Aminolevulinic acid dehydratase (porphobilinogen synthase, or ALA dehydratase, or aminolevulinate dehydratase) is an enzyme (EC 4.2.1.24) that in humans is encoded by the ALAD gene. Porphobilinogen synthase (or ALA dehydratase, or aminolevulinate dehydratase) synthesizes porphobilinogen through the asymmetric condensation of two molecules of aminolevulinic acid. All natural tetrapyrroles, including hemes, chlorophylls and vitamin B12, share porphobilinogen as a common precursor. Porphobilinogen synthase is the prototype morpheein. Function
It catalyzes the following reaction, the second step of the biosynthesis of porphyrin:

2 5-Aminolevulinic acid 
  
    
      
        ⇌
      
    
    {\displaystyle \rightleftharpoons }
   porphobilinogen + 2 H2OIt therefore catalyzes the condensation of 2 molecules of 5-aminolevulinate to form porphobilinogen (a precursor of heme, cytochromes and other hemoproteins). This reaction is the first common step in the biosynthesis of all biological tetrapyrroles. Zinc is essential for enzymatic activity. Structure
The structural basis for allosteric regulation of Porphobilinogen synthase (PBGS) is modulation of a quaternary structure equilibrium between octamer and hexamer (via dimers), which is represented schematically as 6mer* ↔ 2mer* ↔ 2mer ↔ 8mer. The * represents a reorientation between two domains of each subunit that occurs in the dissociated state because it is sterically forbidden in the larger multimers. PBGS is encoded by a single gene and each PBGS multimer is composed of multiple copies of the same protein.
This includes the development of (1) a mottled appearance of the retina and retinal pigment epithelium (RPE) that gives the same visual appearance of Bone Spicule patterns (but are not Bone Spicules), (2) a waxy yellow appearance of the optic disk, and (3) the attenuation of blood vessels in size and Arterial/Venous ratio as they enter and exit the Optic Disk of the retina and transverse it.Non-syndromic RP (RP appears alone without other co-morbidities) usually presents a variety of the following symptoms:
Night blindness
Tunnel vision (due to loss of peripheral vision)
Latticework vision(due to patchy loss of peripheral vision)

loss of depth perception
Photopsia (Spontaneously occurring flashes/blinking/swirling/shimmering lights)
Photophobia (aversion to bright lights)
Development of the appearance of melanin pigment in a bone spicule pattern in the fundus (not bone tissue)
Slow adjustment from dark to light environments and vice versa
Blurring of vision
Poor color separation
Loss of central vision is the last to go, because this is a disease of the rods and not the cones which are the highest in number in the Central Vision (Macula and Fovea)
Eventual blindness (legally defined as 20 degrees or less in the best seeing eye or visual acuity of 20/200 or worse. Majority of patients do not go totally Blind, they quite often retain limited non-functional vision. Causes
RP may be:
(1) non-syndromic, that is, it occurs alone, without any other clinical findings,
(2) syndromic, with other neurosensory disorders, developmental abnormalities, or complex clinical findings, or
(3) secondary to other systemic diseases. RP combined with deafness (congenital or progressive) is called Usher syndrome.