Biochemistry of saliva

Question 1

What is the function of the saliva?

Answer 1

• Saliva is a clear liquid secreted by the salivary

1) Tasting & moistening of food

2) Bolus formation

3) Facilitation of the speech and swallowing

4) Buffering (the saliva contains proteins and phosphate buffers and bicarbonate which stabilizes the pH)

5) Facilitates digestion via the amylase and lipase enzyme

6) Contains antimicrobial (lysozymes, peroxidase, histatins)

7) Inhibits calcium deposition

8) Protects the tooth from decay

Question 2

What is the composition of the saliva?

Answer 2

• The saliva is generally hypotonic

1) Water 98%

2) Electrolytes (Low Na, Low Cl, High K, High HCO3)

3) Mucous (mucin as a glycoprotein)

4) Enzymes (a-amylase “starch to sugar”, Lingual lipase “begins fat digestion”, salivary peroxidase “protects against bacteria”, Lysozymes “breaks the bacterial cell wall”)

5) Proteins (like histatins)

6) IgA (anti-body, which has an immune function of the mucous membranes)

Question 3

Describe the process of the saliva production

Answer 3

1) Initially the saliva is isotonic from the plasma

2) Then as the secretion moves down the ducts, the cells lining the duct will modify the composition of the secretion by reabsorbing Na+ and Cl- and secreting HCO3- and K+

• The ducts are impermeable to water which increases the solute concentration of the saliva, giving us a final product that is hypotonic

Question 4

What determines the tonicity of the saliva?

Answer 4

It depends on the time it takes the saliva to pass through the ducts and gets released

• At high flow rate the saliva will be isotonic as there is less time for the cells to modify it, in low flow rate the saliva is hypotonic

Question 5

What are the salivary proteins the prevents the precipitation of the calcium phosphate?

Answer 5

1) Proline rich protein

2) Statherins

Question 6

What are the salivary proteins the acts as an anti-microbial?

Answer 6

1) Histatins

2) Cystatins

3) IgA

4) Mucin

5) Lysozymes and peroxidase

Question 7

How are monosaccharides broken down?

Answer 7

Sugars like glucose, fructose and galactose have no bonds and they are absorbed directly

Question 8

How are disaccharides broken down?

Answer 8

Sugars like maltose, sucrose and lactose, who have two monosaccharides linked together via a glycosidic bond are broken down to monosaccharides via specific enzymes like maltase, sucrase, and lactase

Question 9

How are polysaccharides broken down?

Answer 9

Sugars like starch, glycogen, and cellulose, these are long chains with variable bonds (starch has a-1,4 and a-1,6 and it is broken down by amylase into maltose and glucose), (glycogen has the same glycosidic bond and it is broken down by glycogen phosphorylase)

• Cellulose however has a b-1,4 glycosidic bond which makes it indigestible for humans

Question 10

What forms the glycosidic bonds?

Answer 10

They are formed by a condensation reaction between the hydroxyl residue on carbon-1 and the C-4 on two molecules

Question 11

How does a-amylase work?

Answer 11

• a-amylase works in the mouth only and it gets destroyed in the stomach (denatured)
• In the S.I the pancreatic amylase continues to breakdown starch into maltose, where then enzyme like maltase breaks it to a monosaccharide

Question 12

Describe the salivary amylase

Answer 12

• It is a calcium metalloenzyme (enzyme containing a metal ion like zinc, iron or copper)
• It breaks a-1,4 glycosidic bonds of the starch, it converts starch into maltose, maltotriose and dextrin
• Salivary amylase is a endoglycosidase (which is an enzyme that cuts sugar chains in the middle)
• Works in a pH of 5.6-7

Question 13

What secretes the lingual lipase?

Answer 13

• It is secreted by Von Ebner’s glands of the tongue

Question 14

What are the functions of the lingual lipase?

Answer 14

• Similar to pancreatic lipase it digests lipids

1) Involved in the first phase of fat digestion

2) It is involved in the digestion of milk fat in newborns

3) It does not need bile salts and works in a wide range of pH

4) It converts T.G to monoacylglycerol & FFA

Question 15

What makes the lingual lipase unique?

Answer 15

It has the ability to catalyze reactions without bile salts and it works in a wide range of pH environment

Question 16

What is the mechanism of action of the lingual lipase?

Answer 16

It Hydrolyzes the first fatty acid then the third producing 2-monoacylglycerol and two fatty acid molecules (bonds being broken are called ester bonds)

Question 17

What is the salivary peroxidase system?

Answer 17

• It is a system that prevents the accumulation of hydrogen peroxide (H2O2) and it inactivates a lot of carcinogenic and mutagenic compounds, it does this through the enzyme salivary peroxidase

Question 18

What is a lysozyme?

