【正文】 dehydration （脫水） synthesis (also called a condensation reaction) in which a covalent bond is formed between the two monomers while a water molecule is also formed from the OH groups. ? This reaction is catalyzed by enzymes. ? This same type of condensation reaction can occur to form different macromolecules, including proteins, nucleic acids, polysacchrides and lipids. Molecular Biology 大連理工大學(xué) 30 Most Common Monomers in Cells Molecule No. Present Names of Molecules Role in cell Amino acid 20 Ala (A) Leu (L) Arg (R) Lys (K) Asp (N) Met (M) Asp (D) Phe (F) Cys (C) Pro (P) Gln (Q) Ser (S) Glu (E) Thr (T) Gly (G) Trp (W) His (H) Tyr (Y) Ile (I) Val (V) Monomers of proteins nucleotides 5 Adenine Thymine Cytosine Uracil Guanine Monomers for nucleic acids Sugars 2 Ribose Component of nucleic acids Glucose Component of polysaccharides Energy metabolism Lipids 3 Choline膽堿 Components of phospholipids Glycerol Palmitate棕櫚酸 Molecular Biology 大連理工大學(xué) Proteins Molecular Biology 大連理工大學(xué) Proteins are Linear Chains of Linked Amino Acids Exception: proline only has an imino group Molecular Biology 大連理工大學(xué) A Common Thread but a Unique Identity Molecular Biology 大連理工大學(xué) Amino acids ? 60 kinds in a cell, but only 20 involved in protein synthesis ? The 20 essential amino acids can be categorized into three groups – Nonpolar (hydrophobic)（疏水） – Polar uncharged (hydrophilic)（親水） – Polar charged (hydrophilic) Molecular Biology 大連理工大學(xué) Molecular Biology 大連理工大學(xué) Form salt bridges Contain groups that form hydrogen bonds with water Molecular Biology 大連理工大學(xué) Polar uncharged Cysteine ? Cysteine (Cys, C，半胱氨酸 ) has a thiol (硫醇 ) group (SH)， which is often oxidizes（氧化） to cystine（光氨酸） . Disulfide bond Molecular Biology 大連理工大學(xué) Amino acids aromatic (芳香族， 3) ? Accounts for most of UV absorbance of proteins at 280 nm Molecular Biology 大連理工大學(xué) Chirality 手性 ? ? carbon is chiral (asymmetric 不對稱的 ) except in glycine (甘氨酸， R is H) – The two asymmetric forms are referred to as D and L ? D es from dextrorotatory(右旋，right turning) ? L es from levorotatory(左旋， left turning) – All biologically active amino acids are of L form. Molecular Biology 大連理工大學(xué) Molecular Biology 大連理工大學(xué) Peptide bond ? Polypeptides are built through a series of dehydration (condensation) reactions – The OH and H groups are removed from the carboxyl group of one amino acid and the amino group of the next amino acid – The removal of water forms a covalent peptide bond Peptide bonds Molecular Biology 大連理工大學(xué) ? Polypeptides contain N termini (amino terminus) and C termini (carboxyl terminus ), usually ranging from 1001500 aa. Peptide bonds Nterminus and Cterminus N terminus C terminus Molecular Biology 大連理工大學(xué) Some of the Diverse Functions of Proteins Molecular Biology 大連理工大學(xué) Primary Structure ? Amino acid sequence of a protein ? Written from amino terminus to carboxyl terminus ? Coded for by nucleotide sequence from the mRNA, therefore represents the mands of the DNA sequence ? Sequence determines secondary, tertiary and quaternary structures, as well as proteinprotein and proteinnucleic acid recognitions ? A single substitution may result in a protein that is not folded properly (hemoglobin) Molecular Biology 大連理工大學(xué) Secondary structure ? Folding into ? helix, b sheet or random coil ? Involves local interactions between amino acids ? Interactions lead to the formation of the ? helix or b sheet 己糖激酶 Molecular Biology 大連理工大學(xué) ? helix ?Helix is derived from repeating polymers and results in amino acids per turn, bringing every 4th amino acid in proximity ?L (leu), M(met), and E(glu) are strong helix formers ?G(gly) and P(pro) are helix breakers and are involved in bends and turns in helices Molecular Biology 大連理工大學(xué) The b sheet ?Maximized by hydrogen bonding, but between two polypeptides or two different segments of one polypeptide ?I(ile), V(val), and F(phe) are strong sheet formers ?If the two strands linked are aligned with both amino and both carbonyl groups running in the same direction, then this is a parallel sheet ?If the two strands run in opposite directions, this is an antiparallel sheet Molecular Biology 大連理工大學(xué) Motifs Motifs: monly occurring secondary structures – b?b motif – Hairpin turn motif – Helixturnhelix motif Molecular Biology 大連理工大學(xué) Tertiary structure ? depends more on the R groups than the amino or carboxyl groups ? Hydrophobic domains tend to associate with one another and are often found in the inner sections of the protein Molecular Biology 大連理工大學(xué) Stabilization of tertiary structures ?Noncovalent bonds ?Hydrogen bonds between appropriate R groups ?Electrostatic interactions between charged R groups ?Hydrophobic interactions between nonpolar R groups ?Covalent bonds ?Disulfide bond between two cysteine residues ?Disulfide bonds are important for stabilizing both tertiary and quaternary structures Molecular Biology 大連理工大學(xué) Molecular Biology 大連理工大學(xué) Protein domain ? Most globular(球狀 ) proteins consists of several domains(結(jié)構(gòu)域 ) ? A domain is a discrete, locally folded unit of tertiary structure – 50350 residues – Each domain has a specific function within the protein, such as metal binding domain of enzymes – Small globular proteins tend to have a single domain – Large globular proteins have multiple domains Molecular Biology 大連理工大學(xué) Quaternary Structure Level of protein anization related to subunit interactions and asse