Protein

A representation of the 3D structure of the protein myoglobin showing turquoise α-helices. This protein was the first to have its structure solved by X-ray crystallography. Toward the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red).
John Kendrew with model of myoglobin in progress
Chemical structure of the peptide bond (bottom) and the three-dimensional structure of a peptide bond between an alanine and an adjacent amino acid (top/inset). The bond itself is made of the CHON elements.
Resonance structures of the peptide bond that links individual amino acids to form a protein polymer
A ribosome produces a protein using mRNA as template
The DNA sequence of a gene encodes the amino acid sequence of a protein
The crystal structure of the chaperonin, a huge protein complex. A single protein subunit is highlighted. Chaperonins assist protein folding.
Three possible representations of the three-dimensional structure of the protein triose phosphate isomerase. Left: All-atom representation colored by atom type. Middle: Simplified representation illustrating the backbone conformation, colored by secondary structure. Right: Solvent-accessible surface representation colored by residue type (acidic residues red, basic residues blue, polar residues green, nonpolar residues white).
Molecular surface of several proteins showing their comparative sizes. From left to right are: immunoglobulin G (IgG, an antibody), hemoglobin, insulin (a hormone), adenylate kinase (an enzyme), and glutamine synthetase (an enzyme).
The enzyme hexokinase is shown as a conventional ball-and-stick molecular model. To scale in the top right-hand corner are two of its substrates, ATP and glucose.
Ribbon diagram of a mouse antibody against cholera that binds a carbohydrate antigen
Proteins in different cellular compartments and structures tagged with green fluorescent protein (here, white)
Constituent amino-acids can be analyzed to predict secondary, tertiary and quaternary protein structure, in this case hemoglobin containing heme units

Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues.

- Protein
A representation of the 3D structure of the protein myoglobin showing turquoise α-helices. This protein was the first to have its structure solved by X-ray crystallography. Toward the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red).

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Distillation apparatus

Residue (chemistry)

Whatever remains or acts as a contaminant after a given class of events.

Whatever remains or acts as a contaminant after a given class of events.

Distillation apparatus
3D image of Aflatoxin

In biochemistry and molecular biology, a residue refers to a specific monomer within the polymeric chain of a polysaccharide, protein or nucleic acid.

Structure of a generic L-amino acid in the "neutral" form needed for defining a systematic name, without implying that this form actually exists in detectable amounts either in aqueous solution or in the solid state.

Amino acid

Amino acids are organic compounds that contain amino (\sNH3+) and carboxylate (\sCO2-) functional groups, along with a side chain (R group) specific to each amino acid.

Amino acids are organic compounds that contain amino (\sNH3+) and carboxylate (\sCO2-) functional groups, along with a side chain (R group) specific to each amino acid.

Structure of a generic L-amino acid in the "neutral" form needed for defining a systematic name, without implying that this form actually exists in detectable amounts either in aqueous solution or in the solid state.
The 21 proteinogenic α-amino acids found in eukaryotes, grouped according to their side chains' pKa values and charges carried at physiological pH (7.4)
Structure of -proline
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Ionization and Brønsted character of N-terminal amino, C-terminal carboxylate, and side chains of amino acid residues
Composite of titration curves of twenty proteinogenic amino acids grouped by side chain category
Share of amino acid in various human diets and the resulting mix of amino acids in human blood serum. Glutamate and glutamine are the most frequent in food at over 10%, while alanine, glutamine, and glycine are the most common in blood.
The Strecker amino acid synthesis
The condensation of two amino acids to form a dipeptide. The two amino acid residues are linked through a peptide bond
Catabolism of proteinogenic amino acids. Amino acids can be classified according to the properties of their main degradation products: 
 * Glucogenic, with the products having the ability to form glucose by gluconeogenesis
 * Ketogenic, with the products not having the ability to form glucose. These products may still be used for ketogenesis or lipid synthesis.
 * Amino acids catabolized into both glucogenic and ketogenic products.
Composite of titration curves of twenty proteinogenic amino acids grouped by side chain category

More than 500 naturally occurring amino acids are known to constitute monomer units of peptides, including proteins, although only 22 appear in the genetic code, 20 of which have their own designated codons and 2 of which have special coding mechanisms: Selenocysteine which is present in all eukaryotes and pyrrolysine which is present in some prokaryotes.

Fig. 1 N-terminal acetylation

Protein primary structure

Fig. 1 N-terminal acetylation

Protein primary structure is the linear sequence of amino acids in a peptide or protein.

DNA replication: The double helix is un'zipped' and unwound, then each separated strand (turquoise) acts as a template for replicating a new partner strand (green). Nucleotides (bases) are matched to synthesize the new partner strands into two new double helices.

DNA replication

Biological process of producing two identical replicas of DNA from one original DNA molecule.

Biological process of producing two identical replicas of DNA from one original DNA molecule.

