proteins polysaccharides and lipids Flashcards
(39 cards)
what are examples of secondary structures?
- a helix
- b pleated sheet
- triple helix
what is the a-helix?
Rod-like, right-handed
Found in strong, extensible proteins
Stabilised by hydrogen bonds
CO of each amino acid is H-bonded to NH of amino acid four residues ahead in sequence
3.6 residues/helix turn
what are examples of where alpha helixes are found?
haemoglobin, myoglobin, keratins, fibrins, myosin
what is a b-pleated sheet?
Zigzag chains
Place several chains side by side, CO and NH groups align, H-bonding occurs → sheet-like structure
Parallel: chains run in same direction
Anti-parallel: chains run in opposite directions
Found in proteins where flexibility needed
where is an example of where b- pleated sheets are found?
silk fibroin
what is a triple helix?
Collagen only
Major component of connective tissue (skin, bone, tendon)
Very strong, water-insoluble fibres
Three chains wound round each other (rope) → tropocollagen
~ 1000 amino acids per chain
No H bonds in each individual chain
Each chain has repeating structure:
X-Pro-Gly or X-Hyp-Gly
Three strands held together by H bonds between Hyp and hydroxylysine residues
Small Gly residue sits inside helix; bulky R groups either side of Gly project outwards
Intra- and inter-molecularly cross-linked by covalent bonds between Lys and His
what is a triple helix?
Collagen only
Major component of connective tissue (skin, bone, tendon)
Very strong, water-insoluble fibres
Three chains wound round each other (rope) → tropocollagen
~ 1000 amino acids per chain
No H bonds in each individual chain
Each chain has repeating structure:
X-Pro-Gly or X-Hyp-Gly
Three strands held together by H bonds between Hyp and hydroxylysine residues
Small Gly residue sits inside helix; bulky R groups either side of Gly project outwards
Intra- and inter-molecularly cross-linked by covalent bonds between Lys and His
what are fibrous proteins?
Insoluble
Metabolically unreactive
Principally structural proteins
what are examples of fibrous proteins?
collagen
keratin (skin, hair, nails, fur, wool)
fibrin (bloot clots)
elastin (elastic fibres of connective tissue eg. arterial walls)
myosin (muscle)
what are examples of fibrous proteins?
collagen
keratin (skin, hair, nails, fur, wool)
fibrin (bloot clots)
elastin (elastic fibres of connective tissue eg. arterial walls)
myosin (muscle)
what are globular proteins?
Spherical
Backbone folds on itself
Water-soluble, compact structures
Usually have tertiary and quaternary structures eg. myoglobin and actin (3º), haemoglobin (4º)
what is an example of a globular protein?
myoglobin, haemoglobin
what is myoglobin?
- Oxygen storage in muscle
- Globular, associated with 3º structures
- Single chain (153 aa)
- Contains eight helical regions (75% of all aa)
- No β-pleated sheet regions
- Helical segments joined by regions of random coiling where chain makes a major directional change
- Interior contains entirely non-polar residues except for two polar His residues (attachment and function of haem group)
- Prosthetic haem group: in hydrophobic pocket, held in position by hydrophobic interactions between haem porphyrin ring and non-polar side chains of aa in surrounding helical segments
- Absence of haem group → apoprotein, not as tightly folded
what is haemoglobin?
- Oxygen transport
- Associated with 4º structures
- Two pairs of polypeptide chains (2α, 2β) folded in a shape similar to myoglobin
- Spheroidal molecule
- Four haem groups lie on surface of molecule in individual pockets, far apart
- α-chain = 141 aa; β-chain = 146 aa
- Each α subunit is in contact with both β chains
- Few interactions occur between the two α or the two β chains
- α1β1 and α2β2 half-molecules irregular in shape → central open channel when fitted together
what are the types of membrane proteins?
peripheral- membrane surface
integral- within lipid bilayer
channel proteins
carrier proteins
what is the function of a channel protein?
- forms a channel through the membrane
- facilitates movements of small molecules across the membrane (simple diffusion)
what is the function of carrier proteins?
bind to transported molecules (facilitated diffusion)
what is an example of a messenger protein?
hormones
what is the function of messenger proteins?
allows cells to communicate with each other
describe the modes of action of messenger proteins
- Influence rate of synthesis of enzymes and other proteins
- Affect rate of enzymatic catalysis
- Alter permeability of cell membranes
describe the stages of hormones acting as messenger proteins
Each hormone has its own corresponding membrane-bound receptor
Hormone binds receptor → message relayed to inside of cell → cascade of events → cellular action
what are examples of hormone messenger proteins and what are their functions?
Insulin: sugar uptake by cells from bloodstream
Glucagon: sugar release by cells into bloodstream
Human growth hormone
Hormones can be protein / polypeptide,
amino acid derivatives or steroid
what are enzymes?
Biological catalysts
Globular proteins
Increase reaction rates by up to 1020
Highly specific (reaction, substrate)
what is the difference in roles of myosin and actin during muscle contraction?
- myosin is fibrous
- actin is globular
