Biochemistry and Metabolism- Lens Flashcards
What is the lens index of refraction
1.390, which is higher than that of the surrounding media and a result of its high protein content (SOLUBLE PROTEIN CALLED CRYSTALLINS)
How much do proteins constitute of the lens weight
33% of the weight of the lens, which is 2–3 times higher than their concentration in other tissues in the body
How does the lens generate ATP
glycolysis. Alterations in this metabolic pathway have been implicated in the development of congenital cataract as well as diabetic cataract
What type of collagen does the lens capsule comprise of
type IV collagen
Which layer of the capsule covering the lens has an epithelial layer
Anterior layer.
Posterior layer does not have an epithelium. It is very thin layer
Where do the epithelial cells divide
At the germinative zone anterior to the equator
Pathophysiology of posterior subcapsular cataracts
In the adult lens, lens epithelial cell migration posterior to the equator results in the development of posterior subcapsular cataract
What is the lens composed of
lens fibres which are long ribbonlike cells
Where are the oldest and youngest cells located in the lens
The lens structure can be equated with the growth rings of a tree: the oldest cells are in the center, and the progressively younger layers, or shells, of fiber cells are toward the periphery.
What are the 2 types of crystallin protein in the lens
α-crystallin and the β,γ-crystallin family, both of which seem to be present in all vertebrate lenses but have also been demonstrated in other ocular tissues.The second group consists of the taxon- specific crystallins, which are present only in certain species.
What is alpha crystallin
a member of the small heat shock protein family. Heat shock proteins are molecular chaperones; they stabilize partially folded proteins and prevent them from aggregating.
Mutations in the α-crystallin gene result in premature cataract development
What is beta,gamma Crystallins
β-Crystallins exist as polymers, and γ-crystallins are monomeric. The specific functions of the β,γ-crystallins are unknown.Gamma crystallins are concentrated in the nucleus of the lens.
Given their compact and symmetric structures (which can pack very densely), γ-crystallins tend to be highly con- centrated in aged, hard lenses, which have little to no accommodative ability.
What are taxon specific crystallins
Taxon-specific crystallins have not been demonstrated in humans. Reduced nucleotides absorb ultra- violet (UV) light and may protect the retina from UV-induced oxidation.
Which 2 proteins form a cytoskeletal structure of the lens
filensin and phakinin
Which wavelengths of light does the cornea and aqueous humour protect the eye from
wavelengths below the visible spectrum (mainly UV). Wavelengths of 300 nm or below are blocked by the cornea and by ascorbate (vitamin C), which is present at high levels in the aqueous humor. Wavelengths of 360 nm or below are blocked by the lens
Lens fibre packaging
hexagonal
packing of lens fibers
Which pump in the lens maintains the ionic balance
Na+,K+-ATPase (also called sodium-potassium pump). Functional Na+,K+-ATPase pumps are found primarily at the anterior surface of the lens, in the epithelium and the outer, immature fibers
Which family do gap junctions in the lens originate from
Connexins
Composition of electrolytes in the lens
sodium (Na+) levels are low (≈10 mmol/L), and potassium (K+) levels are high (≈120 mmol/L). When normal regulatory mechanisms are abrogated, K+ leaks out of the lens and Na+ floods in, followed by chloride (Cl–). Water then enters in response to the osmotic gradient, causing loss of transparency by disrupting the normally smooth gradient of refractive index, as can occur following traumatic violation of the lens capsule.
How does oxygen reach the lens
diffusion from the aqueous humour
What happens in glycolysis
2 molecules of ATP are formed per original molecule of glucose. A small proportion of glucose-6-phosphate is metabolized through the pentose phosphate pathway
What is the pentose phosphate pathway
This path- way is activated under conditions of oxidative stress because it is responsible for replenishing the supply of nicotinamide adenine dinucleotide phosphate (NADPH) that becomes oxidized through the increased activity of glutathione reductase under such conditions
Which enzyme is affected in galactosaemia
deficiency of galactose-1-phosphate uridyltransferase
What is the sorbitol pathway
Aldose reductase is the key enzyme for the pathway, and it converts the sugars into the corresponding sugar alcohols. Because aldose reductase has a very high Km (apparent affinity constant) value—that is, low affinity—for glucose under normal conditions little or no activity occurs through this pathway
