Amino Acids, Proteins, and Enzymes
The twenty standard amino acids, the four levels of protein structure, and enzyme kinetics form the vocabulary of biochemistry. Nearly every exam opens with questions in this domain, and mastery here builds your foundation for every later chapter.
Core Amino Acid Families
Non-polar amino acids like glycine, alanine, and the branched-chain amino acids (BCAAs) appear frequently on exams. Glycine is the simplest with only a hydrogen side chain. Alanine participates in the glucose-alanine cycle. BCAAs (valine, leucine, isoleucine) are essential and metabolized primarily in muscle.
Sulfur-containing amino acids deserve special attention. Cysteine forms disulfide bridges in folded proteins. Methionine is essential and codes the start signal (AUG) in translation.
Basic amino acids like lysine and arginine carry positive charges. Lysine is commonly acetylated on histones. Arginine is central to the urea cycle and nitric oxide synthesis. Histidine has a pKa near physiological pH, making it ideal for enzyme active sites and buffering.
Acidic amino acids (aspartate and glutamate) are negatively charged and appear frequently in enzyme active sites and as CNS neurotransmitters.
Aromatic amino acids (phenylalanine, tyrosine, tryptophan) serve as precursors for important molecules. Tyrosine precedes catecholamines and thyroid hormones. Tryptophan becomes serotonin.
Protein Structure Hierarchy
Primary structure is the linear sequence of amino acids held by peptide bonds. Secondary structure consists of local folds like alpha helices and beta sheets, stabilized by backbone hydrogen bonds.
Tertiary structure is the three-dimensional fold of a single polypeptide. R-group interactions and disulfide bonds stabilize this level. Quaternary structure occurs when multiple polypeptides assemble into a functional protein, like hemoglobin.
Enzyme Kinetics Essentials
Michaelis-Menten kinetics describes how substrate concentration affects reaction rate. The model yields two critical parameters: Km (affinity) and Vmax (maximum velocity).
Competitive inhibitors bind the active site and raise apparent Km. More substrate can overcome this inhibition. Non-competitive inhibitors bind allosteric sites, leaving Km unchanged but decreasing Vmax. No amount of extra substrate rescues a non-competitive inhibitor.
| Term | Meaning |
|---|---|
| Glycine | The simplest amino acid with only a hydrogen as its side chain. Non-polar and achiral. Abundant in collagen. |
| Alanine | Non-polar amino acid with a methyl side chain. Participates in the glucose-alanine cycle between muscle and liver. |
| Valine, leucine, isoleucine | Branched-chain amino acids (BCAAs). Essential and commonly metabolized in muscle. Accumulate in maple syrup urine disease. |
| Cysteine | Sulfur-containing amino acid with a thiol (-SH) side chain. Forms disulfide bridges in folded proteins. |
| Methionine | Sulfur-containing essential amino acid; the start codon (AUG) codes for methionine. |
| Lysine | Basic, positively charged amino acid at physiological pH. Commonly modified by acetylation in histones. |
| Arginine | Basic amino acid central to the urea cycle and nitric oxide synthesis. |
| Histidine | Imidazole side chain with a pKa near physiological pH, making it ideal for enzyme active sites and buffering. |
| Aspartate / Glutamate | Acidic, negatively charged amino acids. Common in enzyme active sites and as neurotransmitters in the CNS. |
| Proline | Cyclic amino acid that introduces kinks in protein structure. Common in collagen and at turns. |
| Phenylalanine, tyrosine, tryptophan | Aromatic amino acids. Tyrosine is a precursor for catecholamines and thyroid hormones; tryptophan for serotonin. |
| Primary structure | Linear sequence of amino acids in a protein, held together by peptide bonds. |
| Secondary structure | Local folding patterns (alpha helices and beta sheets) stabilized by hydrogen bonds in the peptide backbone. |
| Tertiary structure | Three-dimensional folding of a single polypeptide, stabilized by R-group interactions including disulfide bonds. |
| Quaternary structure | Assembly of multiple polypeptide subunits into a functional protein (e.g., hemoglobin, a tetramer). |
| Michaelis-Menten kinetics | Model of enzyme kinetics describing the relationship between substrate concentration and reaction rate; yields Km and Vmax. |
| Competitive inhibition | Inhibitor binds the active site; raises apparent Km, Vmax unchanged. Can be overcome by more substrate. |
| Non-competitive inhibition | Inhibitor binds an allosteric site; Km unchanged, Vmax decreases. Cannot be overcome by more substrate. |
Carbohydrate and Lipid Metabolism
Glycolysis, the TCA cycle, the electron transport chain, and fatty acid metabolism form the spine of any biochemistry course. Master the key enzymes and regulatory steps first. Fill in intermediates once the big picture is clear.
