What do ?-helices and ?-sheets have in common? In a metabolic pathway, What do ?

Question

What do ?-helices and ?-sheets have in common?

In a metabolic pathway,

What do ?-helices and ?-sheets have in common?

Both are stabilized by hydrogen bonding involving carbonyl oxygens and amide nitrogens. The same amino acids stabilize both forms of secondary structure. The length of a 10-amino acid ?-helix and ?-sheet strand will be the same. Both are stabilized by glycine and proline residues.

In a metabolic pathway,

a reaction with a positive ?G can occur if coupled to a reaction with a greater negative ?G. a reaction with a positive ?G cannot occur. it is ?Go, not ?G, which determines whether a reaction can occur. a reaction with a positive ?G can occur if there is an increase in the concentration of the reaction's products.

Explanation / Answer

1) What do ?-helices and ?-sheets have in common?

Both are stabilized by hydrogen bonding involving carbonyl oxygens and amide nitrogens

Reason: Both alpha helix and beta sheets are different secondary structures of a protein. Alpha helix is a right handed helical coil, while beta sheet is a relatively flat structure. Peptide bond components of both the structures i.e., carbonyl oxygen and amide carbon participate in hydrogen bonding thereby imparting stability to the structure. In alpha helix, each turn constitutes of 3.6 amino acid residues, thus the 1st amino acid will form intrachain hydrogen bonding with the 4th residue lying directly above it. While for beta sheets, the hydrogen bonding can be both intra and interchain, but mostly the latter. Interchain hydrogen bonding occurs with amino acids of two strands lies adjacently. While majorly the amino acid composition is similar for both the secondary structures, but amino acids that are too small (e.g., glycine) or too large (e.g., tryptophan, tyrosine) or with irregular geometry (e.g., proline) destabilise the alpha helix by hindering the hydrogen bond formation in the helix.

2) In a metabolic pathway, a reaction with a positive ?G can occur if coupled to a reaction with a greater negative ?G.

Reason: Metabolic reactions primarily consists of two pathways: (a) Catabolic: Involves release of energy by breakig down of molecules into smaller subunits which may be subsequently oxidised, hydrolysed etc. Such reactions are exergonic and spontaneous. (b) Anabolic: pathway involves using cellular energy to synthesise maromolecules such as lipids, proteins, carbohydrates etc. Such reactions are endergonic and thus non-spontaneous.

Thus, anabolic reactions alone is unfavorable due to negative gibbs free energy. But such reactions when coupled to exergonic catabolic reactions become thermodynamically favourable due to overall negative gibbs free energy change.

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