Author ORCID Identifier

https://orcid.org/0000-0002-2285-2357

Year of Publication

2020

Degree Name

Master of Science (MS)

Document Type

Master's Thesis

College

Agriculture, Food and Environment

Department/School/Program

Animal and Food Sciences

First Advisor

Dr. Surendranath P. Suman

Abstract

Surface color of fresh beef is the major trait influencing consumers’ purchase decisions. Fresh beef color is determined by the myoglobin (Mb) redox stability. Post-translational modifications (PTMs) play a critical role in regulating Mb structure and functionality. This thesis focuses on the PTMs in Mb and their impact on fresh beef color stability.

In the first experiment, Mb PTMs in beef longissimus lumborum (LL) muscle during postmortem aging and their influence on fresh beef color stability were examined. Beef LL muscle from nine (n = 9) beef carcasses (24 h postmortem) were subjected to wet-aging for 0, 7, 14 and 21 d. On each aging day, steaks were fabricated. Instrumental color and biochemical attributes of aerobically packaged steaks were evaluated on d 0, 3, and 6 of storage. Mb PTMs were analyzed on 0, 7, 14 and 21 d of wet-aging using two-dimensional electrophoresis and tandem mass spectrometry. Aging decreased (P < 0.05) surface redness, color stability, and Mb concentration. Gel image analyses identified six Mb spots with similar molecular weight (17 kDa) but different isoelectric pH. Tandem mass spectrometry identified multiple PTMs (phosphorylation, methylation, carboxymethylation, acetylation, and HNE alkylation) in these isoforms. The amino acids susceptible to phosphorylation were serine, threonine, and tyrosine, whereas other PTMs are detected in lysine, arginine, and histidine residues. Overall, Mb PTMs increased with aging. The aging-induced PTMs, especially those occurring close to hydrophobic heme pocket, could disrupt Mb tertiary structure, influence heme affinity, and compromise oxygen binding capacity, leading to surface discoloration.

The second experiment was carried out to characterize the influence of vitamin E supplementation to beef cattle on the Mb PTMs in post-mortem LL muscle. Beef LL muscle samples (24 hours postmortem) were obtained from the carcasses of nine (n = 9) vitamin E-fed (VITE; 1000 IU vitamin E for 89 days) and nine (n = 9) control (CONT; diet without supplemental vitamin E) heifers. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to separate Mb from other sarcoplasmic proteins of beef LL muscle. Tandem mass spectrometry identified multiple PTMs (phosphorylation, acetylation, alkylation, methylation, dimethylation, trimethylation, and carboxymethylation) in protein bands (17 kDa) representing Mb. Differential occurrence of acetylation, methylation, dimethylation and trimethylation were identified in Mb from CONT and VITE samples. Additionally, PTMs at lysine residues (K87, K96, K98 and K102) were unique to CONT, whereas PTMs at K118 were unique to VITE. Overall, supplementation of vitamin E decreased the numbers of post-translationally modified residues in myoglobin. These findings suggested that dietary supplementation of vitamin E in beef cattle might protect residues in Mb, especially those located spatially close to proximal histidine, from undergoing PTMs, and thereby improving Mb redox stability.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.476

Funding Information

This study was supported by the Agriculture and Food Research Initiative Grant 2016-67018-24614 from the USDA National Institute of Food and Agriculture in 2016.

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