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Protein is the second most abundant substance in the human body. Molecules called amino acids are linked together by peptide bonds, to form polymers ranging from just a few amino acids, to thousands of amino acids. The term polypeptide usually refers to an amino acid polymer with molecular weight less than 10000. Proteins have molecular weights above 10000 and some consist of thousands of amino acid residues.

Proteins have a primary, secondary, tertiary and quaternary structure. The linear sequence of amino acids is referred to as the proteins primary structure. The secondary structure refers to a common, stable, molecular arrangement of one or more of the primary structures. Tertiary structure refers to the 3 dimensional arrangement of multiple secondary structures. Multiple tertiary structures are sometimes combined in the form of subunits to form a quaternary structure.

Proteins are extremely abundant in the body and fulfill a wide variety of functions. Proteins can be enzymes, membrane channels, receptors, hormones, ligands, or structural molecules such as collagen or keratin.

Proteins are synthesized on ribosomes within the cell. When transcription factors are activated within the cell nucleus, the DNA segments which code from a certain protein are used to produce RNA. Each codon(three nucleotides) on the RNA codes for one amino acid on the ribosome. As the RNA goes through the ribosome, the ribosome links together the amino acids in the proper sequence to form a protein.

Protein is an essential part of the human diet and must be consumed daily. The human body can not synthesize some amino acids such as histidine, isoleucine, leucine, arginine, tryptophan, valine, phenylalanine, methionine and threonine, therefore dietary intake is required. Protein is hydrolyzed in the GI tract by enzymes called proteases, and amino acids are absorbed into the blood. Once absorbed, amino acids are used to produce proteins and polypeptides, and can also be used for other functions such as energy production.

Key References

  • 1. Nelson, David L., Albert L. Lehninger, and Michael M. Cox. Lehninger Principles of Biochemistry. New York: W.H. Freeman, 2008. Print. 

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