B1.2 Protein Flashcards
What are amino acids?
The building blocks of proteins.
Each amino acid has a central carbon atom called the alpha carbon.
Amino acids are amphiprotic, carboxyl group is acidic as it can donate a proton, amine group is basic as it can accept one.
What is bonded to the alpha carbon in the amino acid?
Single covalent bonds to nitrogen atom of an amine group (H2N), another carbon of carboxyl group (COOH), hydrogen atom and a side chain called the R group, which can one be a wide range of other possibilities.
What is a dipeptide?
Two amino acids linked by a condensation reaction,
What is a oligopeptide?
Chains of fewer than 20 amino acids. Polypeptides can contain any number of amino acids. Linked by peptide bonds
What are peptide bonds?
C-N bonds formed by condensation between amine of one amino acid and the carboxyl group of another at end of growing chain. This is catalysed by ribosomes. Peptide bonds are made using groups part of all amino acids, bond is the same, no matter what the R groups are.
How do plants obtain amino acids?
There are two different amino acids used by ribosomes to make polypeptides. Plants make these by photosynthesis.
How do animals obtain essential amino acids?
Obtained by food, as animals cannot synthesise in sufficient quantities.
What is an essential amino acid?
Amino acids that cannot be synthesised in sufficient quantities by the animal, so must be obtained from diet. There are 9 amino acids essential to us.
What are non essential amino acids?
Amino acids that can be synthesised by an animal using metabolic pathways, transforming one amino acid into another.
Foods vary in…
Amino acid content, possible to eat a protein rich diet and be deficient in an essential amino acid. Have a balance of amino acids similar to what is needed in the human diet.
What amino acids are gained from plant based foods?
Different balance and efficient in specific amino acids, cereals and wheat with low lysine content, peas and beans low in methionine. Both are essential to humans.
Function of ribosomes?
Linking amino acids together one at a time until a polypeptide is fully formed, can make peptide bonds between any amino acid pair. Ribosomes do not make random sequences, receive instructions by genetic code.
How do we calculate possible amino acid sequences?
Starting with dipeptides, both amino acids can be any of the 20, so there are 20^2 possible sequences. There are 20^3 possible tripeptide sequences. For a polypeptide of n amino acids, there are 20^n possible sequences.
What are some examples of polypeptides
- Beta endorphin, painkiller secreted by pituitary gland,31 amino acids.
- insulin, small protein, two short polypeptides, one with 21, the other with 30.
- alpha amylase, enzyme in saliva that starts the digestion of starch, 496 amino acids, with one chloride ion and one calcium ion associated.
- titin is the largest polypeptide discovered. It is part of a structure of muscle. In humans, 34,350 amino acids
How is the 3d dimensional conformation of proteins stabilised?
By bonds between R-group amino acids within the molecule e.g hydrogen bonds, disulfide bonds, ionic bonds etc. Most bonds are weak, and can be disrupted/broken, resulting a change to conformation. This is known as denaturation. Unable to return to former structure. Soluble proteins often become insoluble and form a precipitate.
Why is a precipitate formed?
Due to hydrophobic R-groups in the centre of the molecule becoming exposed to water by change in conformation.
Why heat causes denaturation?
Causes vibrations within the molecule that can break intermolecular bonds or interactions. Proteins vary in their heat tolerance.
What other factors cause proteins to denature?
Extremes of pH, can cause denaturation, because positive and negative charges of R groups are changed, breaking ionic bonds within the protein/causing new ionic bonds to form. As with heat, 3d structure is altered, protiens that are dissolved in water become insoluble.
What remains of carboxyl and amine groups after they have been used to make peptide bonds? (secondary structure)
there are C=O and N-H bonds, both are polar, with oxygen having a slight negative charge, hydrogen, slight positive charge.
hydrogen bonds can form between this groups, although weak, the frequency of C=O and N-H groups along chains allow many of them to form and collectively are strong enough to stabilize distinctive conformational structures within protein molecules.
How do Channel proteins in membranes allow hydrophilic solutes or water to diffuse across hydrophobic core of membrane?
They have hydrophilic regions with hydrophobic region in between, holds them in transmembrane position. In addition they have a tunnel lined with hydrophilic amino acids through centre of protein. Width and charge distribution of this channel allows specific hydrophilic ions/molecules to pass through.
Conjugated proteins have one or more? haemoglobin as an example
non polypeptide subunits in addition to their polypeptides. Haemoglobin consists of polypeptide chains, each associated with a haem group. Inclusion of non polypeptide components increases chemical and functional diversity of proteins. Haem group binds oxygen, allowing haemoglobin to transprot oxyygen.
Explain collagen as an example of a fibrous protein.
- Collagen, fibrous protein, the quaternary structure is three polypeptides, wound together into a triple helix.
- Primary structure is a repeating sequence of P-G-X.
- P as in (hydroxy)proline, which has a special property in preventing formation of a alpha helix. The winding together of polypeptides would be impossible if they were alpha helices.
- R group of every third amino acid faces inwards towards centre of triple helix and glycine is the only amino acid with an R group small enough to fit. It is a single hydrogen atom.
- rope like structure of collagen gives high tensile strength
- R group of amino acid X faces outwards and is variable, allowing many variations of collagen to be produced for variety of uses in skin, tendons, ligaments etc.
Globular proteins consist of?
A rounded shape, formed by folding up of polypeptides. Shape is very intricate and is stabilized by bonds between the R groups of the amino acids that have been brought together by the folding.
- An insulin molecule has conformation needed to bind to a specific site on the insulin receptor. This allows a specific and unambiguous signal to be sent to body cells when blood sugar concentration is too high.
