Chromatographic Methods Three polypeptides, the sequences of...

Chromatographic Methods Three polypeptides, the sequences of which are represented below using the one-letter code for their amino acids, are present in a mixture: 1. ATKNRASCLVPKHGALMFWRHKQLVSDPILQKRQHILVCRNAAG 2. GPYFGDEPLDVHDEPEEG 3. PHLLSAWKGMEGVGKSQSFAALIVILA Of the three, which one would migrate most slowly during chromatography through: (a) an ion-exchange resin, beads coated with positively charged groups? (b) an ion-exchange resin, beads coated with negatively charged groups? (c) a size-exclusion (gel-filtration) column designed to separate small peptides such as these? (d) Which peptide contains the ATP-binding motif shown in the following sequence $\log _{0} ?$

Chromatographic Methods Three polypeptides, the sequences of which are represented below using the one-letter code for their amino acids, are present in a mixture:
1. ATKNRASCLVPKHGALMFWRHKQLVSDPILQKRQHILVCRNAAG
2. GPYFGDEPLDVHDEPEEG
3. PHLLSAWKGMEGVGKSQSFAALIVILA
Of the three, which one would migrate most slowly during chromatography through:
(a) an ion-exchange resin, beads coated with positively charged groups?
(b) an ion-exchange resin, beads coated with negatively charged groups?
(c) a size-exclusion (gel-filtration) column designed to separate small peptides such as these?
(d) Which peptide contains the ATP-binding motif shown in the following sequence $\log _{0} ?$

Key Concepts

Size-Exclusion (Gel Filtration) Chromatography

This chromatographic method separates molecules based on their sizes (or more precisely, their hydrodynamic volume). In a gel-filtration column, larger molecules are excluded from entering the pores of the resin beads and thus travel a shorter path, eluting faster; whereas smaller molecules penetrate the pores and experience a larger volume, eluting more slowly.

ATP-Binding Motif (Walker A Motif)

ATP-binding motifs, such as the Walker A motif, are conserved sequences found in nucleotide-binding proteins. They typically feature a glycine-rich loop that interacts with phosphate groups of ATP. Recognizing these motifs is important for predicting ATP-binding properties of peptides and understanding their functional roles in cellular processes.

Ion-Exchange Chromatography

This technique separates molecules based on differences in their net charge by using a resin with ionically charged groups. In cation exchange, the resin is negatively charged and binds positively charged analytes, while in anion exchange, the resin is positively charged and binds negatively charged analytes. The overall net charges of peptides, determined by the ionizable side chains of amino acids and the solution pH, influence how strongly they interact with the resin and thus their elution order.

Net Charge and Amino Acid Composition

The net charge of a peptide is dictated by the distribution of acidic and basic amino acids in its sequence. By considering the pKa values and the environmental pH, one can predict whether a peptide is overall positively or negatively charged. This prediction is crucial for understanding its behavior in ion-exchange chromatography, as the affinity of the peptide for a given charged resin will depend on its net charge.

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