The conformational change within the hinge region in response to heparin binding results in the expulsion of P14 and P15 from the main body of the protein and it has been shown that by preventing this conformational change, increased Factor IXa and Xa inhibition does not occur. It is thought that the increased flexibility given to the reactive site loop as a result of the hinge region conformational change is a key factor in influencing increased Factor IXa and Xa inhibition. It has been calculated that in the absence of the pentasaccharide only one in every 400 antithrombin molecules (0.25%) is in an active conformation with the P14 and P15 amino acids expelled. Non-allosteric activation
Increased thrombin inhibition requires the minimal heparin pentasaccharide plus at least an additional 13 monomeric units. This is thought to be due to a requirement that antithrombin and thrombin must bind to the same heparin chain adjacent to each other. This can be seen in the series of models shown in Figure 5. In the structures shown in Figure 5 the C-terminal portion (P side) of the reactive site loop is in an extended conformation when compared with other un-activated or heparin activated antithrombin structures. The P region of antithrombin is unusually long relative to the P region of other serpins and in un-activated or heparin activated antithrombin structures forms a tightly hydrogen bonded β-turn.
Dry skin and atopic dermatitis may co-exist. The patches may become more apparent after sun exposure, when the normal surrounding skin is tanned. The role of ultraviolet radiation, bathing or not bathing, low serum copper and Malassezia yeasts is not clear. Diagnosis
Diagnosis is mainly done by clinical examination. Shining a Woods light over the skin may reveal further lesions not obviously visible otherwise. Differential diagnosis
Pityriasis versicolor and leprosy. Treatment
No treatment is required and the patches in time will settle. The redness, scale and itch if present may be managed with simple emollients and sometimes hydrocortisone, a weak steroid, is also used.As the patches of pityriasis alba do not darken normally in sunlight, effective sun protection helps minimise the discrepancy in colouration against the surrounding normal skin. Cosmetic camouflage may be required. Tacrolimus has been reported as speeding resolution.In exceptionally severe cases PUVA therapy may be considered. Prognosis
The patches of pityriasis alba may last from 1 month to about one year, but commonly on the face last a year. However it is possible that the white patches may last for more than 1 year on the face. Epidemiology
It occurs in mainly children and adolescents of all races, particularly people with dark skin. The worldwide prevalence is 5% in children, with boys and girls affected equally. adults can also have this disease.Up to a third of US school children may at some stage have this condition. Single-point prevalence studies from India have shown variable rates from 8.4%
to 31%.