DNA replication: The double helix is un'zipped' and unwound, then each separated strand (turquoise) acts as a template for replicating a new partner strand (green). Nucleotides (bases) are matched to synthesize the new partner strands into two new double helices.
DNA polymerases adds nucleotides to the 3′ end of a strand of DNA. If a mismatch is accidentally incorporated, the polymerase is inhibited from further extension. Proofreading removes the mismatched nucleotide and extension continues.
Overview of the steps in DNA replication
Steps in DNA synthesis
Role of initiators for initiation of DNA replication.
Formation of pre-replication complex.
Scheme of the replication fork.
a: template, b: leading strand, c: lagging strand, d: replication fork, e: primer, f: Okazaki fragments
Many enzymes are involved in the DNA replication fork.
The assembled human DNA clamp, a trimer of the protein PCNA.
E. coli Replisome. Notably, the DNA on lagging strand forms a loop. The exact structure of replisome is not well understood.
The cell cycle of eukaryotic cells.
Dam methylates adenine of GATC sites after replication.
Replication fork restarts by homologous recombination following replication stress
Epigenetic consequences of nucleosome reassembly defects at stalled replication forks

A number of proteins are associated with the replication fork to help in the initiation and continuation of DNA synthesis.

A series of codons in part of a messenger RNA (mRNA) molecule. Each codon consists of three nucleotides, usually corresponding to a single amino acid. The nucleotides are abbreviated with the letters A, U, G and C. This is mRNA, which uses U (uracil). DNA uses T (thymine) instead. This mRNA molecule will instruct a ribosome to synthesize a protein according to this code.

Genetic code

A series of codons in part of a messenger RNA (mRNA) molecule. Each codon consists of three nucleotides, usually corresponding to a single amino acid. The nucleotides are abbreviated with the letters A, U, G and C. This is mRNA, which uses U (uracil). DNA uses T (thymine) instead. This mRNA molecule will instruct a ribosome to synthesize a protein according to this code.
The genetic code
Reading frames in the DNA sequence of a region of the human mitochondrial genome coding for the genes MT-ATP8 and MT-ATP6 (in black: positions 8,525 to 8,580 in the sequence accession NC_012920 ). There are three possible reading frames in the 5' → 3' forward direction, starting on the first (+1), second (+2) and third position (+3). For each codon (square brackets), the amino acid is given by the vertebrate mitochondrial code, either in the +1 frame for MT-ATP8 (in red) or in the +3 frame for MT-ATP6 (in blue). The MT-ATP8 genes terminates with the TAG stop codon (red dot) in the +1 frame. The MT-ATP6 gene starts with the ATG codon (blue circle for the M amino acid) in the +3 frame.
Examples of notable mutations that can occur in humans.
Axes 1, 2, 3 are the first, second, and third positions in the codon. The 20 amino acids and stop codons (X) are shown in single letter code.
Genetic code logo of the Globobulimina pseudospinescens mitochondrial genome by FACIL. The logo shows the 64 codons from left to right, predicted alternatives in red (relative to the standard genetic code). Red line: stop codons. The height of each amino acid in the stack shows how often it is aligned to the codon in homologous protein domains. The stack height indicates the support for the prediction.

The genetic code is the set of rules used by living cells to translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets, or codons) into proteins.

Peptide bond.

Peptide bond

Peptide bond.
Peptide bond formation via dehydration reaction.
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In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another, along a peptide or protein chain.

A tetrapeptide (example Val-Gly-Ser-Ala) with green marked amino end ( L-Valine ) and
blue marked carboxyl end ( L-Alanine ).

Peptide

Peptides are short chains of amino acids linked by peptide bonds.

Peptides are short chains of amino acids linked by peptide bonds.

A tetrapeptide (example Val-Gly-Ser-Ala) with green marked amino end ( L-Valine ) and
blue marked carboxyl end ( L-Alanine ).
Solid-phase peptide synthesis on a rink amide resin using Fmoc-α-amine-protected amino acid
A tripeptide (example Val-Gly-Ala) with
green marked amino end ( L-Valine ) and
blue marked carboxyl end ( L-Alanine )

A polypeptide that contains more than approximately fifty amino acids is known as a protein.

Post-translational modification of insulin. At the top, the ribosome translates a mRNA sequence into a protein, insulin, and passes the protein through the endoplasmic reticulum, where it is cut, folded, and held in shape by disulfide (-S-S-) bonds. Then the protein passes through the golgi apparatus, where it is packaged into a vesicle. In the vesicle, more parts are cut off, and it turns into mature insulin.

Post-translational modification

Post-translational modification of insulin. At the top, the ribosome translates a mRNA sequence into a protein, insulin, and passes the protein through the endoplasmic reticulum, where it is cut, folded, and held in shape by disulfide (-S-S-) bonds. Then the protein passes through the golgi apparatus, where it is packaged into a vesicle. In the vesicle, more parts are cut off, and it turns into mature insulin.
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Post-translational modification (PTM) refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis.

The succinate dehydrogenase complex showing several cofactors, including flavin, iron–sulfur centers, and heme.

Cofactor (biochemistry)

The succinate dehydrogenase complex showing several cofactors, including flavin, iron–sulfur centers, and heme.
A simple [Fe2S2] cluster containing two iron atoms and two sulfur atoms, coordinated by four protein cysteine residues.
The redox reactions of nicotinamide adenine dinucleotide.

A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction).

Chemical structure of a polypeptide macromolecule

Macromolecule

Chemical structure of a polypeptide macromolecule
Raspberry ellagitannin, a tannin composed of core of glucose units surrounded by gallic acid esters and ellagic acid units
Structure of a polyphenylene dendrimer macromolecule reported by Müllen, et al.

A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid.