Biochemistry

• Optical clarity and refractive power
• Ocular surface comfort; lubrication
• Protection
• Environment for corneal epithelium

• multifactorial disease of tears and ocular surface
• Symptoms: discomfort, visual disturbance, tear instability, and potential damage to ocular surface
• Increased osmolarity of tear film
• inflammation of the ocular surface

• Increase in the osmotic gradient between the tear film and the ocular surface
• Increases conjunctival cell desquamation.
• Disease first affects the conjunctival morphology while the corneal morphology initially remains normal.

• attachments between corneal cells loosen
• increase in corneal desquamation
• severe corneal epithelial changes: glycoprotein loss and destabilization of the cornea-tear interface

• replenish eyes' tears
• delay evaporation of patients' tears

• Demulcent (hydrogels)
• Preservative
• Buffer (oil gelling agent ions)
• does not contain growth factor or anti-inflamatory components

• Pros:
• viscosity of polymers minimizes thinning of tear film
• Provide temporary palliative relief
• avoid repeated use of banzalkonium chloride (BAK), which is toxic with multiple doses per day
• Cons:
• fail to break the cyclic inflammatory nature of dry eye
• lack proteins and nutrients found in normal, healthy tears

• Prescribed in pulsated, tapered therapeutic modality (fancy word for "treatment")
• start 3-4 times a day and taper to 2x/week
• mild steroid is Loteprednol (Lotemax by Bausch and Lomb)

• cyclosporine opthalmic elulsion 0.05%
• topical anti-inflammatory
• for moderate to severe dry eye
• Side effects: burning, conjunctival heperemia, discharge, epiphora (overflow of tears), eye pain, foreign body sensation, pruritus (itch), stinging, and blurring
• Function: prevents T cells from releasing cytokines
• Dosing: one drop twice/day
• Takes 3-4 months for relief to start

• amino acids are fundamental building blocks
• created from peptide bonds
• 3 divisions: Peptides (molecular weight below 10,000), Proteins (molecular weight above 10,000), Polypeptides (basically another name for protein)

• Mechanical support
• Clarity of cornea and refraction properties
• Electrical signaling
• Protecting eye through lysis of bacteria (tears)

• Acidic (more than one carboxylic group; ex: aspartic acid, glutamic acid)
• Basic (more than one amino group; ex: lysine, arginine)
• Neutral (equal number of amino and carboxylic acid groups; ex: alanine, glycine)

• Unsubstituted
• Dicarboxylic
• Diamino
• Amido
• Hydroxy
• Aromatic
• Sulfur
• Cyclic

• Glycine (can give tighter "twist" to proteins such as collagen)
• Alanine
• Valine
• Leucine
• Isoleucine
• impart hydrophobic characteristics to proteins

• Aspartic (aspartate)
• Glutamic (glutamate)
• Impart negative charges to proteins due to ionization of one carboxylate.

• Lysine
• Arginine
• Histidine
• Impart positive charges to proteins due to ionization of one amine

• Asparagine
• Glutamine
• derived from aspartic and glutamic; hydrophilic character

• Serine
• Threonine
• hydroxy groups give them hydrophilic character

• Phenylalanine
• Tyrosine (less hydrophobic due to OH group)
• Tryptophan
• impart hydrophobic characteristics to proteins

• Cysteine (forms disulfide bonds between polypeptide chains and is hydrophilic
• Methionine (hydrophobic)

• Proline
• nonaromatic and is found in collagen, elastin, and mucin

• dicarboxylic
• diamino
• amido
• hydroxy
• Cystine in Sulfur group
• Important in pH control

• Aromatic
• Methionine
• Proline

• Metal chelation (ex: histidine in hemoglobin)
• Chain binding (ex: cystene in ribonuclease)
• Tight helical turns (ex: glycine in collagen)
• function of protein dependent on number of times each amino acid occurs

• alpha-helical domains
• beta-helical domains
• alpha/beta domains which consist of "beta-alpha-beta" structural units or motifs that form parallel beta-sheats
• alpha and beta domains formed by independent alpha helices and mainly antiparallel beta-sheets
• multi-domain proteins
• membrane and cell surface proteins and peptides
• "small" proteins
• coiled-coil proteins
• low-resolution protein structures
• peptides and fragments
• designed proteins of non-natural sequence

• Water soluble (albumin)
• Lipid soluble (rhodopsin)
• Insoluble (collagen)

protein that possesses its unique biological ability

Protein that has lost its property

normal sequence of amino acids

• beta-sheets are sequences of primary structure connected by hydrogen bonds (ex: crystallins)
• beta-turns connect beta-sheets
• random coils are haphazard forms that connect secondary structures

• alpha-hlix (ex: rhodopsin)
• alpha-helix defined as 1.2-angstrom rise for 100 degree rotation
• stabilized by hydrogen bonds

Structural or functional part of a chain containing secondary structures or a part of tertiary structure

a simplified version of domain (contains few parts of domain; ex: helix-turn-helix)

entire shape or configuration of one polypeptide chain constitutes tertiary structure

two or more polypeptide chains connected

Group of proteins found in epithelium and in the fiber cells of ocular lens. Primarily eye protein but found other places as well. They help maintain shape of lens fibers and thus the structure of the lens.

beta and gamma crystallins same family on functional classification.

• further division:
• α-crystallins has αA and αB
• β-crystallins has βH and βL (H = heavy, L = light)
• γ-crystallins have six subtypes γA to γF

• Molecular chaperones- that is help to maintain normal molecular confirmation of other lens crystallins
• prevents crystallin aggregation- process in formation of senile cataract (age related cataract).

α-crystallins are the only crystallin in lens epithelium

The lens epithelium

Glucose in aqueous higher than lens hence glucose diffuses readily

They're the bulk of lens cortex and nucleus, high in number, take up minimal space (why?).

65% water and 35% organic matter (33% is structural proteins)

• Soluble lens - cortex
• Insoluble lens - nucleus
• the nucleus is more dehydrated than the cortex

the water equilibrium is of prime importance, why?

High K+

Low Na+ and Cl- and water

If sodium moved into the lens that would increase the solutes and the water content will increase (osmotically)

Concentration gradient: Aqueous has more Na+ and low K+

Electrical gradient: lens is electronegative inside

Electrolyte balance maintained by the sodium pump

operates in the epithelium

for every 3Na+ removed, 2K+ are allowed

it requires energy

• In general increase in phosphorylated chains of α-crystallins in lens fiber
• Disulfide bond formation (crosslinking)between different crystallins increases
• Peptide bond disruption in both β and γ crystallins

• General effect is change in amount of soluble and insoluble lens protein
• Lens grows with age hence the protein content also increases with age.
• The process of change of soluble to insoluble is by cross linking

• HM1, HM2, HM3 and HM4
• HM1 and HM2 soluble kind
• HM3 and HM4 insoluble and found in cataractous lens
• HM4 occurs exclusively in nuclear cataract

HM fractions grow into the cell cytoplasm

range: 300-400 nm

the lens absorbs these wavelengths

• increased calcium
• affects glucose metabolism,
• inhibition of protein synthesis,
• induction of HM aggregates
• Direct loss of transparency
• Inhibit sodium pump

• Hypocalcemia
• related to the dependency of the membrane permeability to levels of calcium
• Marked electrolyte and water imbalance in low calcium environment

• Complications of diabetes and glactosemia
• Conversion of excess glucose or galactose to respective sugar alcohols via sorbitol pathway.
• Sorbitol cannot readily diffuse out of intracellular compartment.
• Osmotic gradient favors influx of H2O
• Sugar alcohols and sugar itself will lead to protein aggregation

• Neonates and premature infants at greatest risk
• Remember ocular lens needs energy

• X-ray radiation (some humans as low as 200 rads)
• Microwave animal experiments shown to cause cataract.
• Mechanism for microwave cataract is ----?

• Radiation retinopathy requires 3000-3500 rads
• 1-2 years after therapy
• Endothelial cell DNA damage of blood vessels
• Patients with diabetes grater risk

It is a intrinsic membrane protein found in discs and to some extent the plasma membrane or rod outer segment

• Rhodopsin is represented as egg shaped with hook.
• Rhodopsin more prevalent in discs
• Rhodopsin has vitamin A
• Vitamin-A is bound to protein at amino acid lysine #296
• Vitamin A is present rhodopsin as an aldehyde (retinal) and consists of 11-cis-form

Not a part of visual transduction process per se

Called bleaching of rhodopsin

• Alpha peak- 11 cis retinal
• Beta peak- All trans retinal
• Gamma peak- opsin

• A=[abc]
• A= Absorbance
• a=absorptivity
• b=light pathlength
• c= concentration

• Retina: the point of contact between radiated energy, light and nervous system
• Rod cells:

Scotopic (dim-light) vision

Cone cells:

Photopic (bright-light)



color vision



Less sensitive than rods,

• but they mediate
• differentiation of colors

• FOVEA: cone cells have “private line communication”; no divergence
• PERIPHERY: mostly rod cells greatly outnumber the ganglion cells, convergence is the rule

