206 SSNS - Biochemistry

Question 1

What’s the function of retinal pigment epithelium (RPE)?

Answer 1

1. Helps to absorb scattered light
2. Maintains redox homeostasis
3. Resposible for POS turnover
4. Facilitates transepithelial transportation
5. Regulation of immune system & secretion
6. Visual cycle (Vitamin A metabolism)

Question 2

Where is melanin synthesized?

Answer 2

Melanosomes

Question 3

Photooxidation constantly destructs photoreceptor outer segments (POS), how is it renewed?

Answer 3

Constant renewal is required: by shedding destroyed tips of POS (daily)

Regulated by circadian rhythm and coordination between RPE & photoreceptors

Question 4

Mechanism of RPE phagocytosis

Answer 4

3 surface receptors are involved: αVβ5, MerTK, CD36

These receptors interplay regulate and cooridnate phagocytosis - recognized before engulf

Question 5

What is transported from blood to retina?

Answer 5

Nutrient supply & delivery of essential metabolites

Glucose, Retinal, Omega-3 fatty accids (mainly retina)

Question 6

What is transported from subretinal space to blood?

Answer 6

Metabolic waste

Water, ions, metabolic end products

Question 7

Why use a mutienzyme complex?

Answer 7

The multienzyme complex retains the product of one reaction and transfers it to the active site of another enzyme by the movement of a flexible group. This improves catalytic efficiency by increasing the chances of contact with the next active site and by providing immunity from other reactions.

Question 8

Why can’t human body synthesize vitamin A?

Answer 8

bc human lack the enzyme that synthesizes vitamin A

Question 9

Xerophthalmia

Answer 9

Severe vitamin A deficiency

• Dry out of tear ducts
• Characterised by Bitot’s spots on the temporal side of the conjunctiva
• Conjunctiva becomes dry, thick, wrinkled
• cornea ulceration & secondary infection → blindness

Question 10

Where does re-isomerase of visual cycle take place?

Answer 10

RPE

• bc rods do not express re-isomerase for all-trans retinal

Question 11

Canonical visual cycle (cone-specific visual cycle)

Answer 11

Involves similar enzymes/proteins

Müller cells act in the place of the RPE for the storage of 11-cis retinol

Question 12

Canonical visual cycle for ______;

non-canonical visual cycle for ______

Answer 12

cones; rods

Question 13

Sources of retinal for vision

Answer 13

1. Recycle from visual cycle
2. Regeneration from RPE phagocytosis
3. Food source: (animal and plant sources of vitamin A)

Question 14

3 types of provitamin of vitamin A

Answer 14

α, β, and γ carotenes

• α & γ yield 1 vitamin A precursor

Question 15

Metabolism of vitamin A

Answer 15

1. Absorbed in the proximal small intestine
2. Transported as retinyl esters in chylomicrons (CM)
3. Secreted into lacteals
4. Through venous return to circulation
5. Triglyceride components are hydrolyzed
   6a. Supplied to tissues
   6b. CM remnants are formed
6. CM remnants taken up by hepatocytes

Question 16

Night-blindness

Answer 16

Vitamin A deficiency

• ↓ dark adaptation
• ↓ regeneration of functional visual pigment - retina problem