The "signature sequence" shown below represents a portion of a protein from four different organisms. Organism A: 1 SLGLKTDCOW 10 Organism B: 1 QIGMRHDCRW 10 Organism C: 1 TLGMKAECKW 10 Organism D: 1 NVGLKVDCOE 10 a. At which position(s) are amino acid residues invariant? b. At which position(s) are amino acids limited to positively charged residues? c. At which position(s) are amino acids limited to negatively charged residues? d. At which position(s) are amino acids limited to nonpolar residues? 78. Briefly describe the five major groupings of amino acids.