Covalent and Noncovalent Bonds Stabilize Biologic Molecules
The covalent bond
is the strongest force that holds molecules together (Table 2–1). Noncovalent
forces, while of lesser magnitude, make significant contributions to the
structure, stability, and functional competence of macromolecules in living cells.
These forces, which can be either attractive or repulsive, involve interactions
both within the biomolecule and between it and the water that forms the
principal component of the surrounding environment.
Most biomolecules are
amphipathic; that is, they possess regions rich in charged or polar functional
groups as well as regions with hydrophobic character. Proteins tend to fold
with the R-groups of amino acids with hydrophobic side chains in the interior.
Amino acids with charged or polar amino acid side chains (eg, arginine,
glutamate, serine, see Table 3–1) generally are present on the surface in
contact with water. A similar pattern prevails in a phospholipid bilayer where
the charged “head groups” of phosphatidylserine or phosphatidylethanolamine
contact water while their hydrophobic fatty acyl side chains cluster together,
excluding water (see Figure 40–5). This pattern maximizes the opportunities for
the formation of energetically favorable charge-dipole, dipole-dipole, and
hydrogen bonding interactions between polar groups on the biomolecule and
water. It also minimizes energetically unfavorable contacts between water and
hydrophobic groups.
Hydrophobic Interactions
Hydrophobic
interaction refers to the tendency of nonpolar compounds to self-associate in
an aqueous environment. This self-association is driven neither by mutual
attraction nor by what are sometimes incorrectly referred to as “hydrophobic
bonds.” Self-association minimizes the disruption of energetically favorable
interactions between the surrounding water molecules. While the hydrogens of
nonpolar groups such as the methylene groups of hydrocarbons do not form
hydrogen bonds, they do affect the structure of the water that surrounds them.
Water molecules adjacent to a hydrophobic group are restricted in the number of
orientations (degrees of freedom) that permit them to participate in the
maximum number of energetically favorable hydrogen bonds. Maximal formation of
multiple hydrogen bonds, which maximizes enthalpy, can be maintained only by
increasing the order of the adjacent water molecules, with an accompanying
decrease in entropy. It follows from the second law of thermodynamics that the
optimal free energy of a hydrocarbon-water mixture is a function of both
maximal enthalpy (from hydrogen bonding) and highest entropy (maximum degrees
of freedom). Thus, nonpolar molecules tend to form droplets that minimize
exposed surface area and reduce the number of water molecules whose motional
freedom becomes restricted. Similarly, in the aqueous environment of the living
cell the hydrophobic portions of biopolymers tend to be buried inside the
structure of the molecule, or within a lipid bilayer, minimizing contact with
water.
Electrostatic
Interactions
Interactions
between charged groups help shape biomolecular structure. Electrostatic
interactions between oppositely charged groups within or between biomolecules
are termed salt bridges. Salt bridges are comparable in strength to hydrogen
bonds but act over larger distances. They therefore often facilitate the
binding of charged molecules and ions to proteins and nucleic acids.
van der
Waals Forces
van der Waals forces arise from attractions
between transient dipoles generated by the rapid movement of electrons of all
neutral atoms. Significantly weaker than hydrogen bonds but potentially
extremely numerous, van der Waals forces decrease as the sixth power of the
distance separating atoms, Thus, they act over very short distances, typically
2 to 4 Å.
Multiple
Forces Stabilize Biomolecules
The DNA double
helix illustrates the contribution of multiple forces to the structure of
biomolecules. While each individual DNA strand is held together by covalent
bonds, the two strands of the helix are held together exclusively by
noncovalent interactions such as hydrogen bonds between nucleotide bases
(Watson-Crick base pairing) and van der Waals interactions between the stacked
purine and pyrimidine bases. The double helix presents the charged phosphate
groups and polar hydroxyl groups from the ribose sugars of the DNA backbone to
water while burying the relatively hydrophobic nucleotide bases inside. The
extended backbone maximizes the distance between negatively charged phosphates,
minimizing unfavorable electrostatic interactions.
Daftar Pustaka
Farrier, Dennise .2014.Biokimia Lippincott; Jakarta, Binapura. Ed:6
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