Question

12. Below is a portion of the abstract of a journal article. It describes the isolation, purification, and determination of the peptide sequence as well as, the location of each of the disulfide bridges of an antibacterial peptide from the cowpea plant. Give brief and to the point answers to the questions based upon this information. (2 pts) FEBS Joumal 273 (2006) 3489-3497 2006 The Authors Journal compilation2006 FEBS Identification of a cowpea y-thionin with bactericidal activity Octávio L. Franco, Andre M. Murad, José R. Leite?,Paulo A M. Mendes, Maura V. Prates and Carlos Bloch Jr Laboratorio de Espectrometria de Massa Embrape Recursos Geneticos e Botecnologie Brsla FB Antimicrobial peptides are an abundant group of proteinaceous com- pounds widely produced in the plant kingdom. Among them, the y-thionin family, also known as plant defensins, represents one typical family and comprises low molecular mass cysteine-rich proteins, usually cationic and distributed in different plant tissues. Here, we report the purification and characterization of a novel γ-thionin from cowpea seeds (Vigna angu- culata), named Cp-thionin II, with bactericidal activity against Gram- positive and Gram-negative bacteria. Once the primary structure was Below is a figure from the article which gives the amino acid sequence for the isolated peptide. CpThioninTCMTKKEGW-GRCLIDTTCAHSCRKYGYMGGKCQGITRRCYCLLNC The black lines above the sequence indicate the disulfide bonds in the structure. You may ignore the dashes.

Answer 1

Generally, when an amino acid sequence is denoted, the amino terminal i.e N is noted first and the sequence tends to end with a carboxyl group that i.e C.

So if we now analyse question a:

The N terminal residue should be K as per the figure in the above abstract. Amino acids are generally noted with either three letters or one letter. Now in the sequence the first amino acid is denoted by a K. This amino acid is Lysine and is denoted by Lys (the 3 letter code).

Moving onto question b:

The C-terminal that is the end of the amino acid sequence is C which is the 1-letter notation for Cysteine. However, the 3-letter notation is Cys.

Question C: The three basic amino acids are

Amino acid	pKa value of R group
Lysine (K)	10.5
Arginine(R)	12.5
Histidine(H)	6.0

The R group with a pKa that is greater than 9.0 is Arginine and Lysine. there are 2 strongly basic amino acids.

Question d: the acidic amino acids are:

Amino acid	pKa value of R group
Aspartate(D)	3.9
Glutamate(E)	4.1

Thus the amino acids whose R group has a pKa value less than 5 are Aspartate and Glutamate, therefore, there are 2.

Question e: The isoelectric point(pI) of the protein CpthioninII can be determined by the pH of its environment.

Question f: The 10th residue Tryptophan

Question g: 4 amino acid residues have been identified. They are :

Alanine A Ala  

Leucine L Leu  

Isoleucine I Ile

Glycine G Gly

Question h: there are 2 amino acid residues that have Sulphur, they are Methionine(M) and Cysteine (C). According to the figure in the abstract there are 4 disulphide bridges.