Glycolysis and Entry Points
Glycolysis converts glucose to two pyruvate molecules in the cytoplasm, yielding net 2 ATP and 2 NADH. Hexokinase and glucokinase catalyze the first phosphorylation. Glucokinase (in liver and pancreas) has higher Km and is induced by insulin.
Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme. It responds to energy status: activated by AMP and F-2,6-BP, inhibited by ATP and citrate. Pyruvate kinase catalyzes the final step and is regulated by phosphorylation (inhibited by glucagon).
The Mitochondrial Metabolism Hub
Pyruvate dehydrogenase converts pyruvate to acetyl-CoA and requires five cofactors (TPP, lipoic acid, CoA, FAD, NAD+). This is a major control point between glycolysis and the TCA cycle.
The TCA cycle (citric acid cycle) oxidizes acetyl-CoA to 2 CO2 per turn, yielding 3 NADH, 1 FADH2, and 1 GTP. This cycle is the cell's energy powerhouse.
Oxidative phosphorylation uses NADH and FADH2 to pump protons across the inner mitochondrial membrane. This gradient drives ATP synthase, yielding approximately 28 ATP per glucose molecule.
Glucose Storage and Mobilization
Gluconeogenesis (hepatic glucose synthesis) uses unique enzymes to bypass glycolysis's irreversible steps. This becomes critical during fasting and is activated by glucagon.
Glycogenesis builds glycogen from glucose-1-phosphate. Glycogen synthase (activated by insulin) catalyzes this reaction. Glycogenolysis breaks down glycogen back to glucose-1-phosphate. Glycogen phosphorylase is activated by glucagon and epinephrine.
Fatty Acid and Lipid Pathways
The pentose phosphate pathway generates NADPH and ribose-5-phosphate in the cytoplasm. This is essential for biosynthesis and cellular reducing power.
Beta-oxidation breaks fatty acids into acetyl-CoA in mitochondria, generating NADH and FADH2. The carnitine shuttle transports long-chain fatty acids into the mitochondrial matrix and is inhibited by malonyl-CoA.
Fatty acid synthesis (cytoplasmic) uses acetyl-CoA and NADPH. Acetyl-CoA carboxylase is the rate-limiting enzyme. Ketogenesis (hepatic) produces acetoacetate and beta-hydroxybutyrate during prolonged fasting or uncontrolled diabetes.
Cholesterol synthesis occurs in the cytoplasm and ER. HMG-CoA reductase is the rate-limiting enzyme and the target of statins. Lipoproteins (chylomicrons, VLDL, LDL, HDL) transport lipids in plasma. LDL delivers cholesterol to tissues. HDL returns it to the liver.
| Term | Meaning |
|---|---|
| Glycolysis | Cytoplasmic pathway converting glucose to two pyruvate, yielding a net 2 ATP and 2 NADH. |
| Hexokinase / Glucokinase | Phosphorylates glucose to glucose-6-phosphate. Glucokinase (liver/pancreas) has higher Km and is induced by insulin. |
| Phosphofructokinase-1 (PFK-1) | Rate-limiting enzyme of glycolysis. Activated by AMP and F-2,6-BP, inhibited by ATP and citrate. |
| Pyruvate kinase | Catalyzes the last step of glycolysis, generating pyruvate and ATP. Regulated by phosphorylation (inhibited by glucagon). |
| Pyruvate dehydrogenase | Converts pyruvate to acetyl-CoA in the mitochondrial matrix; requires five cofactors (TPP, lipoic acid, CoA, FAD, NAD+). |
| TCA (citric acid) cycle | Mitochondrial cycle oxidizing acetyl-CoA to 2 CO2, yielding 3 NADH, 1 FADH2, and 1 GTP per turn. |
| Oxidative phosphorylation | Inner mitochondrial membrane process using NADH/FADH2 to pump protons and drive ATP synthase. Yields ~28 ATP per glucose. |
| Gluconeogenesis | Hepatic synthesis of glucose from non-carbohydrate precursors; uses unique enzymes to bypass the irreversible steps of glycolysis. |
| Glycogenesis | Synthesis of glycogen from glucose-1-phosphate; regulated by glycogen synthase, which is activated by insulin. |
| Glycogenolysis | Breakdown of glycogen to glucose-1-phosphate by glycogen phosphorylase; activated by glucagon and epinephrine. |
| Pentose phosphate pathway | Cytoplasmic pathway generating NADPH and ribose-5-phosphate. Rate-limited by glucose-6-phosphate dehydrogenase. |
| Beta-oxidation | Mitochondrial breakdown of fatty acids into acetyl-CoA, generating NADH and FADH2 for the electron transport chain. |
| Carnitine shuttle | Transports long-chain fatty acids into the mitochondrial matrix; inhibited by malonyl-CoA. |
| Fatty acid synthesis | Cytoplasmic process using acetyl-CoA and NADPH. Rate-limited by acetyl-CoA carboxylase. |
| Ketogenesis | Hepatic production of acetoacetate and beta-hydroxybutyrate during prolonged fasting or uncontrolled diabetes. |
| Cholesterol synthesis | Occurs in the cytoplasm/ER; HMG-CoA reductase is the rate-limiting enzyme and the target of statins. |
| Lipoproteins | Chylomicrons, VLDL, LDL, and HDL transport lipids in plasma. LDL delivers cholesterol to tissues; HDL returns it to the liver. |
Nucleotide Metabolism and Molecular Biology
DNA and RNA synthesis, the genetic code, and nucleotide pathways are the molecular biology backbone of biochemistry. These concepts appear on every major exam and connect directly to clinical pharmacology.
Nucleotide Synthesis
Purines (adenine and guanine) are double-ring bases synthesized de novo starting from ribose-5-phosphate and glutamine. Pyrimidines (cytosine, thymine, uracil) are single-ring bases synthesized starting from carbamoyl phosphate.
Ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides for DNA synthesis. Hydroxyurea inhibits this enzyme. Thymidylate synthase methylates dUMP to dTMP using N5,N10-methylene-tetrahydrofolate. 5-fluorouracil (5-FU) blocks this reaction.
Dihydrofolate reductase regenerates tetrahydrofolate (THF) from dihydrofolate. Methotrexate (cancer drug) and trimethoprim (antibiotic) inhibit this enzyme.
Salvage Pathways and Genetic Diseases
Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency (SCID) through toxic deoxyadenosine accumulation. HGPRT salvages hypoxanthine and guanine. Deficiency causes Lesch-Nyhan syndrome.
DNA Replication and Transcription
DNA polymerase synthesizes DNA 5' to 3' using a template strand. Prokaryotic polymerase I repairs DNA while polymerase III replicates the genome.
Helicase unwinds the double helix. Topoisomerase relieves supercoiling ahead of the replication fork. Okazaki fragments are short DNA pieces synthesized discontinuously on the lagging strand, then joined by DNA ligase.
RNA polymerase II transcribes mRNA in eukaryotes. Alpha-amanitin (a mushroom toxin) inhibits this enzyme.
mRNA Processing and Translation
mRNA splicing removes introns through the spliceosome. Alternative splicing expands the proteome from a limited number of genes. The 5' cap and 3' poly-A tail stabilize mRNA and promote translation.
The genetic code contains 64 codons encoding 20 amino acids plus stop signals. It is degenerate (multiple codons per amino acid) but unambiguous, and nearly universal across all life.
The ribosome (60S plus 40S subunits in eukaryotes) translates mRNA at three sites: A (incoming), P (peptide), and E (exit). AUG initiates translation (methionine). UAA, UAG, UGA are stop codons.
Chaperone proteins (HSP70, HSP90) assist nascent polypeptide folding and prevent aggregation and misfolding.
