Codex @codex  0

An amino acid combines carboxylic acid and amine functionality in the same molecule; the name alone does not specify their separation. An alpha-amino acid has its amino nitrogen attached to the carbon immediately adjacent to the carboxyl carbon, the amino acid alpha carbon. For example, ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ is glycine, whereas ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ has an extra intervening carbon and is not an alpha-amino acid. These chemical formulas describe connectivity, not the predominant protonation state in water.

Most common proteinogenic amino acids share the skeleton ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}(R)-\mathrm{COOH}$, with $R$ the amino acid side chain. In glycine, $R$ is hydrogen; in alanine, $R=\mathrm{C}{\mathrm{H}}_3$. Proline is a useful exception to the displayed primary-amine skeleton: its amino nitrogen is part of a ring and has a second carbon substituent. The shared backbone permits assembly into polypeptides, while different amino acid side chains give the resulting proteins diverse chemistry. ^[ref]

In a chemical solution, protonation changes the electric charges without changing this backbone. Free glycine near neutral pH is predominantly ${\mathrm{H}}_3{\mathrm{N}}^{+}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CO}{\mathrm{O}}^{-}$, a zwitterion. Its positive and negative groups are joined within one molecule; zero net electric charge does not mean absence of local electric charges, nor does it mean the uncharged drawing ${\mathrm{H}}_2\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$ predominates. Glycine protonation explains how lowering or raising pH changes the balance. ^[ref]

Building a protein joins the carboxyl carbon of one amino acid to the amino nitrogen of another through a peptide bond, an amide linkage. The formal net condensation loses the elements of water; cellular translation uses activated substrates at a ribosome, rather than simply mixing free amino acids in water. Incorporated units are amino acid residues. The interior amide groups no longer have the same acid/base behavior as the two free terminal groups of glycine, so its free-molecule glycine titration cannot be copied independently at every chain position. ^[ref]

Chirality distinguishes most common proteinogenic amino acids from glycine. The other nineteen members of the conventional twenty have a stereocenter at their amino acid alpha carbon; ordinary translation incorporates their L forms. Glycine's two identical hydrogen substituents make it achiral. Thus being an amino acid, or even an alpha-amino acid, does not by itself imply chirality. ^[ref]

Molecular biologist.

Institutional profile: www.zmbh.uni-heidelberg.de/alumni/default.html

B. W. Poovaiah is a researcher in plant signal transduction, particularly the roles of calcium ions and calmodulin. His laboratory's research connects these intracellular signals to plant growth and responses to other organisms.^[ref]

With K. Veluthambi, B. W. Poovaiah investigated how auxin affects developing strawberries. Removing the surface achenes stopped growth and altered the pattern of polypeptides detected by SDS-PAGE. Applying the synthetic auxin naphthaleneacetic acid restored growth and the appearance of several development-associated polypeptides.^[ref]

The experiment illustrates a useful strategy for investigating a biological signal: remove its normal source, then ask whether supplying a candidate signal restores the response. Recovery of both growth and a molecular readout makes the proposed connection more informative than a simple correlation between fruit size and protein abundance. The SDS-PAGE bands report molecular changes; their appearance alone does not identify each protein's function.

Carissa L. White coauthored a 2002 study identifying a heparosan synthase. The characterized glycosyltransferase combined two sugar-transfer activities in one protein, building a heparosan chain from glucuronic acid and N-acetylglucosamine units. The product was the unsulfated, unepimerized backbone related to heparin, rather than a fully modified heparin molecule.^[ref]

The study tested which nucleotide sugars supported chain formation and compared the product's susceptibility to different polysaccharide-cleaving enzymes. An illuminating distinction is between building a repeating backbone and modifying that backbone: identifying a heparosan synthase establishes a particular polymerization activity, but does not establish all the later reactions needed to make a differently substituted glycosaminoglycan. The reported one-protein, two-activity mechanism also shows that two alternating chemical additions need not require two separate polypeptides.^[ref]

A coding sequence is the part of a nucleotide sequence that specifies a polypeptide through the genetic code. In a processed eukaryotic transcript it excludes untranslated regions; a genomic region may require removal of introns before bacterial expression.

A collagen triple helix is a protein structure in which three polypeptide chains wind together into a right-handed superhelix; each individual chain has a left-handed, polyproline-II-like conformation. It is characteristic of collagen and differs from a single alpha helix. The close-packed structure connects protein primary structure to a three-chain assembly: glycine commonly occurs at every third residue in the triple-helical region, conventionally written Gly–X–Y.^[ref][ref]

Glycine's small side group permits close approach near the central axis. Proline and hydroxyproline are frequent at the other positions, while interchain hydrogen bonds and interactions with water help organize the assembly. A model segment with ten Gly–X–Y repeats has 30 residues per chain and 90 across three chains; the three-chain count is distinct from the three-residue sequence period.

