α-helix

(noun)

secondary structure of proteins where every backbone N-H creates a hydrogen bond with the C=O group of the amino acid four residues earlier in the same helix.

Related Terms

Examples of α-helix in the following topics:

  • Protein Structure

    • The most common forms of secondary structure are the α-helix and β-pleated sheet structures and they play an important structural role in most globular and fibrous proteins.
    • In the α-helix chain, the hydrogen bond forms between the oxygen atom in the polypeptide backbone carbonyl group in one amino acid and the hydrogen atom in the polypeptide backbone amino group of another amino acid that is four amino acids farther along the chain.
    • The R groups (the side chains) of the polypeptide protrude out from the α-helix chain and are not involved in the H bonds that maintain the α-helix structure.
    • The α-helix and β-pleated sheet form because of hydrogen bonding between carbonyl and amino groups in the peptide backbone.
    • Certain amino acids have a propensity to form an α-helix, while others have a propensity to form a β-pleated sheet.

  • Initiation of Transcription in Prokaryotes

    • Four of these subunits, denoted α, α, β, and β', comprise the polymerase core enzyme.
    • Each subunit has a unique role: the two α-subunits are necessary to assemble the polymerase on the DNA; the β-subunit binds to the ribonucleoside triphosphate that will become part of the nascent "recently-born" mRNA molecule; and the β' binds the DNA template strand.
    • The nucleotide pair in the DNA double helix that corresponds to the site from which the first 5' mRNA nucleotide is transcribed is called the +1 site, or the initiation site.

  • The DNA Double Helix

    • The DNA double helix looks like a twisted staircase, with the sugar and phosphate backbone surrounding complementary nitrogen bases.
    • DNA has a double-helix structure, with sugar and phosphate on the outside of the helix, forming the sugar-phosphate backbone of the DNA.
    • The two strands of the helix run in opposite directions, so that the 5′ carbon end of one strand faces the 3′ carbon end of its matching strand.
    • During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand.
    • Native DNA is an antiparallel double helix.

  • Types of Receptors

    • The subunits of the G-protein then split into the α subunit and the β subunit.
    • Later, the GTP on the active α subunit of the G-protein is hydrolyzed to GDP and the β subunit is deactivated.
    • Heterotrimeric G proteins have three subunits: α, β, and γ.
    • When a signaling molecule binds to a G-protein-coupled receptor in the plasma membrane, a GDP molecule associated with the α subunit is exchanged for GTP.
    • The β and γ subunits dissociate from the α subunit, and a cellular response is triggered either by the α subunit or the dissociated β pair.

  • Control of Catabolic Pathways

    • These enzymes are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
    • When more ATP is needed, as reflected in rising ADP levels, the rate increases. α-Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA, a subsequent intermediate in the cycle, causing a decrease in activity.
    • A decrease in the rate of operation of the pathway at this point is not necessarily negative as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.
    • Enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, catalyze the reactions that make the first two molecules of NADH in the citric acid cycle.

  • Epinephrine and Norepinephrine

    • Binding to α-adrenergic receptors inhibits insulin secretion by the pancreas, stimulates glycogenolysis (the breakdown of glycogen) in the liver and muscle, and stimulates glycolysis (the metabolic pathway that converts glucose into pyruvate) in muscle. β-Adrenergic receptor binding triggers glucagon secretion in the pancreas, increased adrenocorticotropic hormone (ACTH) secretion by the pituitary gland, and increased lipolysis by adipose tissue.
    • When norepinephrine acts as a drug, it increases blood pressure by increasing vascular tone through α-adrenergic receptor activation.

  • Intermediate Filaments and Microtubules

    • The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins .
    • Microtubules are hollow, with walls consisting of 13 polymerized dimers of α-tubulin and β-tubulin (right image).

  • The Role of Blood in the Body

    • Medical terms related to blood often begin with hemo- or hemato- (also spelled haemo- and haemato-), which is from the Greek word α (haima) for "blood".

  • The Structure and Sequence of DNA

    • DNA is a double helix of two anti-parallel, complementary strands having a phosphate-sugar backbone with nitrogenous bases stacked inside.
    • Watson and Crick proposed that DNA is made up of two polynucleotide strands that are twisted around each other to form a right-handed helix.
    • The diameter of the DNA double helix is 2 nm and is uniform throughout.
    • Therefore, ten base pairs are present per turn of the helix.
    • DNA has (a) a double helix structure and (b) phosphodiester bonds.

  • Citric Acid Cycle

    • In step three, isocitrate is oxidized, producing a five-carbon molecule, α-ketoglutarate, together with a molecule of CO2 and two electrons, which reduce NAD+ to NADH.
    • Steps three and four are both oxidation and decarboxylation steps, which release electrons that reduce NAD+ to NADH and release carboxyl groups that form CO2 molecules. α-Ketoglutarate is the product of step three, and a succinyl group is the product of step four.