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13/02/2017 (Amino Acids and Polypeptide (Proteins and Proteomes (A…
13/02/2017
Amino Acids and Polypeptide
Drawing Amino Acids
Amino Acid Diversity
R-groups of amino acids varies
Amino Acids with hundreds of different R-groups can be produced in a Lab.
most living organisms include only 20 of them in the polypeptides synthesized by their ribosomes
Case of trends are significant to science, the use of the same repertoire of amino acids is one of the pieces of evidence supporting the theory that all living organisms share common ancestry
Few discrepancies in the trend
Two other amino acids are included in a few polypeptides
Selenocysteine
Pyrrolysine
Only a minority of polypeptide in a minorities of organisms contain either of these amino acids
Special mechanism are needed to incorporate them into polypeptides and it is more likely that there mechanisms evolved after the basic method of making polypeptide from the 20 amino acids
The 2 extra amino acids are therefore extra variations rather than a falsification of the theory that there are twenty basic amino acids in all organisms.
Proteins and Proteomes
A polypeptide is an unbranched chain of amino acids.
The number of amino acids is very variable and can be over 10,000, though most have been between 50 and 2,000 amino acids.
Chains of fewer than 40 amino acids are usually called peptides rather than polypeptides or proteins
Amino acids can be linked together in any sequence giving a huge range of possible polypeptides. If we consider a polypeptide with 100 amino acids, the number of possible sequences is 20^100, which is an almost unimaginable large number
Only a small proportion of the possible sequence of amino acids are ever made by living organisms
Particular sequences are made in very large quantities because they have useful properties
Over two million polypeptides have so far been discovered in living organisms
The amino acid sequence of a polypeptide is coded for by a gene. The sequences of bases in the DNA of the gene determines the sequence of amino acids in the polypeptide
A protein consists either of a single polypeptide of more than one polypeptide linked together
Polypeptides and Proteins
A proteome is all of the proteins produced by a cell, tissue or an organism
By contrast, the genome is all of its genes
Whereas the genome of an organism is fixed, the proteome is variable because different cells in an organism make different proteins
Even in a single cell the proteins that are made vary over time depending on the cell's activities
Proteins that are exacted from a tissue can be separated in a sheet of gel by electrophoresis and identified.
Within a species there are strong similarities in the proteome of all individuals but are also different.
The proteome of each individual is unique, partly because of different of activity but also because of small differences in the amino acid sequence of proteins.
possible exception of identical twins, no one has identical proteins, so each has a unique proteome
Even the proteomes of identical twins can become different with age
Peptide Bonds and Polypeptides
Protein structure and Function
Protein Conformations
The sequence of amnio acids in a polypeptide determines how this folding is done and so determines the conformation of a protein
Each time a polypeptide with a particular sequence of amino acids is synthesized on a ribosome, the conformation will tend to be precisely the same
The polypeptides of most proteins are folded up to produce a globular shape
The structure is stabilized by intramolecular bonds between the amino acids in the polypeptides that are brought together by the folding process
The conformation of a protein is its three-dimensional structure
Denaturation
Heat causes vibration breaking intramolecular bonds and cause the conformation to change.
pH increase by adding alkaline or acid
Functions of Proteins
Living organisms synthesize many different proteins with a wide range of functions
Rubisco
is the
enzyme
with an active site that catalyses the photosynthesis reaction that fixes carbon dioxide for the atmosphere, providing all the carbon needed by living organisms to make sugars
Insulin
is the
hormone
that is carried dissolved in the blood and binds specifically and reversibly to insulin receptors in the membranes of body cells, causing the cells to absorb glucose and lower the blood glucose
Immunoglobulins
are
antibodies
that bind to antigens on pathogens. The immune system can produce a huge range of immunoglobulins, each with a different type of binding site, allowing specific immunity against many different diseases
Rhodopsin
is the
pigment
that makes the rod cells of the retina light-sensitive. It has a non-amino acid part called
retinal
that absorbs a photon of light and when this happens the rod cell sends a nerve impulse to the brain
Collagen
is a
structural
protein. It has 3 polypeptides wound together to form a rope-like conformation and is used in skin preventing tearing, in bones to prevent fractures and in tendons and ligaments to give tensile strength
Spider silk
is a
structural
proteins that is used to make webs for catching prey and lifelines on which spiders suspend themselves. It has a very high tensile strength and becomes stronger when it is stretched so resisting breakage
