Primary structure of goat myoglobin

Abstract

Color stability attributes of goat meat are different from those of sheep meat, possibly due to species-specific differences in myoglobin (Mb) biochemistry. An examination of post-genomic era protein databases revealed that the primary structure of goat Mb has not been determined. Therefore, our objective was to characterize the primary structure of goat Mb. Goat Mb was isolated from cardiac muscles employing ammonium sulfate precipitation and gel-filtration chromatography, and Edman degradation was utilized to determine the amino acid sequence. Sequence analyses of intact Mb as well as tryptic- and cyanogen bromide-peptides yielded the complete primary structure of goat Mb, which shared 98.7% similarity with sheep Mb. Similar to other livestock myoglobins goat Mb has 153 residues. Comparison of the sequences of goat and sheep myoglobins revealed two amino acid substitutions – THRgoat8GLNsheep and GLYgoat52GLUsheep. Goat Mb contains 12 histidine residues. As observed in other meat-producing livestock species, distal and proximal histidines, responsible for stabilizing the heme group and coordinating oxygen-binding, are conserved in goat Mb.

Introduction

Goats (Capra hircus) are well known for their potential to convert weeds and low quality roughage to high quality red meat. Goat meat is a popular red meat in many regions of the world (Webb, Casey, & Simela, 2005), and goats have been exploited as meat animals under harsh environments, which generally are unfavorable to raise cattle and sheep (Alexandre and Mandonnet, 2005, Glimp, 1995, Kadim et al., 2003). In the last four decades, global production of goat meat increased dramatically over 400% (from 1.1 million tons in 1961 to 4.9 million tons in 2006) compared to 200% growth in beef and sheep meat production (FAO, 2008). Although global demand for goat meat exceeds the supply (Simela & Merkel, 2008), the benefits from scientific advances made in biological sciences have not been conceptualized well in goat production compared to other meat-producing livestock species (Shrestha & Fahmy, 2005).

Research in past several decades attempted to characterize goat meat, and differentiate it from sheep meat, based on quality attributes, consumer acceptance, and chemical composition. Sheep meat was more tender and juicy than goat meat (Schonfeldt et al., 1993, Sen et al., 2004, Tshabalala et al., 2003). On the other hand, differences in meat composition between sheep and goat were also reported. Sen et al. (2004) observed that goat meat had greater moisture and lower fat content than sheep meat. Furthermore, Lee, Kannan, Eega, Kouakou, and Getz (2008) reported higher levels of unsaturated fatty acids in goat meat compared to sheep meat. More importantly, several researchers documented that color attributes of goat meat are significantly different from sheep meat. Babiker, El Khider, and Shafie (1990) reported that goat meat was darker (lower L^∗ value), more red (greater a^∗ value), and had higher sarcoplasmic protein content than sheep meat. In agreement, Kadim et al. (2008) reported lower L^∗ values for goat meat than sheep meat. However, other researchers reported lower a^∗ value for goat meat compared to sheep meat (Lee et al., 2008, Sheridan et al., 2003). These observed differences in color attributes of goat and sheep meats indicated possible variations in myoglobin (Mb) chemistry (Brown & Dolev, 1963).

Livestock Mb, the sarcoplasmic heme protein responsible for meat color, is comprised of 153 amino acids, and the primary structure of Mb depends up on species (Livingston & Brown, 1981). Despite differences in the amino acid sequences, many structural and functional properties of Mb are conserved across livestock species. The primary structure of protein dictates the tertiary structure, which in turn influences interactions with ligands and biomolecules. It is known that Mb primary structure influences meat color stability via mechanisms such as autoxidation (Stewart et al., 2004, Tada et al., 1998), heme retention (Grunwald & Richards, 2006), structural stability (Regis, Fattori, Santoro, Jamin, & Ramos, 2005), thermostability (Ueki et al., 2005, Ueki and Ochiai, 2006), and oxygen affinity (Enoki et al., 1995, Marcinek et al., 2001). Furthermore, variations in amino acid sequence of Mb influence meat color stability through species-specific interactions with small biomolecules like lactate (Tamburrini, Romano, Giardina, & Di Prisco, 1999) and aldehydes (Suman et al., 2006, Suman et al., 2007). For Mb, a heme protein with different redox states, this is extremely critical, because Mb stability and aforementioned molecular interactions govern meat color/color stability. In this perspective, information on amino acid sequence of Mb is necessary to further understand species-specificity in meat color attributes.

In the post-genomic era, protein databases have been developed for economically important species across the globe. Utilizing mass spectrometry and Edman degradation, Dosi et al. (2006), recently, characterized the primary sequence of buffalo Mb and illustrated that the uniqueness in buffalo Mb’s primary structure could lead to rapid discoloration of buffalo meat. Myoglobins from food animals such as cattle, pig, horse, sheep, deer, and buffalo are widely studied and well characterized (www.expasy.org; www.ncbi.nlm.nih.gov). On the other hand, a close look at these protein databases revealed that the primary structure of goat Mb has not been determined. Therefore, our objective was to determine the amino acid sequence of goat Mb.