Collagen Structure and Function

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Questions and Answers

What is the primary sequence characteristic of collagen polypeptides?

Random amino acid sequence without repeats

Alternating sequence of alanine and glycine

Repeating tripeptide sequence of X-Y-Gly (correct)

Repeating tripeptide sequence of Gly-X-Y

In collagen's secondary structure, what unique feature contributes to its helical shape?

Presence of phenylalanine in every turn

Hydrogen bonding between all amino acids

Supercoiling of polypeptide chains

Glycine residues found in every turn (correct)

How are the three chains in collagen's tertiary structure organized?

In parallel lines with no interaction

Randomly dispersed without structure

In a right-handed superhelical coiled coil (correct)

In a left-handed helix formation

What consequence does aging have on collagen's structure?

Increases cross-linking frequency, making tissues brittle Signup and view all the answers

Which vitamins are crucial for collagen synthesis through enzyme activation?

Ascorbic acid (Vitamin C) Signup and view all the answers

What structural change occurs in collagens associated with Osteogenesis imperfecta?

Glycine residues are replaced by larger amino acids Signup and view all the answers

What role does prolyl hydroxylase play in collagen synthesis?

It hydroxylates proline residues in collagen Signup and view all the answers

What results from a mutation in collagen genes associated with Ehlers-Danlos syndrome?

Structural abnormalities due to inappropriate amino acid replacement Signup and view all the answers

What is the primary function of myoglobin (Mb)?

O2 storage protein Signup and view all the answers

Which statement correctly describes hemoglobin (Hb)?

Hb functions primarily as an O2 transport protein. Signup and view all the answers

How does the presence of low pH in actively metabolizing tissues affect hemoglobin's function?

It causes Hb to release O2. Signup and view all the answers

What role does vitamin C play concerning prolyl hydroxylase?

It maintains prolyl hydroxylase in its active state. Signup and view all the answers

What characteristic distinguishes elastin's structure?

It has a random coil conformation rich in nonpolar amino acids. Signup and view all the answers

What is a function of the heme group in proteins?

Binding to oxygen in a prosthetic role. Signup and view all the answers

Which structural feature of hemoglobin allows for positive cooperativity?

The tetrameric arrangement of subunits. Signup and view all the answers

What is the impact of bisphosphoglycerate on hemoglobin's affinity for oxygen?

It decreases Hb's oxygen affinity. Signup and view all the answers

What is the primary effect of the hydrophobic patch produced by the hemoglobin defect in sickle cell anemia?

It causes hemoglobin to aggregate. Signup and view all the answers

Which hormone is produced by the β-cells of the Islets of Langerhans in the pancreas?

Insulin Signup and view all the answers

What is the primary function of glucagon?

To elevate blood glucose levels. Signup and view all the answers

What distinguishes Type 2 diabetes from Type 1 diabetes?

Type 1 results from the pancreas failing to produce insulin. Signup and view all the answers

What is the primary structure of an antibody?

Two identical light chains and two heavy chains. Signup and view all the answers

Which antibody is primarily found in mucosal areas and protects against pathogens?

IgA Signup and view all the answers

What role do the variable regions of an antibody serve?

To bind to the antigen. Signup and view all the answers

Which of the following best describes the function of IgE antibodies?

They are involved in allergic reactions and protection against parasites. Signup and view all the answers

Study Notes

Collagen

Most abundant protein in mammals
Insoluble fibers with high tensile strength
Found in connective tissues including skin, bone, tendon, cartilage, and teeth
Three polypeptide chains associate in a right-handed superhelical coiled coil structure to form a triple-helical structure (tropocollagen).
Collagen fibrils are made up of tropocollagen molecules aligned in a staggered fashion and cross-linked for strength.

Collagen: Primary Structure

Repeating tripeptide sequence of X-Y-Gly
X can be any amino acid.
Y is often Pro or Hydroxyproline.
Lys, 5-Hydroxylysine, and His residues present.

Collagen: Secondary Structure

Left-handed helix with 3.3 amino acids per turn.
Gly residues found in every turn, permitting sharp twist of the helix.
Pitch is 10Å.

Collagen: Tertiary Structure

Three parallel polypeptide chains associate in a right-handed superhelical coiled coil structure.
Chains are held together by hydrogen bonds involving hydroxyproline and hydroxylysine.

Collagen: Quaternary Structure

Collagen fibrils are made up of tropocollagen molecules aligned in a staggered fashion and cross-linked for strength.
Individual chains of triple-helical collagen molecules and collagen molecules of fibrils are cross-linked by covalent bonds involving Lys, 5-Hydroxylysine, and His residues.
Increased cross-linking with aging contributes to the brittle properties of aging connective tissue.

