Work Sheet 5

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 -keratin and a simple intervention to disrupt and then reform this type of modification.

Keratins:

structural proteins found in skin, nails and hair

-Keratins: -helix

-Keratins: -sheet

In hair and nails the -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