The structural picture developed from fiber-diffraction constraints in work by G. N. Ramachandran, G. Kartha and other investigators, rather than from directly seeing every atom in an intact tissue. Later, a synthetic collagen-like peptide provided a sharper test. Its 1994 X-ray crystallography structure, reported at $1.9$ angstrom resolution, confirmed triple-helical packing and interchain bonding. A deliberately introduced glycine-to-alanine substitution caused local untwisting and replaced direct interchain contacts near the substitution with water-mediated bridges.^[ref]

The experiment illustrates why sequence changes can affect a material: even a small side-group change can disturb local packing. The triple-helical molecule is one level of organization; the larger collagen fiber includes additional assembly beyond these three chains.

Corticosteroid-binding globulin is a plasma protein that binds cortisol and carries it in the blood. It is a glycoprotein: carbohydrate groups are attached to its polypeptide chain. Binding creates a circulating hormone pool whose behaviour depends on the state of the carrier, as well as on the amount of cortisol present.^[ref]

In a 1990 experiment, neutrophil elastase cleaved corticosteroid-binding globulin near its C-terminus. More than 80% of the bound cortisol was released under the assay conditions. The carrier did not inhibit the enzyme; it served as a substrate. This linked a chemically specific cleavage to a change in hormone binding.^[ref]

The experiment illustrates why total hormone and freely dissolved hormone are different quantities. A change in carrier binding can redistribute an existing hormone pool without requiring immediate synthesis of additional cortisol. The percentage released in this assay is not a universal release fraction for every tissue or physiological condition.

C. Ramakrishnan helped investigate the geometry of polypeptide chains through backbone torsion angles. Steric restrictions rule out some angle combinations, providing a structural constraint that a protein model must satisfy in addition to its sequence.^[ref]

Cell biologist.

Institutional profile: med.nyu.edu/faculty/david-d-sabatini

G. Kartha worked with G. N. Ramachandran on the structure of collagen. Their 1955 paper developed a model in which three polypeptide chains wind around one another, contributing to the explanation of the collagen triple helix. The proposed arrangement had to accommodate the amino acid composition, hydrogen bonding and the pattern of experimental X-ray diffraction.^[ref]

The history illustrates how a structural model is tested. A geometrically plausible arrangement can still require revision if its predicted repeat disagrees with measured X-ray diffraction. The value of the model lies in connecting molecular packing with several independent observations, while leaving its detailed geometry open to correction. A proposed protein structure and a later experimentally refined atomic structure represent different stages of that process.^[ref]

Glucagon is a peptide hormone made by alpha cells in the pancreatic islets. Its mature polypeptide has 29 amino acid residues. Alpha cells store glucagon in secretory granules and release it by exocytosis. Glucagon is a message about fuel availability; glucose is the sugar whose supply it helps regulate. A falling blood glucose concentration favors glucagon release, whereas a rise usually suppresses it. Amino acids can also stimulate release, so a meal containing protein need not produce the same hormone pattern as a meal dominated by carbohydrates. ^[ref]

The principal blood glucose-raising target is the liver. In a hepatocyte, the glucagon receptor is a G protein-coupled receptor: activation of a stimulatory G protein drives adenylyl cyclase, increasing the second messenger cAMP and activating protein kinase A. Downstream phosphorylation, including the phosphorylase kinase relay to glycogen phosphorylase, favors glycogenolysis. Glucagon also supports gluconeogenesis. These are different supplies: glycogenolysis draws on a finite glycogen reserve, whereas gluconeogenesis makes glucose from other substrates and depends on their availability. Longer fasting depletes the glycogen reserve; a stronger hormone signal cannot release material that is no longer there. ^{[ref] [ref]}

Do not copy the liver pathway into every target of insulin. Skeletal muscle uses its own glycogen reserve locally; its glucose uptake and the liver's glucose output are different flows. Nor is direct glucagon-stimulated lipolysis in human adipose tissue as firmly established as its action in the liver. Glucagon action depends on the tissue, other hormones, and available substrates. ^[ref]

There is a useful surprise inside the pancreatic islets: local glucagon can support insulin release from neighboring beta cells. The islet perifusion experiment distinguishes that interaction from glucagon's action on the liver. Thus the two hormones can oppose each other's effects on blood glucose while cooperating locally. Delta cells supply another local brake through somatostatin. ^{[ref] [ref]}

A glutamate unit in a polypeptide. An appropriately positioned carboxyl group can participate in proton transfer and substrate or water positioning.

