Post-Translational Modifications Explained: Types & Functions

Living organisms use only 20 standard amino acids for protein synthesis. They are classified by side chain properties: nonpolar amino acids (8 types) have hydrocarbon-like side chains, making them hydrophobic and causing proteins to fold inward or associate with membranes. Polar amino acids contain OH or amino groups, making them hydrophilic. Charged amino acids include basic (positive charge) and acidic (negative charge) types. Three special amino acids have unique properties: glycine (smallest side chain, H atom), proline (cyclic structure restricting flexibility), and cysteine (thiol group enabling disulfide bond formation).

The 20 standard amino acids are classified by their R group properties: nonpolar (glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan), polar uncharged (serine, threonine, asparagine, glutamine, tyrosine, cysteine), and charged (lysine, arginine, histidine, aspartic acid, glutamic acid). Proline is unique with its cyclic structure containing a secondary amino group. Each amino acid has a three-letter code and single-letter code essential for biochemistry notation. Understanding these structures is fundamental for interpreting biochemistry questions and understanding protein structure.

The 20 standard amino acids are systematically distributed across five classes: (1) Aliphatic amino acids (7): glycine, alanine, valine, leucine, isoleucine, methionine, proline; (2) Aromatic amino acids (3): phenylalanine, tyrosine, tryptophan; (3) Positively charged amino acids (3): lysine, arginine, histidine; (4) Negatively charged amino acids (2): aspartic acid, glutamic acid; (5) Polar uncharged amino acids (5): serine, threonine, asparagine, glutamine, cysteine. Each amino acid has a one-letter abbreviation used in protein sequencing: G, A, V, L, I, M, F, Y, W, K, R, H, D, E, S, T, N, Q, C. Glycine is the smallest amino acid and is optically inactive due to its symmetric structure. Alanine has the highest propensity to form alpha-helices. Valine, leucine, and isoleucine are branched-chain amino acids with isopropyl-like side chains. Methionine contains a methylthio group and serves as the primary methyl group donor in biological systems. Proline is unique because its side chain forms a five-membered ring connecting back to its amino group, creating a secondary amine and restricting conformational flexibility. Phenylalanine, tyrosine, and tryptophan are aromatic amino acids containing benzene rings, with tryptophan absorbing approximately four times more UV light than tyrosine at 280 nm. Lysine contains four methylene groups giving it an epsilon-amino group for ubiquitin conjugation. Arginine contains a guanidinium ring structure. Histidine contains an imidazole ring providing buffering capacity around physiological pH. Aspartic acid and glutamic acid are negatively charged amino acids. Asparagine and glutamine are amide derivatives. Cysteine contains a thiol group capable of forming disulfide bonds.

The 20 standard amino acids are classified into groups based on their side chain properties. Aromatic amino acids (phenylalanine, tyrosine, tryptophan) contain benzene rings and absorb light at 280nm. Sulfur-containing amino acids (cysteine, methionine) have sulfur atoms. Polar alcohol amino acids (serine, threonine) contain hydroxyl groups. Basic amino acids (lysine, arginine, histidine) have amine groups that accept protons. Acidic amino acids (aspartic acid, glutamic acid) have carboxyl groups that donate protons. Amide amino acids (asparagine, glutamine) have amide groups. Proline is unique with its cyclic structure affecting protein folding.

The 20 standard amino acids are classified based on their side chain properties. Acidic amino acids (aspartic acid, glutamic acid) have negatively charged side chains. Basic amino acids (arginine, lysine, histidine) have positively charged side chains. Aromatic amino acids (phenylalanine, tyrosine, tryptophan) contain ring structures. Hydrophobic amino acids have nonpolar side chains that avoid water, while hydrophilic amino acids interact with water. Each classification affects protein structure, function, and interactions. Amino acids are represented using three-letter and single-letter notation for practical protein sequence writing.

This comprehensive lesson covers three essential cellular organelles. Ribosomes are protein-synthesis factories composed of RNA and protein, discovered by George Emil Palade in 1953. They exist as 70S (prokaryotic) or 80S (eukaryotic) structures with small and large subunits, found in nucleus, mitochondria, and chloroplasts. Messenger RNA carries genetic information to ribosomes, and multiple ribosomes on one mRNA form polyribosomes for efficient protein production. The endoplasmic reticulum (ER) is an interconnected membrane network: rough ER with ribosomes for protein synthesis, smooth ER for lipid synthesis and detoxification. Discovered by Camillo Golgi in 1898, the ER is absent in RBCs and sperm cells. The Golgi apparatus, also discovered by Golgi, consists of flattened cisternae that package and modify proteins and lipids for transport. These organelles work together in the cellular protein synthesis and transport pathway.

