Proteins, polysaccharides and lipids Flashcards
What are the two structures found in the secondary structure of proteins?
a-helix and B-pleated sheet
Description of a-helix
A rod-like, right handed coil with 3.6 residues per turn.
How to a-helices form?
Hydrogen bonds between the CO of one amino acid and the NH of an amino acid 4 residues ahead
What proteins contain a-helices?
Strong, extensible proteins e.g. Hb, myoglobin, keratins, fibrins, myosin
Description of B-pleated sheets
Made of several zig-zag polypeptide chains that run either in parallel or antiparallel alongside each other. The CO and NH groups align with hydrogen bonding forming between them.
What proteins contain B-pleated sheets?
Proteins that require flexibility e.g. silk fibroin has the anti-parallel arrangement
Which is the only protein containing a triple helix?
Collagen
What is the term used to describe the three chains winding around each other in collagen (rope-like)?
Tropocollagen
How many amino acids are approximately found in one polypeptide chain of collagen?
~1000 aa per chain therefore about 3000 aa in one collagen molecule
Where do hydrogen bonds exist in collagen?
There are no H bonds within each chain, only between chains. H bonds between Hydroxyproline and Hydroxylysine residues hold the three strands together.
What is the repeating structure in collagen polypeptide chains?
X-Pro-Gly or X-Hyp-Gly
Where do covalent bonds exist in collagen?
Between Lys and His to form inter and intra-molecular cross-links
Which position does Glycine take in collagen?
Small Glycine residue sits inside the helix while the bulky R groups either side project outwards.
What are the features of fibrous proteins?
Insoluble, metabolically unreactive, rope-like, strong
What is the function of fibrous proteins?
Structural function
Examples of fibrous proteins
Collagen, keratin, fibrin, elastin, myosin
What are the features of globular proteins?
Spherical as the backbone folds in on itself. Water-soluble and compact.
What level of protein structure do globular proteins typically exhibit?
Tertiary (e.g. myoglobin and actin) and Quaternary (e.g. Hb)
Function of myoglobin
oxygen storage in muscle
Structure of myoglobin
Globular protein with a tertiary structure (single chain 153aa). 8 helical regions joined by random coiling which produces major directional changes. No B-pleated sheets
Structure of the interior of myoglobin
Contains entirely non-polar residues except 2 polar His residues which allow the attachment of haem group
Where is the prosthetic haem group found in myoglobin?
In the hydrophobic pocket where it is held in position by hydrophobic interactions between the haem porphyrin ring and non-polar side chains of amino acids
What would happen if the haem group in myoglobin was absent?
Myoglobin would become an apoprotein - not as tightly folded
What level of protein structure does Haemoglobin exhibit?
Quaternary structure - 2a and 2B subunits folded similarly to myoglobin (141aa in a-chain and 146 aa in B)
Arrangement of haem groups in haemoglobin
One haem group per subunit. Lie on the surface of the molecule far apart in individual pockets
Arrangement of the subunits in Hb
Hb can be split into 2 halves - a1B1 and a2B2 which are irregular in shape and leave an open central channel when fitted together. Each a subunit is in contact with both B chains. there are few interactions between the 2 a chains or 2 B chains.
Difference between peripheral and integral membrane proteins
Peripheral proteins lie on the membrane surface while integral proteins lie within the lipid bilayer.
What are the two forms of integral membrane proteins that facilitate transport across the membrane?
Channel and carrier proteins
Describe the 2 mechanisms in which carrier proteins transport molecules
- Involves one protein which binds, rotates and releases the molecule
- Shuttle mechanism involving the molecule being transferred along the chain of proteins
Function of messenger proteins
allow cells to communicate with each other e.g. hormones
In what ways can messenger proteins alter cell activity?
- influence rate of synthesis of enzymes and other proteins
- affect rate of enzymatic catalysis
- alter cell membrane permeability
Examples of messenger proteins
insulin, glucagon, human growth hormone
What molecules can hormones be made of?
Proteins, amino acid derivatives or steroid
How much can enzymes increase the rate of reaction?
by up to 10^20
What are the monomers of polysaccharides?
Monosaccharides / sugars
Function of polysaccharides?
Storage e.g. glycogen, starch. Structure e.g. cellulose, chondroitin sulphates, peptidoglycan
What are the two types of monosaccharides?
Aldoses (contain aldehyde -CHO group) and Ketoses (contain ketone -CO group)
How are disaccharides formed?
Condensation reaction of monosaccharides to produce a glycosidic bond
Which monosaccharides make up sucrose?
Glucose and fructose
Which monosaccharides make up lactose?
Glucose and galactose
Why are there a huge variety of polysaccharides?
Monosaccharides have isomers - e.g. starch is made of a-glucose while cellulose is made of B-glucose
Structure of starch
Amylose - unbranched, helical chain of a-glucose joined by a 1,4-glycosidic bonds.
Amylopectin - a-glucose molecules joined by a 1,4-glycosidic bonds and a 1,6-glycosidic bonds every 30 molecules to produce a branched structure
Structure of glycogen
more highly branched than starch - more a 1,6-glycosidic bonds
Structure of cellulose
Made of parallel polymers of unbranched B 1,4 linked glucose molecules which are held together by H bonds between the OH groups in C3 and C6. About 80 cellulose molecules form a microfibril.
Alternative name for fatty acids
acyl lipids
What molecule are glycerides based on?
Glycerol
Which diacylglyceride (2 FA) is a major membrane phospholipid?
Phosphatidylcholine - made up of 2 fatty acids (hydrophobic tails) and a glycerol, phosphate and choline group (hydrophilic head)
Which glycerides are storage molecules?
Triacylglycerides (3 FA)
What are the 3 main types of lipid?
Triacylglycerides (storage), Diacylglycerides (membrane), Sterols (membrane)
What feature affects the physical properties of lipids?
Degree of saturation (affects London forces)
What is the state of saturated fatty acids at room temperature?
Solid - lack of kinks in the FA tails allows molecules to pack closely together
What is the state of unsaturated fatty acids at room temperature?
Liquid - cis C=C cause kinks in the FA tails which prevents the close packing of lipid molecules
Function of sterols
Can form hormones e.g. oestrogen, cortisol, testosterone. Important in membranes e.g. cholesterol
