2.4 - Proteins Flashcards
Polypeptides
Amino Acids are Linked Together by Condensation to Form Polypeptides
Drawing Molecular Diagrams to Show the Formation of Peptide Bond
There is a chain of atoms linked by single covalent bonds forming a backbone with the repeating sequence -N-C-C-
A hydrogen atom is linked by a single bond to each nitrogen in the backbone and an oxygen atom linked by a double bond to one of the carbon atoms
The amine (-NH2) and carboxyl (-COOH) groups are used in forming the peptide bond
20 Amino Acids
There are 20 different amino acids that can be used to build polypeptides, which are synthesized on ribosomes
R groups of the amino acids that give a polypeptide its character, and keep amino acids chemically diverse.
Polypeptides
Amino acids can be linked together in any sequence giving a huge range of possible polypeptides
Peptide bonds can be formed between any pair of amino acids
The number of amino acids in a polypeptide can be anything between 4 and tens of thousands
This gives an almost infinite number of possible unique amino acid sequences
Dipeptide - a molecule consisting of two amino acids linked by a peptide bond
Polypeptide - molecule consisting of many amino acids linked by peptide bonds
For a polypeptide of n amino acids there are 20n possible sequences.
The Amino Acid sequence of Polypeptides is Coded for by Genes
gene: unit of inheritance consisting of a specific DNA sequence
Information for the amino acid sequence of a polypeptide is stored in a sequence DNA sequence
Most genes in a cell store the amino acid sequence of a polypeptide. They use the genetic code to do this.
Levels of Protein Structure
The amino acid sequence determines the three dimensional conformation of a protein
Primary
Primary Structure: The order/ number of amino acids in a polypeptide chain.
Proteins are the stable, folded 3D structure of polypeptides
The primary structure is read from the NH2– terminal to the –COOH terminal.
Each amino acid is identified by its specific R group
Most polypeptides are between 50- 1000’s amino acids long.
Secondary
Local folding of the polypeptide backbone
Stabilized by H- bonds
Three types of secondary structures:
a - helix
Due to hydrophobic amino acids
and hydrophilic amino acid
arrangement
beta - pleated sheet
Due to H - Bonds between
amino acids
open loops
Keratin in hair consists of only primary and secondary structures, not tertiary structures
Collagen - also fibrous, contains a complex of 3 strands so has a quaternary structure.
Tertiary
Tertiary structure is the three dimensional conformation of a polypeptide.
The amino acid chain (in the helical, pleated or loop form) links itself in places to form the unique twisted or folded shape of the protein.
Stabilized by R group interactions
Hydrophobic core
The 3D structure give protein their
functional properties, such as active sites on enzymes
Tertiary structures can be functional
Quaternary
A number of tertiary polypeptides joined together.
Haemoglobin is a quaternary structure.
It is composed of four different polypeptide chains.
Each chain forms a tertiary structure called a haem group.
Examples of proteins with different numbers of polypeptides.
Prosthetic groups:
Proteins are often bound to inorganic groups. e.g. Haemoglobin
has four polypeptide ‘haem’ groups each associated with and Fe2+
lysozyme - Enzyme in secretions such as nasal mucus and
tears; it kills some bacteria by digesting the peptidoglycan in their cell walls.
integrin - Membrane protein used to make connections between structures inside and outside a cell.
collagen - Structural protein in tendons, ligaments, skin and blood vessel walls; it provides high tensile strength, with limited stretching.
hemoglobin - Transport protein in red blood cells; it binds oxygen in the lungs and releases it in tissues with a reduced oxygen concentration.
Protein Shapes
Fibrous
In fibrous proteins the amino acid sequence prevents folding up and
ensures that the chain of amino acids remains in an elongated form.
Globular
the polypeptides gradually fold up as they are made,
to develop the final conformation. This is stabilized by bonds between
the R groups of the amino acids that have been brought together by
the folding.
In globular proteins that are soluble in water, there are hydrophilic
R groups on the outside of the molecule and there are usually
hydrophobic groups on the inside.
In globular membrane proteins there
are regions with hydrophobic R groups on the outside of the molecule,
which are attracted to the hydrophobic canter of the membrane.
Denaturation
Denaturation: process in which the protein unravels and loses its native shape
Protein becomes inactive
Interactions between R groups are interrupted
heat causes denaturation by breaking interactions between R groups
pH affects charges on R groups, which disrupt interactions
Protein Functions
-Catalysis – there are thousands of different enzymes to catalyze specific chemical reactions within the cell or outside it.
● Muscle contraction – actin and myosin together cause the muscle contractions used in locomotion and transport around the body.
● Cytoskeletons – tubulin is the subunit of microtubules that give animals cells their shape and pull on chromosomes during mitosis.
● Tensile strengthening – fibrous proteins give tensile strength needed in skin, tendons, ligaments and blood vessel walls.
● Blood clotting – plasma proteins act as clotting factors that cause blood to turn from a liquid to a gel in wounds.
● Transport of nutrients and gases – proteins in blood help transport oxygen, carbon dioxide, iron and lipids.
● Cell adhesion – membrane proteins cause adjacent animal cells to stick to each other within tissues.
● Membrane transport – membrane proteins are used for facilitated diffusion and active transport, and also for electron transport during cell respiration and photosynthesis.
● Hormones – some such as insulin, FSH and LH are proteins, but hormones are chemically very diverse.
● Receptors – binding sites in membranes and cytoplasm for hormones, neurotransmitters, tastes and smells, and also receptors for light in the eye and in plants.
● Packing of DNA – histones are associated with DNA in eukaryotes and help chromosomes to condense during mitosis.
● Immunity – this is the most diverse group of proteins, as cells can make huge numbers of different antibodies.
Proteome
Proteome: all the proteins produced by a cell, a tissue, or an organism
Every individual has a unique proteome
A proteome is a variable as different cells will make different proteins
Genome - what proteins an organism can make
Environmental Factors - Temp, nutrition can affect what proteins are produced
Whereas the genome of an organism is xed, the proteome is variable because different cells in an organism make different proteins.
A cell can make different proteins over time depending on the activities of the cell
Proteomes are unique due to small differences in the sequences of amino acids
Every individual (even identical twins) has a unique proteome
