Indian Journal of Pathology and Microbiology

ORIGINAL ARTICLE
Year
: 2010  |  Volume : 53  |  Issue : 4  |  Page : 696--698

Comparative analysis of protein structure of common Hb Q variants


Amit K Yadav 
 Department of Pathology, Super Religare Laboratories Ltd., Fortis Escorts Hospital and Research Centre, Faridabad, India

Correspondence Address:
Amit K Yadav
Department of Pathology, Super Religare Laboratories Ltd., Fortis Escorts Hospital and Research Centre, Faridabad
India

Abstract

Context: Hemoglobin (Hb) Q variant is a group of hemoglobinopathies prevalent in south, south-east and western Asia. The primary structure of all of these molecules is well known. However, very little is known about the secondary and tertiary structures of these molecules. Therefore, a study of their secondary and tertiary structures is needed. Aim: The study was aimed at investigating the secondary and tertiary structures of common Hb Q variants using bioinformatics tool. Settings and Design: The secondary and tertiary structures of common Hb Q variants were evaluated using NNPREDICT server and CPHmodels 2.0 server, respectively. Materials and Methods: Amino acid sequence of alpha globin chain was searched using ExPASY and was used for further mutation to Hb Q variants. The derived sequences were further analyzed using NNPREDICT server and CPHmodels 2.0 server to calculate their secondary and tertiary structures, respectively. These were then compared and any differences noted. Results: It was observed that there is no difference between the predicted secondary structures of normal alpha globin and Hb Q-India. Hb Q-Iran carries an extra helix while Hb Q-Thailand carries two extra helices. The results of tertiary structure prediction also support these findings. Conclusions: Differences in secondary and tertiary structure of various Hb Q variants have been observed in the present study. The study provides valuable data for better understanding of these uncommon hemoglobinopathies.



How to cite this article:
Yadav AK. Comparative analysis of protein structure of common Hb Q variants.Indian J Pathol Microbiol 2010;53:696-698


How to cite this URL:
Yadav AK. Comparative analysis of protein structure of common Hb Q variants. Indian J Pathol Microbiol [serial online] 2010 [cited 2019 Dec 16 ];53:696-698
Available from: http://www.ijpmonline.org/text.asp?2010/53/4/696/72039


Full Text

 Introduction



Hemoglobinopathies are important inherited disorders with high prevalence in Asia, especially India. Hb Q variants are rare variants of hemoglobin molecule. The primary structure of all of these molecules is well known. They are all characterized by distinct point mutations in the a globin chain - Hb Q-India (alpha 64 Asp to His), Hb Q-Thailand (alpha 74 Asp to His) and Hb Q-Iran (alpha 75 Asp to His). Very little is known about the secondary and tertiary structures of these molecules. However, secondary and tertiary structures are more important in explaining the function of a protein. [1] Therefore, a study of their secondary and tertiary structures is needed. A greater understanding of this will help to shed light on the pathogenesis of these disorders.

Recent advances in molecular biology allow us to simulate mutations on the basis of their known sequences. The method that the author has employed in this study is relatively new. It has been employed in some recent structural studies of hemoglobinopathies. The main objective of this study is to find the secondary and tertiary structures of Hb Q variants by bioinformatics method. In this study, the author has performed a bioinformatics analysis to study the effect of mutations in common Hb Q variants on the secondary and tertiary structures of alpha globin chain. A computer-based study for amino acid sequence comparison and protein structure modeling was performed and is being presented.

 Materials and Methods



Sequence Acquisition

The protein sequence database ExPASY [2] was used for searching for the amino acid sequence of normal alpha globin chain (primary accession number P69905). Then the mutations for common Hb Q variants alpha 64 Asp to His, alpha 74 Asp to His and alpha 75 Asp to His were experimentally performed.

Protein Structure Modeling

Secondary structure modeling was performed by doing secondary structure prediction in both normal and Hb Q variants from their primary sequence using NNPREDICT server. [3] The calculated secondary structures were then compared.

