Proteins:
Globular, Membrane, Fibrous
1. List the three
main classes of proteins and two distinguishing features of each.
Globular: maintain tertiary
structure through the hydrophobic effect, soluble, most enzymes are globular
Membrane: are anchored to a
membrane, often contain a globular domain
Fibrous (Structural): one
repeating structural pattern, are insoluble, framework of connective tissue
2. Identify the most
significant force in determining the 30 structure of
globular proteins and list one secondary
way in which these proteins maintain their most active
conformation.
Hydrophobic
effect
Salt bridges
and disulfide bridges
3. Describe the main
features of an enzyme and distinguish
between coenzymes and prosthetic
groups.
· Are Globular Proteins.
· Are catalytically active proteins which convert substrate to product.
· Are specific for their substrate and reaction.
· Often require a supplementary chemical group or co-factor
Organic co-factors:
Co-enzymes-soluble, easily removed, usually derived from vitamins
Prosthetic Group- covalently
attached to the enzyme, good example is heme
4. Describe a common
structural modification of circulating
globular proteins and indicate which specific residues are modified.
Glycosylation
or attachment of a sugar
“O” linked:
Ser, thr
“N” linked:
Asn, Gln
5. Compare glycosylation of circulating proteins
that occurs in a specific, enzyme-directed fashion with the glycosylation
that can occur in a spontaneous and uncontrolled (“non-enzymatic) fashion. How does “non-enzymatic” glycosylation provide
useful clinical information
concerning diabetic patients?
Enzymatic: is
enzyme directed, specific, and involves anhydride formation
Non-Enzymatic:
not specific, involves Schiff base formation
Advanced Glycosylation
Endproducts (AGE’s): are glycosylated hemoglobin formed when blood glucose
levels remain high over extended periods of time, i.e. they are a signal of
poorly controlled blood glucose levels seen in diabetics who do not properly
minitor their blood sugar/insulin levels
6. Identify which
domain of a membrane protein is usually glycosylated and how membrane glycoproteins
may contain a vast amount of biological “information”.
Extracellular
domain
Because there is such a wide
variety of sugars, sugar modifications and different linkages possible
glycoproteins have high information content
High
information content allows for cell-cell recognition and communication
7. List three
functional categories of membrane proteins.
Receptors
Transporters
Cell-cell recognition proteins
Proteins-Structural
1. Describe the secondary and tertiary
structure of mature collagen and
explain the importance of the many glycine residues.
Primary
Structure: Gly-X-Y-Gly-X-Y-Gly (Gly very third residue)
Secondary Structure: Collagen
Helix, 3.0 residues per turn, particularly tight helix due to all of the Gly
(very small) residues
Tertiary Structure: Triple Helix,
three collagen helices braided together
Quaternary Structure: Fibril,
arrangement of triple helices held together by lysine allysine covalent cross
links
2. List five different
modifications of amino acid residues in collagen. Identify the cofactors
required for these modification reactions.
1) Proline is very abundant. Are
hydroxylated by prolyl hydroxylase to form hydroxyproline.
Co-factor: ascorbic acid (vitamin C). Talk about -ase.....
Proline --------> Hydroxyproline Enzyme: Prolyl Hydroxylase, Co-factor: Vitamin C
2) Lysine is abundant. Are hydroxylated by lysyl
hydroxylase to form hydroxylysine. Co-factor: ascorbic acid.
Lysine --------> Hydroxylysine Enzyme: Lysyl Hydroxylase, Co-factor: Vitamin C
3) Hydroxylysines can be glycosylated
on their newly acquired -OH groups. (sugar attachment)
HO-Lysine ------> "Glycosylation"
4) Cysteine residues at the ends of the
collagen helices are oxidized to form disulfide bridges, this places the
chains in proper orientation for triple helix formation.
2 Cysteine Residues -------> Disulfide Bridge
5) Covalent crosslinks in fibril formation:
(a) Oxidation of certain lysine and hydroxylysine residues converts them from amines to active aldehydes (allysines). Enzyme is lysyl oxidase, co-factor: copper, Vitamin B6.
Lysine-NH3+ ------> Allysine-C-H Enzyme: Lysyl Oxidase, Co-factor: Vitamin B6
(b) Reaction between these aldehydes and free amino groups on other lysine residues to form a "Schiff" base, then covalent crosslink.
Lysine-NH3+ + Allysine-C-H ---------> "Schiff" Base
3. Delineate the
steps in collagen maturation.
Nucleus:
Transcription
ER:
HO-Pro and HO-Lys
Glycosylation some HO-Lys
Disulfide Formation
Folding "Triple Helix"
Extracellular:
Assembly into Fibrils
Lys, HO-Lys to Allysine (active Aldehydes)
"Schiff" Base Formation (Covalent Crosslinks)
4. Identify the
feature unique to elastin plus one
feature it shares with collagen.
Four lysines are crosslinked in a unique fashion
called desmosine.
Two
features elastin and collagen share:
1) Some prolines are hydroxylated. (not Lysine, so no glycosylation)
2) Some Lysines are oxidized to active aldehydes (allysine).
5. Describe an important
stabilizing modification of a-keratin and a simple
intervention to disrupt and then reform this type of modification.
Keratins:
structural proteins found in skin, nails and hair
a-Keratins: a-helix
b-Keratins: b-sheet
In hair and nails the a-helices of adjacent strands are crosslinked by disulfide bridges.
Nails: numerous disulfide bridges are resistant to chemical alteration ----> "hard"
Hair: relatively few disulfides can be easily disrupted with "reducing" agents.
“a simple
intervention to disrupt and then reform this type of modification”:
Permanent Wave
1) Disulfides are reduced
2) Cosmetic rearrangement (curlers)
3) Free -SH groups are oxidized to reform disulfides