High sensitivity, but low acuity

Additive Primary Colors

• Blue (435 nm), green (545 nm), and red (700 nm)
• Add up to produce white

Subtractive Primary Colors

• Yellow, magenta-red, and cyan-blue
• From the mixing of any 2/3s of the spectrum

• Subtractive color mixing: the mixing of color pigments or dyes to produce full spectrum of colors
• Add up to produce black

• Erythrolabe (red, long)
• Chlorolabe (green, medium)
• Cyanolabe (cyan, short)

• result of three independent comparisons of rates of photon absorption by the three cone types
• dynamic output relates only to the rate of photon capture

• Congenital: inherited, non-progressive nature
• Acquired: not inherited, secondary to disease (blue-yellow)

a person missing one of three photopigments

person missing chlorolabe (green)

person missing erythrolabe (red)

person missing cyanolabe (cyan)

have photopigment but absorption spectrum of one of the pigments is displaced

• X-chromosome
• 3-8% in males
• females less frequent
• color vision deficiency, can be a problem in certain jobs

• worn on only one eye
• reddish brown tint
• patients may suppress the eye

• proteins to which short chains of sugars are bound
• Roles of glycoprotein:
• structural orientation
• immunological recognition
• biological lubrication
• Rhodopsin is a glycoprotein

• made of 55% carbohydrates
• similar to mucins in the GI tract, nasal mucosa, and trachea
• secreted by goblet cells of conjunctiva
• stabilizes tear film by increased viscosity
• biological lubricant
• prevent mechanical shock
• rapid tear film break-up time when amount is low

• Vitamin A deficiency
• Ocular pemphigoid (conjunctival ulcerations)
• Steven-Johnsons syndrome
• Alkali burns

• Xerosis (dryness of conjunctiva in the interpalpebral zone with loss of goblet cells, squamous metaplasia, and keratinization)
• Bitot spots (triangular patches of foamy keratinizes epithelium in the interpalpebral zone thought to be caused by infection w/ Corynebacterium xerosis)

• twice as common as acid burns
• common alkalis are ammonia, sodium hydroxide, and lime
• penetrate deeper than burns because acids coagulate surface proteins

• life-threatening condition affecting the skin in which epidermis separates from the dermis due to cell death
• caused by infections
• hypersensitivity reactions to medication

• autoimmune mucocutaneous blistering disease
• keratinization of the caruncle
• adhesion between the bulbar and palpebral conjunctiva
• hyperaemia and oedema
• chronic conjunctivitis

• extracellular, insoluble molecular complex
• 80-90% of bulk of eye
• 12 different types in eye
• protein complex basic unit consists of triple helices (tropocollagens)
• Roles:
• supporting members or fibers - maintain tissue, repair tissue
• scaffolding - platform on which basement membrane is constructed
• anchoring devices - holds cells onto non cellular area
• semiliquid gel in eye (vitreous body)

disulfide bonds that form at the c-terminal causes the chain to wrap around each other

• units are associated hydrophobically by covalent bonds
• microfibril: 5 rows of cross-linked tropocollagen

• IV
• VIII

• I
• V
• VI

• II
• III

• I
• II

II

III

III

III

IV

V

VII

Fiber sheets

• 70% is type I
• 15% approx type V
• 15% approx type VI

maintain the optical properties of the cornea

Type V collagen oblique running lamellae in stroma

• beaded form
• stabilizing molecule

made of parallel fibers

the vitreous body

vitrosin

99%

types IX and V-XI

type V-XI limits diameter of type II

basement membrane

They are thin structures that occur extracellularly.

support and separation from the other cells

descement's membrane

type VIII and a very geometric pattern

a diabetic state

type VII and type I (more randomly oriented)

osteogenesis imperfecta

• Decreased synthesis of type 1 collagen
• Clinically – fracture of long bones specially prior to puberty
• Blue sclera

• Point mutations and rearrangements in triple helical regions
• Malformed and soft fragile bones and perinatal lethality

almost normal scleral color

nine

• type 6 - associated with ocular issues
• type 4 - rare

skin is hyperelastic, bruise easily, slow to heal, and joints are hypermobile

Ocular fragility with increased vulnerability to mild trauma, high myopia, retinal detachment and keratoconus

Epicanthic folds, microcornea, blue sclera, ectopia lentis and angioid streaks.