For tertiary structure modeling, the author performed protein tertiary structure predictions of both normal and Hb Q variants from their primary sequence using CPHmodels 2.0 server. [4] The calculated tertiary structures were compared.

 Results



Sequence of Alpha Globin in Hb Q Variants

After searching the database ExPASY, sequence of alpha globin chain was derived as shown in [Table 1]. The experimentally mutated alpha globin chain in Hb Q variants was derived as displayed in [Table 1].{Table 1}

Structure Modeling

NNPREDICT server was used to calculate secondary structure of alpha globin chains of normal and Hb Q variants. Alpha globin chain of normal hemoglobin consists of 53 helix and 6 strands, Hb Q-India has 53 helix and 6 strands, Hb Q-Iran has 54 helix and 6 strands and Hb Q-Thailand has 55 helix and 6 strands. The difference in secondary structures of alpha globin chains between normal and Hb Q variants is presented in [Figure 1].{Figure 1}

Using CPHmodels 2.0 server, the calculation for tertiary structure of alpha globin chains of normal and Hb Q variants was carried out. The calculated structure along with the differences between them is shown in [Figure 2].{Figure 2}

 Discussion



Hb Q variants are a group of hemoglobinopathies occurring in Asia mainly. They are alpha globin chain variants due to structural mutations. Three molecular variant types have been documented, namely, Hb Q-India (alpha 64 Asp to His), Hb Q-Thailand (alpha 74 Asp to His) and Hb Q-Iran (alpha 75 Asp to His). Hb Q-India [5] is a rare alpha globin chain variant and usually presents in the heterozygous state. Normally, Hb Q-India is clinically silent. It becomes symptomatic when present in association with other conditions like beta-thalassemia, alpha-thalassemia, HbE and HbH. It presents with a band in the region of HbS or HbD on electrophoresis at alkaline pH. High performance liquid chromatography (HPLC) identifies Hb Q-India with a prominent peak in the unknown window (4.70-4.90 min). [6] Besides India where it was first described, it has also been described in some other Asian countries, particularly Thailand. [7]

Hb Q-Iran is an alpha chain variant that usually presents in the heterozygous state with normal hematologic features. However, recently it has been reported in association with beta-thalassemia minor. [8] Hb Q-Thailand is another alpha chain variant which is usually asymptomatic hematologically. However, it has been reported in association with Hb-H, [9] Hb E and hereditary spherocytosis. HPLC identifies Hb Q-Thailand with a prominent peak in the s window (4.30-4.70 min). [6]

Although the primary structure of these Hb Q variants is well known, the secondary and tertiary structures of these molecules are not known. On the basis of present advances in molecular biology, simulation of the mutation can be performed based on their sequences. The method that the author has used in the present study has been used by some other authors also to carry out structural studies on hemoglobinopathies. [10]

The effect of point mutation in alpha globin chain of Hb Q variants on their secondary and tertiary structures has been studied here. This has been done by calculating for the possible secondary and tertiary structures using standard bioinformatics methods. Based on the findings in the present study, it can be said that there is no difference between the predicted secondary structures of normal alpha globin and Hb Q-India. Hb Q-Iran carries an extra helix while Hb Q-Thailand carries two extra helices. The results of tertiary structure prediction also support these findings. The functional significance of these findings needs further study. The difference in HPLC retention time between Hb Q-India (unknown window 4.70-4.90 min) and Hb Q-Thailand (s window 4.30-4.70 min) could be due to the difference in their secondary and tertiary structures. An extra helix in other hemoglobin variants has been noted by some authors to be correlated with thalassemia. An extra helix in Hb Tunis-Bizerte has been reported to result in alpha-thalassemia trait. [11] Presently, there is no evidence to suggest that this extra helix leads to a thalassemic phenotype in Hb Q variants.

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