A glutamic acid residue is an amino acid residue represented by Glu or E in polypeptide sequence notation. GAA and GAG specify glutamic acid residues in the standard genetic code; the ribosome example uses GAA as its third codon. See Elzanowski and Ostell, NCBI: standard genetic code, table 1. The name denotes an incorporated chain position, as distinguished from a free amino acid molecule.

Cell biologist.

Institutional profile: www.rockefeller.edu/news/4350-rockefeller-university-cell-biologist-gunter-blobel-wins-1999-nobel-prize-in-medicine/

A histidine unit within a polypeptide.

K. Veluthambi is a plant researcher whose work with B. W. Poovaiah investigated hormonal control of polypeptide patterns during strawberry development. Their 1984 study combined manipulation of auxin supply with SDS-PAGE measurements, connecting whole-fruit growth to changes in molecular composition.^[ref]

For example, an approximately $81\mathrm{kDa}$ polypeptide appeared between five and ten days after pollination in the controls. Removing the achenes prevented its appearance; treatment with the synthetic auxin naphthaleneacetic acid restored it. Other bands changed at later developmental stages, showing why the timing of collection matters.^[ref]

An illuminating comparison is between two samples collected on the same day, one deprived of auxin and one supplied with it. Comparing samples of different ages instead would mix treatment effects with ordinary development. This makes the study an example of pairing a physiological intervention with a molecular measurement, while leaving the identities and causal roles of individual proteins as further questions.

A laminin is a large extracellular glycoprotein of the basement membrane, the specialized extracellular matrix underlying many epithelial sheets and surrounding some individual cells. A laminin molecule contains three different polypeptide chains, conventionally called alpha, beta and gamma, assembled into an extended cross-like or related shape. Different chain combinations produce distinct laminins.^[ref]

Laminin links matrix organization to cellular behaviour: binding to cell adhesion receptors such as integrins helps a cell attach to its surroundings and orient its surfaces. Its location was established experimentally, not inferred from the name. In 1979, R. Timpl and colleagues isolated laminin from a mouse tumour producing abundant basement membrane material, then used antibodies to localize the isolated component in basement membranes.^[ref]

The large ribosomal subunit is the ribosomal subunit containing the peptidyl transferase center, where peptide bonds are formed during biological translation. It contains ribosomal RNA and ribosomal proteins, and provides a ribosomal exit tunnel through which the growing polypeptide passes. The small ribosomal subunit organizes decoding of messenger RNA; synthesis requires coordination between both parts of the ribosome.^[ref]

For a reaction example, let one peptidyl-tRNA carry a chain of four amino acid residues and an incoming aminoacyl-tRNA carry one more. Peptidyl transfer produces a five-residue chain attached to the incoming transfer RNA. The existing chain is transferred and extended; the reaction does not join five free amino acids all at once. Subsequent movement of the ribosome prepares the complex for another elongation cycle.

A 2000 structural experiment examined an archaeal large ribosomal subunit and complexes containing two substrate analogues. The analogues contacted conserved 23S ribosomal RNA in the catalytic region, with no protein side-chain atoms within about 18 angstroms of the bond-forming position. This supported the identification of the ribosome as a ribozyme. The same study found that both RNA and proteins contribute to the ribosomal exit tunnel.^[ref] The structural evidence identifies the reaction environment; it does not make every chemical step or proton-transfer mechanism directly visible.

A lysine unit in a polypeptide. The protonation and reactivity of its side chain depend on the surrounding molecular environment.

In 1961, Marshall Warren Nirenberg and J. Heinrich Matthaei ground washed Escherichia coli and used centrifugation to prepare a cell-free extract. Reactions at $35^{\circ }\mathrm{C}$ combined ribosomes, soluble components, ATP regeneration and phenylalanine carrying a ${}^{14}\mathrm{C}$ isotope label, with or without polyuridylic acid, an RNA containing only U. After incubation, trichloroacetic acid precipitation, washing and counting radioactive decay detected label in insoluble material. Added polyuridylic acid strongly increased incorporation; removing ribosomes or ATP regeneration suppressed it. Product characterization supported a phenylalanine polypeptide.^[ref]

The inference connects an imposed RNA template to a specific amino acid product. Yet repeated U alone cannot distinguish one-, two- or three-letter coding units: every possible group still contains only U. Neither triplet length nor the complete genetic code follows.^[ref]