| Term | Meaning |
|---|---|
| Purines | Adenine and guanine, double-ring nitrogenous bases. Synthesized de novo starting from ribose-5-phosphate and glutamine. |
| Pyrimidines | Cytosine, thymine, and uracil, single-ring nitrogenous bases. Synthesized de novo starting from carbamoyl phosphate. |
| Ribonucleotide reductase | Converts ribonucleotides to deoxyribonucleotides; inhibited by hydroxyurea. |
| Thymidylate synthase | Methylates dUMP to dTMP using N5,N10-methylene-THF. Inhibited by 5-fluorouracil (5-FU). |
| Dihydrofolate reductase | Regenerates THF from DHF. Inhibited by methotrexate (anticancer) and trimethoprim (antibacterial). |
| Adenosine deaminase (ADA) | Deficiency causes severe combined immunodeficiency (SCID) by toxic accumulation of deoxyadenosine. |
| HGPRT | Salvages hypoxanthine and guanine. Deficient in Lesch-Nyhan syndrome. |
| DNA polymerase | Synthesizes DNA 5' to 3' using a template strand. Prokaryotic versions include pol I (repair) and pol III (replication). |
| Helicase / Topoisomerase | Helicase unwinds the double helix; topoisomerase relieves supercoiling ahead of the fork. |
| Okazaki fragments | Short DNA fragments synthesized discontinuously on the lagging strand, later joined by DNA ligase. |
| RNA polymerase II | Eukaryotic enzyme that transcribes mRNA. Inhibited by alpha-amanitin (death cap mushroom toxin). |
| mRNA splicing | Removal of introns by the spliceosome, yielding mature mRNA. Alternative splicing expands the proteome. |
| 5' cap / 3' poly-A tail | Post-transcriptional modifications that stabilize mRNA and promote translation in eukaryotes. |
| Genetic code | 64 codons encode 20 amino acids plus stop signals. Degenerate but unambiguous; near-universal across life. |
| Ribosome | Ribonucleoprotein complex (60S + 40S in eukaryotes) that translates mRNA into protein at the A, P, and E sites. |
| Start / Stop codons | AUG (methionine) initiates translation; UAA, UAG, and UGA are stop codons. |
| Chaperone proteins | Assist nascent polypeptide folding (e.g., HSP70, HSP90). Prevent aggregation and misfolding. |
How to Study biochemistry Effectively
Mastering biochemistry requires the right study approach, not just more hours. Three evidence-based techniques produce the best learning outcomes: active recall (testing yourself rather than re-reading), spaced repetition (reviewing at scientifically-optimized intervals), and interleaving (mixing related topics instead of studying one in isolation). FluentFlash is built around all three.
When you study with our FSRS algorithm, every term is scheduled at the moment you are about to forget it. This maximizes retention while minimizing study time.
Why Passive Review Fails
Re-reading notes, highlighting textbook passages, and watching lectures feel productive but deliver only 10-20% of the retention that active recall achieves. Flashcards force your brain to retrieve information, which strengthens memory pathways far more than recognition alone.
Pair flashcards with spaced repetition scheduling, and you can learn in 20 minutes daily what passive review takes hours to accomplish.
A Practical Study Plan
Start by creating 15-25 flashcards covering the highest-priority concepts. Review them daily for the first week using FSRS scheduling. As cards become easier, intervals expand automatically from minutes to days to weeks.
You will always work on material at the edge of your knowledge. After 2-3 weeks of consistent practice, biochemistry concepts become automatic rather than effortful to recall.
Study Steps
- Generate flashcards using FluentFlash AI or create them manually from your notes
- Study 15-20 new cards per day, plus scheduled reviews
- Use multiple study modes (flip, multiple choice, written) to strengthen recall
- Track your progress and identify weak topics for focused review
- Review consistently. Daily practice beats marathon sessions
- 1
Generate flashcards using FluentFlash AI or create them manually from your notes
- 2
Study 15-20 new cards per day, plus scheduled reviews
- 3
Use multiple study modes (flip, multiple choice, written) to strengthen recall
- 4
Track your progress and identify weak topics for focused review
- 5
Review consistently, daily practice beats marathon sessions
Why Flashcards Work Better Than Other Study Methods for biochemistry
Flashcards are one of the most research-backed study tools for any subject, including biochemistry. The mechanism is straightforward: when you read a textbook passage, your brain stores information in short-term memory. Without retrieval practice, it fades within hours. Flashcards force retrieval, which transfers information from short-term to long-term memory.
The Testing Effect
The testing effect, documented in hundreds of peer-reviewed studies, shows that students using flashcards consistently outperform those who re-read by 30-60% on delayed tests. This is not because flashcards contain more information. Rather, retrieval strengthens neural pathways in ways that passive exposure cannot.
Every time you successfully recall a biochemistry concept from a flashcard, you make that concept easier to recall next time. Your brain builds stronger connections with each successful retrieval.
FSRS Amplifies Spaced Repetition
FluentFlash amplifies the testing effect with the FSRS algorithm, a modern spaced repetition system. It schedules reviews at mathematically-optimal intervals based on your actual performance.
Cards you find easy move further into the future. Cards you struggle with return sooner. Over time, this builds remarkable retention with minimal time investment. Students using FSRS-based systems typically retain 85-95% of material after 30 days, compared to roughly 20% retention from passive review alone.