*ectopia lentis displaced lens from its normal position

Android streaks

proteins that have the ability to optimize rates of biochemical reaction

catalysts

energy required for a reaction to occur

they decrease the amount of activation energy required

• A- Substrate
• B - Product

Depending on which direction the reaction goes a biochemical substance can be substrate or a product.

a rate limiting enzyme

a series of enzyme driven reactions, reaction tends to proceed in a given direction regardless of the substrate and product concentration

heat of molecular collision

catalyzed reactions

kinetic basis, michaelis-menten, and allosteric

kinetic and funcational properties described by michealis and menten in 1913

example = aldose reductase

• v= {Vmax*[S]}
• {[S]+Km}

• Vmax is maximum velocity of enzyme
• Km is Michealis-Menten constant

Km=(k2+k3)/k1

the lower the Km value, the greater the affinity of the substrate

Kapparent

• "other site"
• kinetics influenced by substrates that bind to enzymes at locations other than active sites
• quaternary protein structure (more than one polypeptide chain)
• each chain has its active site
• few active sites occupied = low affinity of enzyme and substrate

• conformational change in the protein
• increased binding of substrate
• velocity of enzyme is increased

• activator substances occupy sites on enzyme away from active sites
• change in form T to R form
• Biological way of jumpstart reactions at low concentration of substrate

inhibitor replaces or competes with substrate for active site

inhibitor binds close to active site and prevents reaction despite the binding of the substrate to the active site

• two substrates are required for the reaction
• inhibitor binds close to the active site after the attachment of the first substrate to the site; the inhibitor prevents second substrate from binding
• Example: aldose reductase

• enzyme of tear film
• destroys gram positive bacteria by breaking glycan of peptidoglycan
• levels of lysozyme reflect tear film dysfunction
• aqueous deficiency
• 1.3 mg of lysozyme found /ml of tears

• described by Alexander Flemming in 1992
• first located in nasal mucosa

• tear film on tissue paper
• dish w/ 5x10⁷ microorganisms
• 24 hours at 37 degrees C
• zone of lysis (zone of clearing) is measured and converted to enzyme activity

• an enzyme located in plasma membrane of cells
• ocular functions:
• control of corneal hydration
• production of aqueous humor
• good allosteric enzyme example

• single layer of polygonal cells covering posterior surface of the cornea
• cell density declines with age
• newborn cell density = more than 5500 cells/ square millimeter
• adult cell density = 2500-3000 cells/square millimeter
• minimum cell density = 400-700 cells/mm²

• active in transport, synthesis, and secretory functions
• contain:
• large nucleus
• numerous mitochondria
• prominent endoplasmic reticulum
• Golgi apparatus

• (macula occludens)
• function as a barrier
• do not completely encircle cell like zonula occludens
• they are a leaky barrier between aqueous and stroma!

• function is intercellular communication
• mainly in lateral membranes
• do not contribute to the endothelial barrier

• barrier function of endothelium
• pump function of endothelium
• Water is maintained at 78%

127 micrometers/hour

33 micrometers/hour

swell

• located in the basolateral membrane
• about 1.5x10⁶ pump sites/cell

• inhibits Na+ K+ ATPase pump
• stops sodium transport
• causes corneal swelling
• prevents temperature reversal
• eliminates the transendothelial potential difference

• increases pump site density
• greater capacity for the pump to counteract the leak

decreased pump site density despite increased permeability

• Na ions are transported by pump into channels between endothelial cells
• this creates osmotic pressure and water follows
• directionality is helped by density of sodium ions in Descemets membrane

• functional unit for production of aqueous humor and its secretion
• made of capillaries, stroma, and pigmented and non-pigmented epithelia (deep to superficial)

• Carefully controlled filtrate of blood
• produced in ciliary body
• antioxidant source for the corneal endothelium and the lens
• carries oxygen
• helps maintain shape of globe
• serves as shock absorber

• carbohydrate metabolism (aerobic and anaerobic)
• aerobic is most efficient process
• anaerobic requires more sugar molecules but produces energy quickly

• H - heart; aerobic
• M - muscle; anaerobic
• K - cancer cells, photoreceptor cells; anaerobic
• all called "isozymes"

• closed eye conditions, contact lens wear, or similar unfavorable environments
• low partial pressure of oxygen
• can support cells survival with partial pressures of oxygen of 15-20 mmHg

• energy demand is low
• lack of blood vessels
• deep fibers lack subcellular organelles

• energy demand high
• aerobic tissue (blood supply present)
• Warburg effect: production of lactate coupled with high consumption of glucose and oxygen

• small organic non-protein molecules that carry chemical groups between enzymes
• sometimes called "co-substrates"

Pellagra (dermatitis, depression, and diarrhea)

Skin gland malfunction, dryness/scaling of mouth, photophobia

Berberi (muscle loss, gatigue, depression, loss of eye coordination

similar to others

Pernicious anemia and brain demyelination

Scurvy (incomplete formation of collagen)