Cell organelles are specialized structures within the cytoplasm that perform specific functions: ribosomes (found in all cells) synthesize proteins by linking amino acids according to DNA instructions, forming peptide bonds and releasing water molecules; the endoplasmic reticulum (ER) exists as rough ER (with ribosomes for protein synthesis and processing) and smooth ER (for lipid synthesis and detoxification); and the Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles, working with ribosomes and ER to facilitate protein synthesis, processing, and transport through the secretory pathway.

The endoplasmic reticulum (ER) is involved in transporting substances and synthesizing proteins and lipids. Rough ER has ribosomes for protein synthesis, while smooth ER is involved in lipid synthesis and detoxification. The Golgi apparatus packages and transports materials, modifies substances, and forms lysosomes. It acts as the traffic police of the cell, directing materials to their destinations.

Rough Endoplasmic Reticulum (RER) has ribosomes and is involved in protein synthesis. Smooth Endoplasmic Reticulum (SER) is involved in lipid synthesis and detoxification. The Golgi apparatus (Golgi bodies) is involved in packaging, modifying, and transporting materials, and helps in lysosome formation.

The endoplasmic reticulum (ER) is a network of membrane-bound channels extending throughout the cell, functioning as the cell's internal skeleton; it exists in two forms—rough ER (with ribosomes for protein synthesis) and smooth ER (for steroid and lipid synthesis). The Golgi apparatus, discovered by Camillo Golgi, consists of flattened sacs (cisternae) and vesicles that modify, sort, and package proteins and lipids, and is responsible for forming lysosomes and acrosomes. Ribosomes, discovered by Palade, are cellular structures responsible for protein synthesis, found freely in the cytoplasm or attached to rough ER; prokaryotic cells contain 70S ribosomes (50S + 30S subunits) while eukaryotic cells contain 80S ribosomes (60S + 40S subunits), with the 'S' representing Svedberg units measuring sedimentation rate during centrifugation.

Functional groups are specific atomic arrangements that determine molecular properties and reactivity. The eight key functional groups include: hydroxyl (OH), carbonyl (C=O), carboxyl (-COOH), amino (-NH2), sulfhydryl (-SH), phosphate (PO4^3-), methyl (CH3), and acetyl (CH3-C=O). Each group has distinct structural characteristics and biological roles. Understanding these groups is fundamental to comprehending how biological molecules function and interact in living systems.

Covalent compounds form through electron sharing, with all carbon compounds being covalent. Ethanol (C2H5OH) is an alcohol with the hydroxyl functional group. Hydrocarbons contain only carbon and hydrogen, classified as saturated (alkanes with single bonds) or unsaturated (alkenes with double bonds, alkynes with triple bonds). The N2 molecule has 3 covalent bonds as each nitrogen needs 3 electrons to complete its octet.

This segment covers two essential chemistry concepts. First, covalent bonding occurs when atoms share electrons to form chemical bonds, illustrated through the oxygen molecule (O2) where two atoms share two electrons each. Second, functional groups determine compound properties: ethanol contains the hydroxyl group (-OH) indicated by the suffix '-ol', while ethanoic acid contains the carboxylic acid group (-COOH) indicated by the suffix '-oic acid'. These concepts form the foundation for understanding organic chemistry structure and reactivity.

Functional groups determine compound properties: OH (alcohol), CHO (aldehyde), C=O (ketone), COOH (carboxylic acid), COOCH₃ (ester). Covalent bonds are sigma (head-to-head overlap) or pi (side-wise overlap). Single bond = 1 sigma, double bond = 1 sigma + 1 pi, triple bond = 1 sigma + 2 pi.

This section covers biomolecules fundamentals: (1) Functional groups - hydroxyl, carboxyl, amino, carbonyl, and phosphate groups that determine chemical properties; (2) Covalent bonds connecting monomers - glycosidic (carbohydrates), peptide (proteins), ester (lipids), phosphodiester (nucleic acids), and disulfide bonds; (3) Identifying biomolecules from structural diagrams by analyzing functional groups; (4) Peptide bond structure and formation between amino acids. The instructor emphasizes that understanding these bonding patterns is essential for identifying biomolecule types and their functions.