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s Related Biological Terms:

allostery

Allosteric regulation; the modification of binding or catalytic properties of a protein by binding of a regulator at a site distinct from the ligand- or substrate-binding site. allostery typically results in sigmoid kinetics. (see also heterotropic enzyme; homotropic enzyme)

alpha (α)-isomer

In sugar chemistry, the anomer that places the hemiacetal (or hemiketal) hydroxy group on the side of the pyranose (or furanose) ring opposite the non-ring carbon atom (i.e. C-6 of glucose) that is attached to the carbon whose configuration defines the sugar as having a D- or L-configuration, i.e. C-5 of glucose; thus the mirror image of a-D-glucose is a-L-glucose. For example, for the pyranose form of a-D-glucose, the C-1 hydroxy group is on the opposite side of the ring from C-6. (see also Haworth projection)

alpha/beta (α/β)-hydrolase fold

A structure common to a group of proteins, including esterases, carboxypeptidases and dehalogenases, that have a catalytic triad of nucleophile, histidine and carboxylate positioned on a scaffolding of eight -sheets connected by -helices. (see also topology/packing diagram)Ollis, D.L., Cheah, E., Cygler, M., et al. (1992) Protein Eng. 5, 197-211

alpha1 α1)-cysteine proteinase inhibitor

An enzyme inhibitor that also functions as a kininogen; specifically H-kininogen and, in the rat, also T-kininogen. (see also cystatin)

alpha2 (α2)-cysteine proteinase inhibitor

(= L-kininogen; see kininogen)

altered-self hypothesis

The proposal that helper T-cells recognize a foreign material, e.g. a viral antigen, when it is presented on the surface of an antigen-presenting cell as a complex with the major histocompatibility complex (MHC) class II glycoprotein. This is contrasted with a less favoured explanation, the intimacy or dual-recognition model, which postulates that helper T-cells must simultaneously recognize both the foreign antigen and the separate MHC complex. By a simple extension, the altered-self hypothesis further proposes that a chemically altered MHC may also be recognized by helper T-cells.

alternative splicing

The production of more than one mRNA from a single pre-mRNA due to differences in the excision of introns and/or the use of stop codons. Black, D.L. (2000) Cell 103, 367-370

amino acyl site

The part of a ribosome that binds one amino acyl-tRNA where it will accept the peptidyl group held at the peptide site in the form of its tRNA ester. (see also peptide site)

amniocentesis

A procedure to obtain fetal cells during pregnancy by puncturing the womb with a needle and removing some fluid that surrounds the fetus. The procedure is used, in part, to obtain fetal cells from which DNA can be isolated for prenatal genetic analysis.

amplification of refractory mutation system

(see PCR amplification of specific alleles)

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