Collagen: Diseases caused by defects

Osteogenesis imperfecta and Ehlers-Danlos syndrome are caused by mutant alleles of collagen genes.
Often, an amino acid with a relatively large R group such as Cys or Ser replaces Gly residues in mutant collagens, disrupting their structure and function.
In the synthesis of collagen, ascorbic acid (Vitamin C) is required as a cofactor for the enzyme activities of prolyl hydroxylase and lysyl hydroxylase.
These enzymes are responsible for the hydroxylation of Pro and Lys in collagen.
Scurvy results from a deficiency of vitamin C.

Elastin

Connective tissue protein with elastic properties.
Found in lungs, walls of large blood vessels, and elastic ligaments.
Elastin polypeptide chain consists predominantly of nonpolar amino acid residues.
Elastin fibers associate by desmosine crosslinks.
Can be stretched to several times their normal length and recoil to original shape when relaxed.

Heme

Prosthetic group (non-amino acid component required in protein function).
Consists of Fe2+ bound at the center of protoporphyrin ring.
Fe2+ has six coordination sites.
Contributes to the red color of the blood.

Myoglobin

Single polypeptide chain (153 aa long) with eight -helices.
Globular protein.
Single heme group.
High O2 affinity.
Primary function is O2 storage.
Abundant in skeletal muscle.

Hemoglobin

Tetrameric protein, made up of 2α subunits (141 aa each) and 2β subunits (153 aa each) in a tetrahedral arrangement.
Each monomer is similar to Mb structure.
Each subunit has one heme group.
Hb can bind up to 4 O2 molecules.
Primary function is O2 transport.

Hemoglobin: Function

Binds to O2 reversibly and exhibits positive cooperativity.
Hb’s affinity to O2 increases as more O2 is bound to it.
Relative to Mb, Hb has low affinity to O2 making it a more efficient O2 carrier than Mb (can easily release O2).
Factors affecting Hb affinity to O2: pH, presence of CO2, and bisphosphoglycerate.

Oxygen Transport by Hemoglobin

In actively metabolizing tissue, pH is low decreasing Hb affinity to O2.
Hb releases O2 and binds to both CO2 and H+.
In the lungs, pH is high increasing Hb affinity to O2.
Hb releases both CO2 and H+ and binds oxygen.

Diseases due to Hemoglobin Defects

Sickle cell anemia is caused by the substitution of Val to Glu on the 6th aa of the β subunit.
The changed tertiary structure produces a hydrophobic patch on the surface of Hb.
The hydrophobic patch interacts with other hydrophobic patches causing the Hb to aggregate into strands that align into insoluble fibers.
Less efficient in delivering O2.

Insulin

Produced from β-cells of the Islets of Langerhans in the pancreas.
Made up of two polypeptide chains (51 aa) with inter- and intra-chain disulfide linkage.
Promotes glucose intake from blood into fat, liver, and skeletal muscle cells (hypoglycemic hormone.)
Glucose in the cell is converted to glycogen (glycogenesis) or fats (lipogenesis.)

Glucagon

Produced from α-cells of the Islets of Langerhans in the pancreas.
Made up of a single polypeptide chain (29 aa).
Elevates the blood glucose level by promoting synthesis of glucose (gluconeogenesis) and breakdown of glycogen into glucose molecules (glycogenolysis).
Hyperglycemic hormone.

Insulin Diseases

Diabetes mellitus
- Type 1: failure of the pancreas to produce enough insulin.
- Type 2: a condition in which cells fail to respond to insulin (insulin resistance).

Antibody (Ab)

Also known as immunoglobulins.
Secreted mostly by differentiated B lymphocytes (plasma cells).
Y-shaped molecule consists of two identical light chains (~25kDa) and two identical heavy chains (~50kDa) held together by disulfide bonds.
Each chain contains constant and variable regions.
The variable regions in Ab serve as binding sites (paratope) to the epitope of the antigen.

Antibody Isotypes

IgA: found in mucosal areas and prevents colonization by pathogens.
IgD: functions mainly as an antigen receptor on B cells.
IgE: binds to allergens and triggers histamine release.

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Description

Explore the intricate structure of collagen, the most abundant protein in mammals, and its vital role in connective tissues. This quiz covers the primary, secondary, and tertiary structures of collagen, including its unique polypeptide composition and structural features.