2.4 - Proteins Flashcards
1
Q
Amino acids and polypeptides
A
- Amino acids are linked together by condensation to form polypeptides
- This reaction happens on ribosomes by a process called translation
- Polypeptides are a main component in proteins and in many proteins they are the only component
- Proteins can contain one, two or many polypeptides
- The condensation reaction involves the amino group NH2 of one amino acid and the carboxyl group COOH of another. Water is eliminated and a peptide bond is formed between the two amino acids
- A dipeptide is a molecule consisting of two amino acids linked by a peptide bond
- A polypeptide is a molecule consisting of many amino acids linked by peptide bonds
- Polypeptides can have up to 35,000 amino acids however less than 20 are called oligopeptides
- Twenty different amino acids are used by ribosomes to make polypeptides, the amino groups and the carboxyl groups are used up in forming the peptide bond, R groups give the polypeptide its character. R groups give living organisms a wide range of proteins. Due to the different R groups the 20 amino acids can be very diverse
- Some proteins contain amino acids that are not in the basic 20. This is usually due to one of the amino acids being modified after a polypeptide has been synthesised.
2
Q
Polypeptide diversity
A
- Amino acids can be linked together in any sequence giving a huge range of possible polypeptides
- Ribosomes link amino acids together one by one until a polypeptide is formed. The ribosomes make peptide bonds between the amino acids so any sequence is possible.
- For a polypeptide of N amino acids there is 20^n possible polypeptides, there is almost an infinite amount of possible polypeptides as proteins can have thousands of amino acids
3
Q
Genes and polypeptides
A
- The amino acid sequence of polypeptides is coded for by genes
- Living organisms can only produce a small fraction of the total number of amino acid sequences
- The amino acid sequence of each polypeptide is stored in a coded form in the base of a gene
- Most genes store the sequence of amino acids for a polypeptide using genetic code, three bases of the gene are needed to code for each amino acid in the polypeptide
4
Q
Proteins and polypeptides
A
- A protein may consist of a single polypeptide or more than one polypeptide linked together
- Lysozyme: consists of one polypeptide, kills some bacteria, used in mucus and tears
- Integrin: a membrane protein with two polypeptides, both have a hydrophobic portion embedded I the membrane
- Collagen: consists of three long polypeptides wound together to form a rope-like structure, this structure has a greater strength than the polypeptides would have if they were separate.
- Haemoglobin: consists of four polypeptides with associated non-polypeptide structures, the four parts of haemoglobin interact to transport oxygen more effectively in the tissue.
5
Q
Protein conformations
A
- The amino acid sequence determines the three-dimensional conformation of proteins
- Conformation of protein is a three-dimensional structure
- Fibrous structures such as collagens are often elongated with a repeating structure.
- Many proteins have a globular structure with an intricate shape
- Amino acids are added one by one to form a polypeptide, they are always added in the same sequence to make the same polypeptide, in globular proteins the polypeptide gradually folds up as it is made to develop the final conformation, this conformation is stabilised by bonds between the R groups of amino acids that have been brought together by folding
- Globular proteins that are soluble to water have hydrophilic R groups on the outside of the molecule and often 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 centre of the membrane
- In fibrous proteins the amino acids sequence prevents folding up and ensures that the chain of amino acids remains in an elongated form.
6
Q
Protein functions
A
- Catalysis: there are thousands of different enzymes to catalyse 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 too sick 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 very chemically diverse
- Receptors: binding sites in membranes and cytoplasm for hormones, neurotransmitters, tastes and smell, 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
- Proteins can be used artificially for pregnancy tests, and companies produce different proteins for treating diseases
7
Q
Proteomes
A
- Every individual has a unique proteome
- A protein is all of the proteins produced by a cell, tissue or an organism.
- Whereas the genome is fixed the proteome is variable because different cells in organisms make different proteins, proteins made depend on a cells activity, therefore the proteome reveals what is happening in an organism
- Within a species there are similarities however the proteome of every individual is unique