Answer 18

• It is a protein present in a lot of mucosal secretions (like tears, saliva, and mucus)
• Lysozymes are enzymes that damages the bacterial wall by catalyzing the hydrolysis of 1,4-beta-linkages in the bacterial peptidoglycan component of the bacterial wall

Question 19

What is the main gland that secretes mucin?

Answer 19

The sublingual salivary gland

Question 20

What are the three amino acids that allows glycosylation to occur?

Answer 20

1) Asparagine (contains a amide group which forms a N-glycosidic bond)

2) Threonine (contains a hydroxyl group which forms a O-glycosidic bond)

3) Serine (contains a hydroxyl group which forms a O-glycosidic bond)

• The side chains of proteins that can form glycosidic bonds typically contain amino groups or hydroxyl groups which participates in the formation of the bond with the sugars

Question 21

What is mucin?

Answer 21

• It is a glycoprotein (proteins that has carbohydrate which are linked via a glycosidic bond), mucin is heavily glycosylated with a lot of serine and threonine residues
• It function as a tissue coating
• It blocks the bacterial action by making bacterial aggregates
• It also has a lubricating function

Question 22

Describe the process of glycoprotein synthesis

Answer 22

1) Protein synthesis begins and the polypeptide chain is extruded to the rER

2) A branches oligosaccharide is synthesized on dolichol pyrophosphate

3) The oligosaccharide is transferred from the dolichol to the amide N of an asparagine residue of a growing polypeptide chain

4) As the protein moves through the rER carbohydrate is being trimmed

5) In the golgi apparatus further trimming or addition of monosaccharides occurs

6) Glycoprotein are then directed to the lysosome by the addition of mannose derivatives added at the golgi apparatus via phosphotransferase (N-acetylglucosamine-1-phosphate transferase)

Question 23

What is a lysosome?

Answer 23

They are spherical vesicles that contains hydrolytic enzymes which can break down many kinds of biomolecules

Question 24

What is a lysosomal storage disease?

Answer 24

Lysosomal storage disease are inborn errors of metabolism which are characterized by excess accumulation of substrates in various organs or cells due to a defect in the functioning of the lysosome, leading to the dysfunction of the organ contributing to a great morbidity and mortality

Question 25

What are the different types of lysosomal storage diseases?

Answer 25

1) Fabry's disease 2) I-cell disease (Inclusion-cell disease, mucolipidosis II)

Question 26

What is I-cell disease (Inclusion-cell disease, mucolipidosis II)?

Answer 26

- Rare autosomal recessive llysosomal storage disorder - It is caused by defective mannose-6-phosphate (M6P) tagging, leading to misdirected lysosomal enzymes - Lysosomal enzymes are glycosylated proteins - They are synthesized in the ER and modified at Golgi apparatus. They are directed to the lysosome by addition of mannose derivatives added at Golgi apparatus by phosphotransferase (N-acetylglucosamine-1- phosphate transferase (GlcNAc phosphotransferase), In the state of GlcNAc phosphotransferase deficiency: the hydrolytic enzymes fail to be directed to lysosomes: lysosomes are deficient Accumulation of by products in the lysosomes: formation inclusion cell (I cell)

Question 27

What are the symptoms of I-cell disease (Inclusion-cell disease, mucolipidosis II)

Answer 27

1) High plasma levels of lysosomal enzymes 2) Skeletal abnormalities 3) Restricted join movement 4) Psychomotor retardation 5) Early death 6) Coarse facial features - It has no treatment

Question 28

What is Fabry's diseae?

Answer 28

- Most common lysosomal storage disease - It is a X-linked disease, which occurs due to the deficiency of a-galactosidase (responsible for hydrolysis of the terminal a-D-galactose residue) - Deficiency of the a-galactosidase will lead to glycosphingolipid accumulation particularly ceramide trihexoside, in brain, heart, kidney - Symptoms include (Neuropathic pain and renal failure)

Question 29

What is meant by xerostomia (dry mouth)?

Answer 29

- Salivary gland hypofunction 1) Common causes include - Malnutrition - Dehydration - Psychological - Radiation therapy - Antidepressants - Anti-hypertensives - Anti-emetics 2) Systemic causes - Diabetes - End-stage renal disease - Cystic fibrosis - Sjögren syndrome (a systemic autoimmune disease attacking and destroying exocrine glands)

Question 30

What are the diagnostic role of the saliva?

Answer 30

- It is non-invasive, low-cost, and rapid 1) It can be used to detect alteration in human genetics 2) It can be used to detect biomarkers of cancer, metabolic disorders, alzheimer's, and periodontal disease 3) It can be used to detect hormones (like cortisol "the ones that are secreted into the blood)