ways that bring them to
split into separate pieces in breakdown reactions (catabolism). Enzymes are
usually very specific
for substrates; very few will react with multiple substrates.
This
three-dimensional pickiness, which is also found in things like
receptors and other binding proteins, is called specificity
or
stereospecificity.
Any kind of chemistry that occurs in organisms only occurs because
specific enzymes exist for any given step of any given process. For
example, sucrase
is a digestive enzyme used to break down table sugar, sucrose,
but it cannot react with the similar molecule in sucralose,
known commercially as
SplendaTM.
Enzyme types are usually named with an -ase ending; like
carbohydrates, however, all enzymes don't have this ending, but chemicals
with this ending are all enzymes. Like
any catalyst, when an enzyme has contributed to a chemical change, it
returns to its original form, ready to help again. However, just
because an enzyme emerges from a reaction in the same form it started as
does not mean that it doesn't change while the reaction is going
on. The old image of a solid lock-and-key does not give a truly
accurate idea of how these molecules work.
PRIONS.
These were at one time important functional proteins, but accidentally
became twisted into a dangerous new form that could cause other copies of
the proteins to change to prions - these are the closest thing nature gets
to actual zombies! These can cause diseases by shutting down the
protein's original function, and some cause
infectious diseases.
As mentioned earlier, this is just a list of major functions performed by proteins, without getting into the long list of more minor activity, and proteins often resist categorization and could be put into multiple classes from this list.
As mentioned earlier, this is just a list of major functions performed by proteins, without getting into the long list of more minor activity, and proteins often resist categorization and could be put into multiple classes from this list.
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